Strangest language feature

Question

What is  in your opinion  the most surprising  weird  strange or really  WTF  language feature you have encountered   Please only one feature per answer

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In JavaScript  seeing   a for the first time  as a way to convert to boolean

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In C  arrays can be indexed like so   a 10    which is very common   However  the lesser known form  which really does work   is   10 a    which means the same as the above

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This is one of my favorites  you can do a println in Java without main     This will compile and run  giving the println  but also an exception  java lang NoSuchMethodError  main   class Test       static           System out println  I m printing in Java without main

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In MOD REWRITE  RewriteRule    a-zA-Z0-9 -     php   1   R NC  RewriteRule    a-zA-Z0-9 -       1  php  NC L    Will cause    file php  gt  file  gt  file php  gt  file  gt  file php  gt  file  gt          And finally   Error 500 Too Many Redirects    In general I find editing  htaccess files to be about as tedious as constructing a properly functioning regular expression

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In fortran  77 for sure  maybe in 95 as well   undeclared variables and arguments beginning with I through N  the  in  group  will be INTEGER  and all other undeclared variables and arguments will be REAL  source    This  combined with  whitespace optional in certain cases  resulted in one of the most famous bugs   As told by Fred Webb in alt folklore computers in 1990      I worked at Nasa during the summer of 1963   The group I was working   in was doing preliminary work on the Mission Control Center computer   systems and programs   My office mate had the job of testing out an   orbit computation program which had been used during the Mercury   flights   Running some test data with known answers through it  he was   getting answers that were close  but not accurate enough   So  he   started looking for numerical problems in the algorithm  checking to   make sure his tests data was really correct  etc       After a couple of weeks with no results  he came across a DO   statement  in the form   DO 10 I 1 10       This statement was interpreted by the compiler  correctly  as   DO10I   1 10       The programmer had clearly intended   DO 10 I   1  10       After changing the   to a   the program results were correct to   the desired accuracy   Apparently  the program s answers had been    good enough  for the sub-orbital Mercury flights  so no one suspected   a bug until they tried to get greater accuracy  in anticipation of   later orbital and moon flights   As far as I know  this particular bug   was never blamed for any actual failure of a space flight  but the   other details here seem close enough that I m sure this incident is the   source of the DO story    I think it s a big WTF if DO 10 I is taken as DO10I  and that in turn  because of implicit declarations is taken to be of type REAL   And it s a great story

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I would definitely give Perl the honor of having multiple horrible examples   if    var    or  if  mystring    m   d

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In Ruby  0 evaluates as true in conditional expressions

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Tcl s virtualize the sense of time hooks in the interpreter are pretty weird  http   www tcl tk cgi-bin tct tip 233 html  Basically it allows you to make the interpreter use some other source of time data  e g  to run hardware tests in a simulator first and than later just replace the timer functions and run the identical tests against the real thing

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Haskell s use of Maybe and Just  Maybe a is a type constructor that returns a type of Just a  but Maybe Int won t accept just an Int  it requires it to be a Just Int or Nothing  So in essence in haskell parlance Just Int is about as much of an Int as an apple is an orange  The only connection is that Just 5 returns a type of Maybe Interger  which can be constructed with the function Just and an Integer argument  This makes sense but is about as hard to explain as it can theoretically be  which is the purpose of haskell right  So is Just really JustKindaLikeButNotAtAll yea sorta  and is Maybe really a KindaLooksLikeOrIsNothing  yea sorta again   -- Create a function that returns a Maybe Int  and return a 5  which know is definitly Int able  gt   let x    Maybe Int  x   5   lt interactive gt  1 24      No instance for  Num  Maybe Int         arising from the literal  5  at  lt interactive gt  1 24     Possible fix  add an instance declaration for  Num  Maybe Int       In the expression  5     In the definition of  x   x   5   gt   Just 5   Just 5 it    Maybe Integer      -- Create a function x which takes an Int  gt   let x    Int - gt  Int  x     0  x    Int - gt  Int -- Try to give it a Just Int  gt   x   Just 5                      lt interactive gt  1 4      Couldn t match expected type  Int  against inferred type  Maybe t      In the second argument of        namely  Just 5      In the expression  x   Just 5     In the definition of  it   it   x   Just 5   Good luck reading this  I hope its right

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In JavaScript   alert 111111111111111111111     alerts 111111111111111110000   This was quite damaging to some 64bit keys I passed back and forth in JSON

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In MATLAB  interactive array-oriented language  currently TIOBE 20  there is a keyword end to denote the last element of array  it corresponds to NumPy -1   So this is a well known MATLAB syntax   myVar   myArray end    To get an element from the middle of array one would usually write   myVar   myArray  ceil  length myArray  2       Surprisingly the keyword end is not a keyword at all but is a kind of variable   myVar   myArray  ceil  end 2

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In Forth  anything that does not contains spaces can be an identifier  things that contain spaces take a bit of work   The parser first checks if the thing is defined  in which case it is called a word  and  if not  checks if it is a number  There are no keywords   At any rate  this means that one can redefine a number to mean something else     0 1     Which creates the word 0  composed of 1  whatever that was at the time this was executed  In turn  it can result in the following   0 0     2 Ok   On the other hand  a definition can take over the parser itself -- something which is done by the comment words  That means a Forth program can actually become a program in a completely different language midway  And  in fact  that s the recommended way of programming in Forth  first you write the language you want to solve the problem in  then you solve the problem

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The designers of VB NET did several really dumb things to maintain backwards compatibility with Visual Basic 6 0   Of course  not enough that it actually was compatible   just enough to make things more counter-intuitive   But the worst of them was the fact that you don t have to initialize variables because they already are  except on those rare occasions when they are not         For i As Integer   1 To 3         Try             Dim k As Integer             k    1             MsgBox k          Catch ex As Exception             MsgBox ex ToString          End Try     Next   This will print 1 2 3   Having a feature you can t trust 100  of the time is not a feature  it s a bug   Saying it s as designed just makes it a design bug  not an implementation bug

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Not so long ago  when I first descoverd the C Language in my CS class  it was very strange to see the way pointers behaived  we just wrote programs and guess what it would do  until they get the right behavior

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The implict variables constants and mutable constants in ruby

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javascript   parseInt  06       6 parseInt  08       0

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Easy pickins  Erlang is full of them   For example  3 forms of punctuation    a function SomeVariable  - gt    statements end with commas      case PatternMatching of     0 - gt  now we end with semicolon      true - gt  except the last one   end       Function definitions end with periods

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In C   why is this not legal   public class MyClass lt T gt      where T  Enum        It d be pretty cool to be able to add extension methods on Enum s along with Func lt T gt  where the T would be the enum you re extending so that you can get type inference on that enum   Re the comment  Yes  you can extend an actual enum  but here s the difference   You CAN do this   public static void DoSomethingWithEnum this Enum e         do whatever     but what if you want to take a Func with your method that would be the same type as your enum   public static void DoSomethingWithEnum lt T gt  this T e  Func lt T bool gt  func      where T  Enum        do whatever     That way  you can call your method like so   DayOfWeek today   DayOfWeek Monday  today DoSomethingWithEnum e   gt  e    DayOfWeek Sunday     or something like that  You get the idea    THAT S not possible  and I m not sure why

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PHP backticks  From http   www php net manual en language operators execution php     PHP supports one execution operator  backticks       Note that these are not single-quotes  PHP will attempt to execute the contents of the backticks as a shell command  the output will be returned  i e   it won t simply be dumped to output  it can be assigned to a variable      output    ls -al   echo   lt pre gt  output lt  pre gt      Well it s  quite easy  to spot   instead of   in the code   This is funny  too      After much trouble  I have concluded that the backtick operator  and shell exec  have a limited buffer for the return  My problem was that I was grepping a file with over 500 000 lines  receiving a response with well over 100 000 lines   After a short pause  I was flooded with errors from grep about the pipe being closed

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PL SQL allows to declare variables and function names that are keywords  The following is compilable PL SQL   create or replace    function function    return number  as   return number  begin    function return    4    return   return  end function      This created a function named function  Later    SQL gt  select function from dual     FUNCTION ----------          4

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In C-like languages  including C itself   you can use the  goes down to  operator   for  x   20  x -- gt  0         print x      This will print the numbers from 19 to 0

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ActionScript 3   When an object is used by its interface  the compiler doesn t recognize the methods inherited from Object  hence   IInterface interface   getInterface    interface toString      gives a compilation error  The workaround is casting to Object  Object interface  toString      PHP     and   operators  It has its reasonable explanation  but still  a     5    5 seems awkward   Java  and any implementation of IEEE754    System out println 0 1   0 1   0 1   0 1   0 1   0 1   0 1   0 1   0 1   0 1     Outputs 0 9999999999999999

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I was taken by surprise that you can change a class s inheritance chain in Perl by modifying its  ISA array   package Employee  our  ISA   qw Person     somwhere far far away in a package long ago  Employee  ISA   qw Shape      Now all Employee objects no longer inherit from  Person  but from  Shape

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The absolute worst WTF has got to be Cobol s ALTERED GOTO   The syntax is pretty straight forward    ALTER label1 TO GOTO label2   but the results of debugging run-time spaghetti are mind-boggling

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Common Lisp s format function has an option to print numbers as Roman numerals   In INTERCAL that is the only form of output you ll ever get

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I m fond of the lack of operator precedence in Smalltalk  2   3   4   5   6   4   5   10   5   50   instead of  2   3   4   5   6   4   5   6   20   26  This is due to the object nature of smalltalk and the fact that messages are passed left to right  If the message   is sent to the 2 with the number 3 as a parameter  the response of that message is 6  Pretty awesome  you can even monkey patch it if you re feeling evil

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In JavaScript     5    3 gives  53    Whereas    5  - 3 gives 2

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I added the  format  function to Lisp in about 1977  before  printf  even existed  I was copying from the same source as Unix did  Multics    It started off innocently enough  but got laden with feature after feature  Things got out of hand when Guy Steele put in iteration and associated features  which were accepted into the Common Lisp X3J13 ANSI standard   The following example can be found at Table 22-8 in section 22 3 3 of Common Lisp the Language  2nd Edition    defun print-xapping  xapping stream depth     declare  ignore depth      format stream                     S   - gt  S                         S- gt                    - gt  S  - gt                       xectorp xapping         do   vp  xectorp xapping               sp  finite-part-is-xetp xapping               d  xapping-domain xapping   cdr d               r  xapping-range xapping   cdr r               z      cons  list  if vp  car r   car d    or vp sp   car r   z                null d   reverse z           and  xapping-domain xapping              or  xapping-exceptions xapping              xapping-infinite xapping           xapping-exceptions xapping         and  xapping-exceptions xapping              xapping-infinite xapping          ecase  xapping-infinite xapping            nil  0            constant 1            universal 2          xapping-default xapping         xectorp xapping

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Something bizarre -- VBScript having both a Null keyword and a Nothing keyword  Null is missing data and Nothing is a missing object    Why not just have one keyword      Most other languages seem to do fine with one   Visual Basic 6 0 and of course  Classic ASP  code  because it uses VBScript  have the same bizarrity   And in Visual Basic old and new we also have DBNull   The situation is improving however  as in Visual Basic NET Null has at last gone away so that Null is unused and only Nothing and DBNull are used

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For those who never worked with COBOL  this is a common line of code but it doesn t do what you might be thinking about     PIC XXX

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At Ohio State University they teach programming using a bastard C   language called Resolve C     Resolve C   uses a design-by-contract methodology to everything   It requires you to mathematically model out components and methods within comments that get compiled so that it forces you to maintain a requires ensures relationship between methods and objects

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Tri-valued logic of nulls in ANSI SQL

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Oracle has a couple of SQL WTF issues    Oracle s treatment of empty strings as null  Treatment of null values in a   lt    comparison   create table wtf  key number primary key  animal varchar2 10        insert into wtf values  1  dog    insert into wtf values  2      insert into wtf values  3  cat        select   from wtf where animal  lt  gt   cat      The only row returned is the  1  dog   row

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Perl   s many built-in variables           not a comment   0      and        just like the shell variables by the same name       amp   and        weird matching variables    and        weird variables for list- and output-field-separators        like errno as a number but strerror errno  as a string        the stealth variable  always used and never seen         index number of the last subroutine argument    maybe        the  non names of the current function    maybe        the last-raised exception         the symbol table              -  and        something to do with output formats    and        input line number  output page number    and        input and output record separators        output buffering controller        change your array base from 0-based to 1-based to 42-based  WHEEE         nothing at all  oddly enough    lt     gt              real and effective UIDs and GIDs  ISA     names of current package   s direct superclasses   T     script start-up time in epoch seconds   O     current operating system name   V     what version of Perl this is   There   s a lot more where those came from  Read the complete list here

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In PHP   echo  foo     0        echos  1  echo  foo     true     echos  1  echo 0    true         echos  0   foo    foo   echo  foo  bar         echos  f    PHP has some of the most annoying type coercion

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In Python    gt  gt  gt  x   4  gt  gt  gt  y   1000000  gt  gt  gt  x is 4 True  gt  gt  gt  y is 1000000 False  gt  gt  gt    Just try it if you don  t believe me

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In MySQL string comparisons are case-insensitive     gt  SELECT   FROM blah WHERE foo    BAR    gt  SELECT   FROM blah WHERE foo    Bar    gt  SELECT   FROM blah WHERE foo    bAr     Are all equivelent  Not only that they will match any value of foo that looks like  bar   e g  if foo    bar  it will match for BAR  baR  bAR  etc

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PHP  From the online doc  string implode     string  glue     array  pieces        Join array elements with a string Note  implode   can  for historical reasons  accept its parameters in either order   So this works  implode  someArray   glue   Hope they kill these historical quirks in PHP 6

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In C  a i      i    It compiles  but it rarely does what you think it ought to do   An optimization change leads to producing wildly different results   And it runs differently on different platforms   Yet  the compiler s perfectly happy with it

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The weird things C   templates can be used for  best demonstrated by  Multi-Dimensional Analog Literals  which uses templates to compute the area of  drawn  shapes  The following code is valid C   for a 3x3 rectangle   include analogliterals hpp  using namespace analog literals  symbols             unsigned int c     o-----o                                                                                                                                             o-----o   area    Or  another example with a 3D cube     assert    o-------------o              L                             L                              L                               o-------------o                                                                             o                                L                                L                                L                                o-------------o   volume      o-------------o                                                                                                                                                                                                                                         o-------------o   area   int I-------------I

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In SQL server  MS at least    This will always evaluate to false   IF  someint  lt  gt  NULL   Given   DECLARE  int INT  SET  int   6  IF  int  lt  gt  NULL BEGIN     Print   int is not null  END ELSE BEGIN     Print   int is evaluating to null  END   The output will be    int is evaluating to null   It must be written   IF  someint IS NOT NULL BEGIN END   Who put English majors on the SQL Team

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Weak typing  in general   C   printf   c n    a    3     PHP   echo 5    3     And far too many other languages

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Perl can automatically convert base 26 into base 10  if you can live with yourself in the morning       perl -E  say lc   a  a    asdf    30530

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Another C-ism   int i  0  while  i    12            Some comment      i    1         Another comment        Why doesn t it work   Lint will tell you   The C compiler  however  usually passes over this blithely   As did I   That was a real WTF moment when I figured out what was wrong

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In JavaScript  and Java I think  you can escape funny characters like this   var mystring    hello   world       If you want to put a carriage return into a string though  that s not possible  You have to use  n like so   var mystring    hello   nworld     That s all normal and expected- for a programming language anyway  The weird part is that you can also escape an actual carriage return like this   var mystring    hello    world

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In Common Lisp  arrays with zero dimensions are strange  and naturally  they have read syntax      aref  0A5  5

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In Perl  objects are just blessed refs  so changing the class of an object at run time is a piece of cake   package Foo  sub new   bless        0    package Bar  package main  my  foo   Foo- gt new  ref  foo       gt   Foo  bless  foo   Bar   ref  foo       gt   Bar    I was surprised that other languages can t do this  What a useful feature

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in PHP the strings letters cannot be used like in C  you need to use ord   and chr   in order to convert from number to char and vica versa   a     97  but ord  a      97   Although  there is one exception   for   i    a    i  lt   z    i          print   i        will print letters a to y  just like you would expect as if it was C style datatypes   however if the test condition is changed to  lt   it will not print a to z as you would think  but a to yz   total 676 items printed   also if you change the  z  to  aa  which came out next after  z  in the 676 items list  and change test condition to  lt  again  you will see only  a  being printed out  not a to z as you would expect   And if you change the incrementor to  i  2 it will print only  a  again  only way to do that is to use  i     i   in sequence  and now it works like expected   Nevertheless  this is a nice way in PHP to output combinations of letters a-z  although its very hard to actually use it

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In Java you might expect  byte b   0  b      to be equal to  byte b   0  b   b   1    But it is not  In fact you get a compiler error  as the result of the addition is of type int and therefore not assignable to the byte variable b  When using the compound operator    The compiler automatically inserts a cast here  So   b      becomes   b    byte  b   1

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In two words  multiple inheritance  It makes no sense  and creates nothing but trouble   Edit - I am referring to MI in C    not mixins and the like in Java and other languages

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I always wondered why the simplest program was   class HelloWorldApp       public static void main String   args            System out println  Hello World               Whereas it could be   print  Hello World     Maybe this is to frighten computer science students in the first place

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Found while learning PowerShell   Try to guess what the resulted array look like    a   1  2  b   1  2 3  c   1  2 3   Answers   1  2 1  2  3 1  2  1  2  1  2   Ouch  It shakes my faith in PowerShell and people behind it

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shift    sometimes you see it in the very first line of a perl method to get read of self pointer

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Perl has the yada yada operator            The so called    yada yada    operator of Perl 6 heritage is a shortcut to mark unimplemented code    if   condition               is the same as   if   condition    die  not yet implemented

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This is a lack of a feature which is weird  Python has no switch statement  although workarounds exist

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This is nothing strange or surprising  but it is something that made me always say WTF   Case sensitivity in syntax  or in identifier names   Most languages that have it just seem to have it because C has it   There is no good reason for it

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In Python   abs  10 5j - 25 -5j     Returns  18 03  which is the distance between the points  10 5  and  25 5  by the Pythagoras theorem  This fact happens because Python has native language support to complex numbers in the form of 2 2j for example  Since the absolute value of a complex number in form of a bj   sqrt a 2 b 2   we get the distance while subtracting one complex number from another and then apply the abs  absolute  function over it

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SQLite lets you declare columns with whatever data type you want   It looks for a few particular substrings   INT    REAL    TEXT   etc   to determine the affinity   This makes it possible to lie in your type declarations   CREATE TABLE Quirks      X    FLOATING POINT   --   INTEGER affinity because of the  INT     Y    STRING           --   NUMERIC affinity

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A Fortran compiler that I used years ago had the interesting feature that   a  Numbers were stored high-byte first   b  Numbers were passed to subroutines by address   c  There was no compile-time checking of length   So you could write a program like this   Excuse me if I mess up the syntax  It s been a long time since I ve written Fortran    INTEGER 2 FUNCTION TIMESTWO  INTEGER 2 N  RETURN N 2      THEN CALL THIS SOMEWHERE WITH A LONG INTEGER      INTEGER 4 I  J  I 42 J TIMESTWO I    The final value of J is     zero    Why  Because the passed in value is 4 bytes  but the called function looks at only the first two bytes  As the first two are zero  it doubles the zero and returns it  This return value is then converted back to four bytes   This was very mysterious when I first encountered it  Almost every number I passed in to certain functions got interpreted as zero

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Reverse Polish Notation  RPN   That means the arguments precede the function  Or  in other words  you add two and two by writing 2 2      Languages featuring that WTF include Forth  Postscript  yes  of laser printers  and Factor

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The C   templating mechanism is Turing-complete  As long as you don t need input at run time  you can do arbitrary calculations at compile time  Arbitrary  Or you can easily write a C   program that never compiles - but is syntactically correct

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Perl filehandle-style operator calls   In the beginning  there was   print  foo    bar    baz     to stdout print STDERR  foo    bar    baz     Notice the ostentatious lack of a comma so that you know that s a filehandle to print-to  not a filehandle to print in a stringified manner  It s a dirty hack   Language upgrade rolls around  they make proper OO filehandles and turn x FOO y  z  abc into FOO- gt x y  z  abc   Kinda cute  The same print statement effectively runs  STDERR- gt print  foo    bar    baz      Mostly you notice this when you miss a comma  or try to run something like hashof  a   b   c  subroutine call without parentheses  and forget to import the hashof function into your namespace from its utility package  and you get a weird error message about  Can t call method  hashof  via package  contents of string  a

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And again Haskell   In Haskell you can handle an arbitary size file  as if it is a simple String  The file will be only read  if the string is actually used  Because of the incredible laziness of Haskell  a program like this will run in constant space  regardless of the file s size   main   interact   gt  gt    x - gt  if x      n  then   r n  else  x      This program convert s a file from stdin to stdout and replace LF by CRLF  the interact function input s the whole stdin to a function and moves the output to stdout    This laziness may also cause problems  because if you close a file handle  you cannot be completely shure  whether lazy Haskell has already parsed all the data from it

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either in java  this is an if statement that results in assignment   result    Boolean condition     if Boolean is true     if Boolean is false     or   data Nat   Z S Nat deriving Show nattoInt Z   0 nattoInt  S a    1   nattoInt a  buildNat 0   Z buildNat a     S  buildNat  a - 1     in Haskell    I still don t quite get how this defines the natural numbers  I understand the THEORY perfectly  -p

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Feature  Bash  the Korn shell  ksh93  and the Z shell each allow subscripting arrays with variables with or without a dollar sign   array index   value array  index   value   This  with the dollar sign  will produce the expected value of 10000   unset array for i in  1  10000  do       array  RANDOM 6 1      done unset total for count in   array     do       total    count   done echo  total   Strangeness  If you remove the dollar sign from RANDOM  the total will vary randomly  even to be greater than 10000    Similarly  this produces 3 instead of 2   a 1    r a         echo  a   And you can t use a dollar sign there because it s an assignment  a is on the lhs   although you could do it if you were using indirection  but the double evaluation still occurs   The Reason  With the dollar sign  the variable expansion is performed before the arithmetic evaluation so only gets done once  Without the dollar sign  it s performed twice  once to calculate the index for the lookup and again to calculate the index for the assignment  so  in effect  an assignment at one step in the loop might look like array 4     array 6    1 which totally scrambles the array

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In awk  arrays start at index 1  which is confusing the least

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Perl   It s possible to write a program consisting entirely of punctuation   How does this even work

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In JavaScript  void is not a keyword  it is not a type declaration  nor is it a variable name  and it is also not a function  nor is it an object  void is a prefix operator  similar to -  --      and    You can prefix it to any expression  and that expression will evaluate to undefined    It is frequently used in bookmarklets  and inline event handlers  as in this somewhat frequent example    lt a href  javascript void 0   gt do nothing lt  a gt    The way it s used in that example makes it look like a function invocation  when really it s just an overly clever way of getting the primitive undefined value  Most people don t really understand the true nature of void in JavaScript  and that can lead to a lot of nasty bugs and weird unexpected things happening   Unfortunately  I think the void operator is the only truly guaranteed way to get the undefined value in JavaScript  since undefined  as pointed out in another answer  is a variable name that can be reassigned  and    a can be messed up by Object prototype a    foo   Update  I thought of another way to generate undefined    function          Eh  a bit verbose though  and it s even less clear that returning  undefined  is its purpose

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In MAXScript  all operators are treated equal   So  a   b   c sets a equal to b  then calculates the sum a c  and discards the result

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Another vote for JavaScript   parseInt  08      0   because anything with a leading 0 is interpreted as octal  weird   and invalid octal numbers evaluate to zero  BAD   I discovered this one August when code I hadn t touched in months broke on its own  It would have fixed itself in October  as it turns out   Octal support has apparently been deprecated  so future generations of JavaScripters will not have this rite of passage

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In Scheme there are no reserved identifiers   So  the following expression evaluates to 1     lambda  lambda  lambda   let   let 1   let     Note that there is a restriction on definitions within a given scope  no definition may redefine an identifier used to define identifiers within that scope  so the following is a syntax error    begin  define define 1  define

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In PHP  you can reference variables using a sigil and a string literal or variable containing the name of the variable  for example      foo      test   echo  foo    This will print  test   The strange thing about this behavior is that you can also use non-strings as variable names  for example     array       test   echo   array       NULL     test   echo   NULL     Now we have variables named array   and even NULL  All containing the string  test

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This C program prints a different result on x86 vs  x86-64    include  lt stdio h gt  int main  void      long a   -1     unsigned b   1     printf    d n   a  gt  b      return 0

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In Visual Basic 7 and above I found the implementation of short-circuit logical evaluation to maintain compatibility with legacy Visual Basic  lt  6 code a bit of a WTF      AndAlso  MSDN    OrElse  MSDN

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In Python  the  compile time   or declaration time  evaluation of function arguments can be confusing   def append v  l            l append v      return l   print append 1  print append 2    gt  gt  gt   1   gt  gt  gt   1 2    The intention might have been   def append v  l   None       if l is None          l          l append v      return l  print append 1  print append 2    gt  gt  gt   1   gt  gt  gt   2    This behavior is useful for things like caching  but it can be dangerous   A bonus feature  tuples with mutable contents   a    1 2  3   a 2        4    OK a 2     2    crashes

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In J  foreigns      are various functions bunched together  The left argument is a category  where as the right are often  but not always  incremental values for different    stuff  For example        2  55 NB  Close console     9  10 NB  Set print precision     6  0  NB  Actual time     6  2  NB  Execution time     4  3  NB  Loaded scripts   Of course  the smart thing is to wrap them  but some you just commit to memory  BTW  all of those are  come to think of it  triadic  with 2 arguments to the right and one to the left  None of the above will work unless you give them a final valid argument

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Perl s CORE  open and standard library having elements of object orientation masked with a procedural interface  open   my  fh    gt     foobar     open is a constructor that operates on the reference returned by my    and takes the arguments      and  foobar   Moreover  that being an object that is a blessed typeglob  meaning it can t hold state inside the object    More information on my perlmonks post IO  File vs CORE  open here  http   www perlmonks org  node id 763565

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C  yield statement  not weird but pretty useful   http   msdn microsoft com en-us library 9k7k7cf0 VS 80  aspx

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In J  most primitives  a k a  functions  are monadic  one argument  or dyadic  two arguments  one to the left  one to the right   But the amend primitive takes 3  I think it s the only one  besides foreigns   It s understandable that it would take 3  but it just looks    wrong at first   vector    i  10   NB  Vector will be 0 1 2 3 4 5 6 7 8 9  10   0    vector NB  Will yield 10 1 2 3 4 5 6 7 8 9

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You can throw anything throwable in Java   class YourBoss extends Throwable     public class Main    public void main String   s  throws YourBoss      try      throw new YourBoss        catch Exception e       catch Error e

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PHP  again    First   unset  type casting    a   1   b    unset  a  var dump  a      1 var dump  b      NULL   Usage  http   www php net manual en language types type-juggling php 89637  Second  difference between   NULL and the unset   function    c   10   d    amp  c   c   NULL  var dump  c      NULL var dump  d      NULL    e   10   f    amp  e  unset  e   var dump  e      NULL var dump  f      10 - WTF

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In Java  Actually  I have wrote this on different SO post recently         int x   1                           1      System out println x

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Well  this one s also my all-time-favorite hard to find bug    treating integers beginning with a zero as octal numbers  This led to a bug that would only show between 8 and 10 in the morning   Once  I helped building an automated regression test to be executed via cron at night  It worked nearly for everyone in a 20 person team - expect one developper complained every once in a while the automatic test had failed  but when run manually  everything worked fine  Not even once this could be reproduced manually   Well  the reason was  we did some calculation  in bash  for statistics based on the output of the date command  and this failed only from 8 00 till 9 59 in the morning because we d read the hour value as  08   which is an illegal octal value  whereas  01  -  07  are valid octal values  and from  10  onwards everything is treated as decimal again

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I came across this one while trying to figure out a MACRO that made absolutely no sense but worked anyway  This is true for objective-c but might be also true for other flavors of C  or at least the gcc compiler   NSString  oneString     This     is     just     one     normal      string     equals  NSString  oneString     This is just one normal string     It s also true for C style strings  char  str    this    also    works

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Ruby  Time parse often pretends that the parsing did not fail  returns now instead  require  time   Time parse  2000-01-01 12 00 00    - gt  2000-01-01 12 00 00  0100  Time parse  2000-99-01 00 00 00    - gt  ArgumentError  argument out of range      Time parse  now    - gt  2010-08-13 21 26 13  0200  Time parse  yesterday    - gt  2010-08-13 21 26 18  0200  Time parse  billion years ago    - gt  2010-08-13 21 26 37  0200

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In C  a b  c  is exactly the same thing as c b a

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In Bash  variables can appear to be both scalars and arrays     a 3   echo  a 3   echo   a          treat it like an array 3   declare -p a      but it s not declare -- a  3    a 1  4            treat it like an array   echo  a           acts like it s scalar 3   echo   a          but it s not 3 4   declare -p a declare -a a    0   3   1   4      a 5               treat it like a scalar   echo  a           acts like it s scalar 5   echo   a          but it s not 5 4   declare -p a declare -a a    0   5   1   4      ksh does the same things  but uses typeset instead of declare   When you do this in zsh  you get substring assignment instead of arrays     a 3   a 2  4            zsh is one-indexed by default   echo  a 34   a 3  567   echo  a 34567   a 3  9   echo  a 34967   a 3  123           here it overwrites the first character  but inserts the others   echo  a 3412367   a  1 2 3    echo  a 1 2 3                it s an array without needing to use   a      but it will work    a 2  99            what about assignments    echo  a 1 99 3

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PHP s handling of numeric values in strings   See this previous answer to a different question for full details but  in short      01a4      001a4    If you have two strings that contain a different number of characters  they can   t be considered equal   The leading zeros are important because these are strings not numbers    01e4      001e4    PHP doesn   t like strings  It   s looking for any excuse it can find to treat your values as numbers  Change the hexadecimal characters in those strings slightly and suddenly PHP decides that these aren   t strings any more  they are numbers in scientific notation  PHP doesn   t care that you used quotes  and they are equivalent because leading zeros are ignored for numbers   To reinforce this point you will find that PHP also evaluates  01e4      10000  as true because these are numbers with equivalent values   This is documented behaviour  it   s just not very sensible

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Don t know if it is a feature or not  For some  yes  but for others it might be an annoying behavior  Anyway  I think it s worth mentioning   In Python  the builtin function round   behaves a bit differently between Python 2x and Python 3x   For Py 2x    gt  gt  gt  round 0 4  0 0  gt  gt  gt  round 0 5  1 0  gt  gt  gt  round 0 51  1 0  gt  gt  gt  round 1 5  2 0   For Py 3x    gt  gt  gt  round 0 4  0  gt  gt  gt  round 0 5  0  gt  gt  gt  round 0 51  1  gt  gt  gt  round 1 5  2   I m just not familiar with the way round   in Py 3x works with 0   Docs for round   in Py 2x and Py 3x

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Processing  processing org  is a language based on Java  In simple terms  processing compiler is Java preprocessor that translates Processing-specific syntax into Java    Due to the language s design  it has a few surprises   Processing s class are compiled into Java inner class  this causes some annoyance  like private variables that isn t really private  class Foo     private int var   0     compiles fine    void setup       Foo f   new Foo      print f var      but does not causes compile error     also missing draw   function causes event handlers to not be called      void draw         if you forgot to include this void mousePressed       print  this is never called

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MySQL enums  specifically their ability to confuse the living hell out of unprepared coworkers   CREATE TABLE foo                dispatched ENUM  0   1   NOT NULL DEFAULT  0       Then   UPDATE TABLE foo SET      dispatched   1    Oops  dispatched was set to ZERO instead  because the 1 wasn t quoted  This really annoyed someone who worked on my code  I use plain old INTs now   On a related note  even if you add an empty string option to your enum  e g   blah ENUM     A   B   NOT NULL    If you assign an invalid value to blah  MySQL will use a secret hidden empty string value to represent the invalid value  which will be difficult to distinguish from the one you added yourself  Yay

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In C or C   you can have a lot of fun with Macros   Such as   define FOO a b   a b   1-a    if FOO bar   4  is passed in it ll increment a twice

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String math in Perl is pretty weird     perl -E   string    a    string    say  string  b    perl -E   string    abc    string    say  string  abd    perl -E   string    money    string    say  string  monez    perl -E   string    money    string--  say  string  -1

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I find Javascript Date Object s love for the year 110 delightful   Try it    lt Script language   JavaScript  gt   lt  -- var now   new Date   var dia   now getDay   var mes   now getMonth   var fecha    Day of the week if dia  0    fecha  Domingo      else if dia  1    fecha  Lunes      else if dia  2    fecha  Martes      else if dia  3    fecha  Mi  rcoles      else if dia  4    fecha  Jueves      else if dia  5    fecha  Viernes      else   fecha  S  bado        fecha   fecha   now getDate       de     Which month is it  if mes  0    fecha fecha    Enero   else if mes  1    fecha fecha    Febrero   else if mes  2    fecha fecha    Marzo   else if mes  3    fecha fecha    Abril   else if mes  4    fecha fecha    Mayo   else if mes  5    fecha fecha    Junio   else if mes  6    fecha fecha    Julio   else if mes  7    fecha fecha    Agosto   else if mes  8    fecha fecha    Septiembre   else if mes  9    fecha fecha    Octubre   else if mes  10    fecha fecha    Noviembre   else   fecha fecha    Diciembre       Year fecha   fecha     del     now getYear    document write fecha     -- gt   lt  Script gt    Script is in Spanish - sorry if you don t understand the code   The idea is for you to see the year 110 result

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In Python    gt  gt  gt  x 5  gt  gt  gt  1 lt x lt 10 True  gt  gt  gt  1 lt x lt 3 False   Not a WTF  but a useful feature

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I ve always been a huge fan of the PHP error thrown when using two colons in a row out of context      Parse error  syntax error  unexpected T PAAMAYIM NEKUDOTAYIM in  path to file error php on line 3   The first time I encountered this I was absolutely befuddled

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I ve written a programming language for a client  used for experimentally driving custom hardware  with some custom types  Curl  Circuit       that each have only 2 values  They are implicitly convertible to boolean  but  at the request of the client  the exact boolean value of a constant of such a type can be changed at runtime   E g   The type Curl has 2 possible values  CW and CCW  clockwise and counterclockwise   At runtime  you could change the boolean value by a simple assignment statement   ccw    true   So you could change the boolean meaning of all values of those types

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JavaScript                   false

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In C   this should at least generate a compiler warning  but it doesn t   public int Something       get   return Something        set   Something   value        When called  it causes your app to crash  and you don t get a good stack trace  since it s a StackOverflowException

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In JavaScript  undefined is a global variable whose default value is the primitive value undefined  You can change the value of undefined   var a       a b     undefined     true because property b is not set undefined   42  a b     undefined     false   Due to the mutability of undefined  it is generally a better idea to check for undefined-ness through typeof   var a       typeof a b     undefined      always true

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In earlier version of Visual Basic  functions without a  Return  statement just  Return None   without any kind of compiler warning  or error    This lead to the most crazy debugging sessions back when I had to deal with this language on a daily basis

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Go s pseudo-constant Iota   type ByteSize float64 const       iota       ignore first value by assigning to blank identifier KB ByteSize   1 lt  lt  10 iota   MB  GB  TB  PB  YB

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JavaScript truth table                   0               false 0                               true 0               0               true false           false           false false           0               true false          undefined        false false          null             false null           undefined        true    t r n       0                true      Source  Doug Crockford

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In C   int main       int i   0    int array      1 2      return  i array    1    array i        This program will return 1  true

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not that this is heavily used  but syntax of C   s  return reference to static-size array  is weird   struct SuperFoo     int   amp getFoo   const  10        static int foo 10       return foo          ofc  in above case method can be declared as static const

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The bigest collection  today 1313  of decent and weird programming languages I know  you will find here  http   99-bottles-of-beer net  be prepared to see real weird stuff  -  Everybody should make his one choice

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Anything will autometic pluralizes or singularizes any class and member names   Linq-to-Sql  for example

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RSL programming language is used in one strange banking system  There is built-in class TArray for arrays  But if you inherit from it every instance variable become an element of the array   class  TArray  DerivedArray   var someField   56  end  var a   DerivedArray    PrintLn a Size            gt  1 PrintLn a 0               gt  56

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s a  a   a     a      a     2N     a     c           22      a   a    a a   a   a c    S    c e  T   a  c     a  this is a nice one-liner in COS  cache objectscript   The funny thing to note here are 5 different modes of code-indirection  G

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The following C  code throws NullReferenceException rather than print 1       static void SomeMethod string format  params object   args                Console WriteLine args Length              static void Main string   args                SomeMethod  blabla   null   Ok here       print 2         SomeMethod  blabla   null      exception

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in X    Microsoft Dynamics AX      1  the need of a semi-colon     on a separate line to separate variable declaration from statements  at least up to version 4 0         int i      int myArray 5             i   1      2  array indexes are 1-based  so you are not allowed to read from an array using index 0  zero  like in        int myArray 5             print myArray 0         runtime error   this is not strange  but you are allowed to use the zero index on the left hand side of an assigment  like in      int myArray 5             myArray 2    102      myArray 0    100        this is strange     print myArray 2         expcting 102    what happens  The array gets initialized to it s default value  no matter what value was used in the assignment  The above code outputs 0  zero

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My favorite little C   syntax trick is that you can put URL s  with some restrictions  directly into the code   int main  int argc  char  argv           int i 10       http   www stackoverflow com     return 1      This compiles just fine   Syntax highlighting kind of spoils the joke  but it s still fun

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Well the first thing that came to my mind was  noop   my brain did the same thing when I first came across it

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Ok  since question will be in intermittent mode  I ll join to the  fun    Go   aka Issue9    use of upper case for visibility    If you name something with uppercase it will have public access  If you use lower case it will be package-protected    Visible outside the package   func Print v             Not visible outside the package  func print  v             You can find more in this original answer

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Might have already been said  and maybe this isn t so strange to some  but I thought this was pretty cool   In Javascript  declaring the parameters a function accepts is only a convenience to the programmer  All variables passed through the function call are accessible by the keyword  arguments   So the following would alert  world     lt script type  text javascript  gt   function blah    alert arguments 1       blah  hello    world      lt  script gt     Note  that while it may seem like these arguments are stored in an array  since you can access object properties in much the same way as array elements   they are not   arguments is an Object  not an Array  so  they are Object properties stored with numeric indices   as the following example illustrates  typeOf function taken from Crockford s remedial JavaScript page    argumentsExample   function        console log typeOf arguments         anArray           console log typeOf anArray         anObject           console log typeOf anObject       function typeOf value        var s   typeof value      if  s      object             if  value                if  typeof value length      number   amp  amp                        value propertyIsEnumerable  length     amp  amp                      typeof value splice      function                     s    array                           else               s    null                       return s     argumentsExample  a    b

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In Java   int   numbers       return null      Can be written as   int numbers          return null

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In JavaScript   2     2      Even stranger 2       2        And down-right nutty var a      abc    1    a     abc          a  abc       this is also true   Luckily the kind folks at stackoverflow com explained the whole thing to me   Why does 2     2  in JavaScript

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I love the fact that this sort of thing is fine in JavaScript   var futureDate   new Date 2010 77 154   alert futureDate     and results in a date 77 months and 154 days from the 0th day of 0th month of 2010 i e  Nov 1st 2016

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The C preprocessor and its usages  Specifically preprocessor metaprogramming and using the preprocessor to generate portable code -- total mindfcuk

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LOLCODE   The whole language itself  While not exactly a WTF thing  I ve never come across a language which plays out in my head in a squeeky cartoony voice  Nor have I ever looked at code before and want to exclaim  aaaawwww cuuute    This program displays the numbers 1   10 and terminates  HAI CAN HAS STDIO  IM IN YR LOOP UPPIN YR VAR TIL BOTHSAEM VAR AN 10     VISIBLE SUM OF VAR AN 1 IM OUTTA YR LOOP KTHXBYE

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Duff s device in C   In C one can interlace a do while with a switch statement  Here an example of a memcpy using this method   void duff memcpy  char  to  char  from  size t count         size t n    count 7  8      switch  count 8         case 0  do   to      from        case 7       to      from        case 6       to      from        case 5       to      from        case 4       to      from        case 3       to      from        case 2       to      from        case 1       to      from                 while --n gt 0

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In PHP    lt  php  o   new stdClass  echo count  o     gt      prints 1   Never figured out why

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Python s ternary operator  In Python  the C ternary operator  C   example  bool isNegative   i  lt  0   true   false   is available as syntactic sugar    gt  gt  gt  i   1  gt  gt  gt   It is positive  if i  gt   0 else  It is negative    It is positive   gt  gt  gt  i   -1  gt  gt  gt   It is positive  if i  gt   0 else  It is negative    It is negative     It s not really strange but a feature  The odd thing is the changed order  A if CONDITION else B  in comparison to the  IMO more logical  order in C  CONDITION   A   B

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Inform 7  An example of a valid program        Chomsky is a room       A thought is a kind of thing       Color is a kind of value       The colors are red  green and blue       A thought has a color  It is usually Green       A thought can be colorful or colorless  It is usually colorless       An idea is a thought in Chomsky with description  Colorless green ideas sleep furiously        A manner is a kind of thing       Furiously is a manner       Sleeping relates one thought to one manner       The verb to sleep  he sleeps  they sleep  he slept  it is slept  he is sleeping  implies the sleeping relation       Colorless green ideas sleep furiously     Other silliness like this Turing machine simulator can be found

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More of a platform feature than a language feature  on the iPhone  create an infinite loop with a few computations inside and run your program   Your phone will heat up and you can use it as a hand-warmer when it s cold outside

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Since I haven t seen anyone mention it    RPG 2 or 3  Report Program Generator    aka Rocket Propelled Garbage  is by far the screwyest language I ve ever used  It combines almost no control over program flow  Enter at the top of the file  Exit at the bottom  and programming statements are defined based on characters defined in specific columns using a fixed font  think PUNCH CARDS      To be really FUBAR you have to attempt to program in DYL-280  It combined RPG flow and logic with COBOL syntax   Look here for RPG  wikipedia org  wiki IBM RPG  An example of DYL-280  http   99-bottles-of-beer net language-dyl-280-224 html

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in C   int a     amp a  0    10     will assign value 10 to a       amp a  0  is equivalent to    amp a  0   which gives us    amp a   which is nothing but a

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Variable function declarations in Javascript   var x   1  function weird      return x    var x   2      weird   returns undefined    x is  taken  even though the assignment never happened   Similarly  but not so unexpectedly  function weird2       var x     return x       function x    return 2        returns 2

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In PHP  you have to explicitly reference globals and explicitly use this-  for class variables  Makes refactoring fun  You cannot promote a variable argument to a global or a class member without finding all points of usage

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In PHP  a string is as good as a function pointer    x    foo   function foo    echo  wtf      x       wtf    Unfortunately  this doesn t work    foo

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Not so much a weird feature  but one that s really irritating from a type-safety point of view  array covariance in C    class Foo     class Bar   Foo     class Baz   Foo      Foo   foo   new Bar 1   foo 0    new Baz       Oh snap    This was inherited  pun intentional  from Java  I believe

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Perl s sub not having a real parameter list  just the    array  Also  sub s auto-flattening the parameters that are passed into it   I don t understand why this is a persistent feature  this reflects what I had to do as a kludge on my TI-86 BASIC years ago because the language wasn t featured enough

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One of my C   favorites    include  lt iostream gt  using namespace std  int main         cout  lt  lt   3  lt  lt  5   lt  lt  endl      cout  lt  lt   3  lt  lt  5   lt  lt  endl      return 0      Of course  this is easily explainable  but it has beginning programming students scratching their heads

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Early FORTRAN where whitespace was not significant    The anti-Python    DO 20 I   1  10   Meaning  loop from here to line 20 varying I from 1 to 10   DO 20 I   1  10   Meaning  Assign 1 10 to the variable named DO20I   Rumors are that this bug crashed a space probe

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Fortran s special meaning of different columns   Probably completely natural if you grew up with punchcards    One side effect of this is that e g  variable names are truncated after column 72  Combined with IMPLICIT NONE this then silently introduces a new variable when such a variable name is started close to column 72    You ll need   to be aware of this an editor which highlights the comment part  after column 72  in a different color than the part before

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In C or C    parentheses are optional for the argument to sizeof     provided the argument isn t a type   void foo       int int inst        usual style - brackets       size t a   sizeof int     size t b   sizeof int inst     size t c   sizeof 99         but       size t d   sizeof int inst     this is ok      size t e   sizeof int     this is NOT ok   size t f   sizeof 99     this is also ok     I ve never understood why this is

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In C    int x   1   int y   x       x   printf   d   y     Is ambiguous  what gets printed depends on the compiler  The compiler could store the new value of x   before   x is evaluated  or at the end of the statement

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C   s most vexing parse   struct S       S        default constructor     int main          S s       this is not a default construction  it declares a function named s that takes no arguments and returns S

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In PHP function names are not case sensitive  This might lead you to think that all identifiers in php are not case sensitive  Guess again  Variables ARE case sensitive  WTF   function add  a   b        return  a    b      foo   add 1  2    Foo   Add 3  4    echo  foo is  foo      outputs foo is 3 echo  Foo is  Foo      outputs Foo is 7

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In javaScript  NaN is a global variable

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In PHP   for   s  a   s lt   z   s    echo  s        This will write   a b c d e     w x y z aa ab ac ad    ay az ba bb bc     by bz ca cb     yz za zb     zx zy zz

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Looking for a function  Why not a language   I love PHP but it always seems to be built like this  Oh s   t  I forgot this  Let s just add another argument to the function  which result in this    str replace  search   replace   subject       strstr  subject   search        Notice the extra underscore and the different order for the arguments   Here is something else   a   array   a    b    c    d     print r  a     Prints array  0   gt   a   1   gt   b      2   gt   c   3   gt   d    unset  a 2      Destroys the element 2 of the list print r  a     Prints array  0   gt   a   1   gt   b      3   gt   d

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Digraphs and alternative tokens  C  ISO IEC 9899 1999  6 4 6 3  and C    ISO IEC 14882 2003  2 5  have a feature that is rarely used  called  digraphs  by C and  alternative tokens  by C     These differ from trigraphs mainly because string literals containing them will never be interpreted differently     include  lt stdio h gt   int main    lt       int a lt  10  gt     lt  0  gt       printf  Here s the 5th element of  a    d n   a lt  4  gt        puts  Evil  eh     gt         return 0    gt    C   has many more  including and  or  and not which are required to behave as  amp  amp       and     C has these too  but requires that  lt iso646 h gt  be included to use them  treating them as macros rather than tokens   The C   header  lt ciso646 gt  is literally an empty file   It s worth noting that GCC implements support for this weird language feature  but lots of other compilers choke and die when trying to compile the above segment of code

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Not really a language feature  but an implementation flaw  Some early Fortran compilers implemented constants by using a constant pool  All parameters were passed by reference  If you called a function  e g   f 1    The compiler would pass the address of the constant 1 in the constant pool to the function  If you assigned a value to the parameter in the function  you would change the value  in this case the value of 1  globally in the program  Caused some head scratching

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Forth can change the base of the numbers at any time   HEX 10 DECIMAL 16 -   0 Ok   It need not be one pre-defined one either   36 BASE   1Z DECIMAL   71 Ok

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The most weird feature I know of is from C   world   SFINAE   The worst is that it happens to actually be very usefull  extensive use of SFINAE in BOOST is proof enough for me

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INTERCAL is probably the best compendium of strangest language features   My personal favourite is the COMEFROM statement which is  almost  the opposite of GOTO      COMEFROM is roughly the opposite of   GOTO in that it can take the execution   state from any arbitrary point in code   to a COMEFROM statement  The point in   code where the state transfer happens   is usually given as a parameter to   COMEFROM  Whether the transfer happens   before or after the instruction at the   specified transfer point depends on   the language used  Depending on the   language used  multiple COMEFROMs   referencing the same departure point   may be invalid  be non-deterministic    be executed in some sort of defined   priority  or even induce parallel or   otherwise concurrent execution as seen   in Threaded Intercal  A simple example   of a  COMEFROM x  statement is a label   x  which does not need to be   physically located anywhere near its   corresponding COMEFROM  that acts as a    trap door   When code execution   reaches the label  control gets passed   to the statement following the   COMEFROM  The effect of this is   primarily to make debugging  and   understanding the control flow of the   program  extremely difficult  since   there is no indication near the label   that control will mysteriously jump to   another point of the program

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JCL Conditional execution     STEP02 EXEC PGM PROG02 COND  4 GT STEP01      This features allows you to run or not run a step depending on the return code from previous steps  Quite a nice feature really   Except for a couple of small features which turn the logic inside out and backwards   First the step does NOT run if the condition is true   Secondly the 4 GT STEP01 actually means  if the return code from STEP01 is greater than 4   So the whole thing means  Do not run this step if the return code from STEP01 is greater than 4   Which is the almost but not quite the same as a naive interpretation  Run the step if 4 is greater than the return code from STEP01     Given that only time you ever look at these things seriously is about 2 30 am with a frantic nightshift operator at the other end of the line this double ambiguity leads to serious headaches

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Dozens of things in Javascript can make your eyes water   The scoping of local variables  as just one simple example   function foo obj      for  var n   0  n  lt  10  n            var t            Here is a  t                t    okay          And here s the same  t

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else in Python s for loops   From the Python documentation   for n in range 2  10       for x in range 2  n           if n   x    0              print n   equals   x       n x             break     else            loop fell through without finding a factor         print n   is a prime number    Output   2 is a prime number 3 is a prime number 4 equals 2   2 5 is a prime number 6 equals 2   3 7 is a prime number 8 equals 2   4 9 equals 3   3

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In ColdFusion text values are converted to various data types automatically for various purposes  I hit an odd problem where  00A  and  000  were being return as equal  It turned out that ColdFusion was interpreting  00A  as a time  converting to some sort of numeric time format  and converting it to 0   000  was being converted to 0  So they were both considered equivalent  That is when I learned about the compare function for strings

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In MUMPS you can have a GOTO with offset  If you have  my MUMPS is rusty      some label if x 1 do something            else  do something else   Then the code             goto some label 1   Will jump to the ELSE statement

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Variable variables in PHP  An odd feature in PHP which allows you to create and assign variables from the content of other variables  warning  untested code     a    Juliet     a    awesome      assigns a variable named  Juliet with value  awesome   echo   a            prints Juliet echo     a          prints awesome echo   Juliet       prints awesome   Alright  let s say we have something like this    bob    I  m bob    joe    I  m joe    someVarName    bob     someVarName    Variable   bob   changed     How about some fun with all kinds of indirection    juliet    Juliet is awesome     func    getVarName   echo     func         prints  Juliet is awesome    function getVarName     return  juliet

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This may have been already mentioned  but --  PHP s handling of octal values    a   07     7  as it should be   b   08     0  would be an error in any sensible language   c   018     1  again  should have been an error   d   0A     error  as it should be    See here  http   bugs php net bug php id 29676  Also note the comments on the bug - Derick calls it a feature  as shown by quoting  fix    not a bug and he claims it would  slow down PHP dramatically in all cases where numbers are used inside scripts  - but then  why does PHP raise an error for 0A   I think one could make a whole book about the weirdness of PHP

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I built a language with a BUT clause once  a long time ago

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In JavaScript  you can use a double bitwise negation    n  as a replacement for Math floor n   if n is a positive number  or parseInt n  10   even if n is negative   n n and n amp n always yield the same results as   n   var n   Math PI  n     3 141592653589793 Math floor n      3 parseInt n  10      3   n     3 n n     3 n amp n     3       n works as a replacement for parseInt   with negative numbers       -n      -3  -n   -n      -3  -n  amp  -n      -3 parseInt -n  10      -3       although it doesn   t replace Math floor   for negative numbers Math floor -n      -4   A single bitwise negation     calculates - parseInt n  10    1   so two bitwise negations will return - - parseInt n  10    1    1    Update  Here   s a jsPerf test case comparing the performance of these alternatives

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Commodore BASIC s command shortcuts  Basically most commands had an abbreviated form which was usually the first letter    shift 2nd letter   But because the character set on a C64 was by default in all uppercase  these commands would look like bizarre symbols  Here s a short hello world example     Maybe someone has a better example actually with more meat to it  but for long programs this looked completely ridiculous   Here is a list of abbreviations  http   www c64-wiki com index php BASIC keyword abbreviation

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Bracket identifiers in VBScript  VBScript has so-called bracket identifiers  which are identifiers defined enclosed in square backets  like this    Foo    They re quite handy  actually  as they allow you to name variables and routines after reserved words  call methods of third-party objects whose names are reserved words and also use almost any Unicode characters  including whitespace and special characters  in identifiers  But this also means that you can have some fun with them    2 2    5                        Hello  world          Looks like my name is an empty string  isn t that cool    For For i 0   0 To To To 0     Next To   For i 0    For i 0  Next   On the other hand  bracket identifiers can be a gotcha in case you forget the quotes in a statement like this   If MyString     Something   Then   because If MyString    Something  Then is a perfectly legal syntax   And that s why an IDE with syntax highlighting is a must     More info on bracket identifiers in Eric Lippert s blog    VBScript Trivia  Bracket Identifiers and Reserved Word Incompatibilities VBScript Quiz Answers  Part Six

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How about the neat system-dependent overflows causing year rollovers in  MRI C  Ruby and MacRuby  but not in JRuby  followed by localtime errors for a larger number  Not a common issue  but it is strange     ruby -version ruby 1 8 7  2009-06-12 patchlevel 174   universal-darwin10 0    irb  gt  gt  Time at 67767976233550799    gt  Tue Dec 31 23 59 59 -0500 2147483647  gt  gt  Time at 67767976233550800    gt  Wed Jan 01 00 00 00 -0500 -2147483648  gt  gt  Time at 67768036191694799    gt  Wed Dec 31 23 59 59 -0500 -2147481749  gt  gt  Time at 67768036191694800  ArgumentError  localtime error     Maybe IRB bug     This may be specific to 64-bit environments  though

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In Perl  without  use strict  or  use warnings     if true  undef        print  True n     else      print  False n     if undef        print  True n     else      print  False n     if true        print  True n     else      print  False n       Prints   True False True

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In C    I find it strange and distasteful that  virtual  MI  multiple inheritance  allows the  diamond-shape  class hierarchy to  work    A   Base class  e g   Object  B  C  both derive  virtually or not  from Object and  D  derives from both B and C    Problem   normal  inheritance causes D to be 2 ambiguous kinds of A   virtual  MI collapses B s A and C s A to a single shared base A object   So  even if your Wheel is an Object and your Left Front Wheel is a Wheel and your Car inherits four kinds of Wheel  your Car is still only one kind of Object with virtual MI  Otherwise  your Car is not an Object  but 4 Wheel-y Objects   This is one language feature that rewards poor class design  punishes compiler writers  and leaves you wondering at run-time where the heck the Object really is - and if any virtual MI baggage was misplaced   If you really need the diamond pattern in your class hierarchy  it can be accomplished with regular MI and an  AProxy  that delegates to the single A base      A   Base class  e g   Object  AProxy  Base class  constructs with other A to bind to B   derives from A C   derives from AProxy D   derives from both B and C  passing B s A to C s AProxy at construction     This requires a little more work for those that really like diamond MI and leaves the rest of us in peace with a more tractable set of language features

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C   namespace reslove order  for example   namespace foo bar xyz    public class Foo      Exception e       you ll get compile time error here             Because  namespace foo bar Exception    class HowDoMyWayException   ApplicationException         because someone did this

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Java  making all object instances be mutexes

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PHP     php -r      Parse error  syntax error  unexpected T PAAMAYIM NEKUDOTAYIM   WTF  http   en wikipedia org wiki Scope resolution operator  Why not say unexpected T SCOPE RESOLUTION OPERATOR

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this made me stunning    define   -F lt 00  --F-OO--  int F 00 OO 00 main   F OO   printf   1 3f n  4  -F OO OO   F OO                  - - -          - - - - - - - -       - - - - - - - - - - -     - - - - - - - - - - - - -    - - - - - - - - - - - - - -    - - - - - - - - - - - - - -  - - - - - - - - - - - - - - -   - - - - - - - - - - - - - - -   - - - - - - - - - - - - - - -   - - - - - - - - - - - - - - -    - - - - - - - - - - - - - -    - - - - - - - - - - - - - -     - - - - - - - - - - - - -       - - - - - - - - - - -          - - - - - - -              - - -

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The fact that there is no encapsulation in C    or Java    Any object can violate the encapsulation of any other object  mess around with its private data  as long as it s the same type   For example    include  lt iostream gt  using namespace std   class X     public         Construct by passing internal value     X  int i    i  i             This breaks encapsulation     void violate  X  amp  other                other i    i             int get      return i       private      int i      int main  int ac  char   av          X a 1   b 2   c 3        a violate  c       b violate  c       cout  lt  lt  c get    lt  lt  endl         6

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MUMPS  There are lots of WTF features  I ve picked one  the if statement   Note that I m using a rather verbose coding style below in order to accomodate those who don t know the language  real MUMPS code is usually more inscrutable to the uninitiated    if x gt 10 do myTag x       in MUMPS  tag  means procedure function else  do otherTag x    This is similar to saying in Java   if  x  gt  10      myMethod x     else     otherMethod x       Except that in MUMPS  the else statement isn t syntactically part of the if block  it is a separate statement that works by examining the built-in variable  TEST  Every time you execute an if statement it sets  TEST to the result of the if statement  The else statement actually means  execute the rest of line if  TEST is false  otherwise skip to the next line    This means that if x was greater than 10 and thus the first line called myTag  and myTag contains if statements  then the behavior of the else depends not on the if in the line above it but on the last if evaluated inside of myTag  Because of this  feature   MUMPS coders are generally taught write the above code like this to be safe   if x gt 10 do myTag x  if 1 else  do otherTag x    The if 1 at the end of the first line ensures that  TEST is set correctly before control proceeds to the next line   BTW  the spacing here has to be just so  with two spaces after the else and one space in all the other places  The spacing is odd but at least it s very orthogonal once you understand the pattern

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Try  except  else  try      pass except   pass else     pass finally  pass   If no exception was caught the else part is executed   Makes sense  but at first I really hadn t any clue what it does   Example   def show square string     try      n   int string  10    except ValueError      print  I can t do that  Dave     else      print n   n

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Subjunctive case in English    Oh wait  did you mean programming languages  Then using  macro  in C to bypass the preprocessor  define of macro    E g   if someone has  define free        free       will not be the same as free

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var a   Double Parse  10 0   CultureInfo InvariantCulture      returns 10 var b   Double Parse  10 0   CultureInfo InvariantCulture      returns 100   In invariant culture comma is not decimal point symbol  but group separator   As I know  it s common mistake for novice programmers from some locales

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In c    Math Round 2 5   2

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In Java    int x   010   This assigns x to have the value 8    Any integer preceded with a zero in Java is presumed octal

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Call cc  Call cc passes a function representing the rest of the program to its body

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In Haskell   let 2   2   5 in 2   2   yields 5

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In PHP  the following    lt  php  foo    abc   echo    foo     is a syntax error   If you actually wanted    followed by the contents of  foo  you d have to use      lt  php  foo    abc   echo        foo

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VBScript s With blocks   With xml appendChild xml createElement  category       setAttribute  id  id     setAttribute  keywords  keywords    With  appendChild xml createElement  item         setAttribute  count  count       setAttribute  tip  tip       appendChild xml createTextNode text     End With End With

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Trigraphs in C and C     int main        printf  LOL           This will print LOL   because the trigraph     is converted to

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Piet is an esoteric programming language designed by David Morgan-Mar  whose programs are bitmaps that look like abstract art      Piet program that prints Piet

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Python 2 x   gt  gt  gt True   False  gt  gt  gt True False   You can really make someone become crazy with this one

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Forth has some strange things about its control structures  First  because it is a reverse polish notation language  the condition precedes the IF  as in   x 0   IF   Now  to close the conditional block  one uses the keyword THEN   x 0   IF    Equals zero   THEN   Now the real WTF begins  What IF does is compile a conditional forward jump  and place on a stack the address of the jump offset  When THEN is found  it pops that address from the stack  computes the actual offset  and then compile that  The ELSE  on the other hand  compiles an inconditional forward jump  pops an address from the stack  pushes a new address on the stack  computes the offset for the popped address  and then compiles that offset  Meaning the syntax is this   x 0   IF    Equals zero   ELSE    Not equal to zero   THEN   The first and second statements are compiled like this   x LITERAL 0    0BRANCH  LITERAL offset SLITERAL  Equals zero    DOTQ  x LITERAL 0    0BRANCH  LITERAL offset SLITERAL  Equals zero    DOTQ  BRANCH LITERAL offset SLITERAL  Not equal to zero    DOTQ    To compound the weirdness  that behavior is not hidden  It is part of the ANSI specification of the language  and can be freely be taken advantage of  either by constructing custom flow control structures or by combining them in interesting ways  For example  take Forth s WHILE loop   BEGIN x 10  lt  WHILE x 1  to x REPEAT   The part between BEGIN and WHILE is arbitrary code  so you can actually have code execute before and after the conditional test in a single control structure  That s by design  but the following  though allowed  is not   BEGIN DUP 2  gt  WHILE DUP 5  lt  WHILE DUP 1  REPEAT 123 ELSE 345 THEN    Which takes advantage of how each control flow word works to combine two WHILE statements  and  to boot  add a different post-loop code for each exit  And just to show I m not kidding  I just copied that small snippet from a code on the Internet  with minor modifications to simplify it

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In all languages today   TypeA a    TypeA some operation returning TypeB 1 2 3      TypeB is not inheriting TypeA   fails on runtime with  Cast to TypeA failed exception -message  or similar   What this tells us is just how lazy programmers really are  There s no way for them to produce message  Failed to assign variable  a  of TypeA with a value  some operation returning TypeB 1 2 3   of TypeB   Noooo   their motto is  thy who makes mistakes must suffer

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In Ruby  you can do some weird things with heredocs  Consider   a    lt  lt ONE This is one     lt  lt TWO  This is two     lt  lt THREE  This is three  THREE TWO ONE  p a     gt   This is one  This is two  This is three  n n n

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In JavaScript  the following construct  return       id   1234      title    Tony the Pony       returns undefined is a syntax error due to the sneaky implicit semicolon insertion on the newline after return  The following works as you would expect though   return       id   1234      title    Tony the Pony       Even worse  this one works as well  in Chrome  at least    return            id   1234      title    Tony the Pony       Here s a variant of the same issue that does not yield a syntax error  just silently fails   return     2   2

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In Java  if the value of x is NaN then x    x returns false and x    x returns true

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Variables everywhere are taken as globals in Coldfusion  no matter where they are placed    lt cffunction name  one  returntype  void  gt       lt cfset var wtf  coldfusion  gt       lt cfinvoke method  second  gt   lt  cffunction gt    lt cffunction name  two  returntype  void  gt       lt cfoutput gt  wtf  lt  cfoutput gt   lt  cffunction gt

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For those who didn t know  PostScript is actually a programming language   I ve gotten a bit insane with it -- I wrote a PostScript program that computes a Mandelbrot fractal to a very high level of detail   It s really printable PostScript  though it will crash a lot of print drivers     Anyway  where to start with PostScript     Here s one   You can actually create a variable whose identifier is     nothing      cvn 5 def     Assign the number 5 to    nothing  PostScript is a stack-based language      puts an empty string on the stack   cvn converts it to a name       if you print it  because all names in PS are preceded by a slash    Then 5 def assigns the value 5 to it      is the comment character   You can t directly get it back  e g  if I say    print   this will not print the number 5   But you can get it back indirectly      cvn load print    this will print the number 5  What else     PostScript has dictionaries as a native type  and you can use an array reference as a key to the dictionary     but it is the REFERENCE that is the key  not the array   So    myDict 100 dict def   0  dup myDict exch 42 put myDict exch get       prints 42  myDict  1  42 put myDict  1  get     throws an undefined error  Edit   Oh yeah  one more fun thing     Try the following at a Ghostscript prompt   1 array dup dup 0 exch put      D oh

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in Java  String  aaa    String  aaa     false   you need to use String  aaa   equals String  aaa       true

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This is not a strange feature  in fact it makes total sense if you think about it  but gave me a WTF moment nevertheless   In C   and in C    subclasses of a base cannot access private and protected members on the instance of the base   class Base   protected   m fooBar      class Derived  public Base   public   void Test Base amp  baseInstance      m fooBar 1    OK   baseInstance m fooBar   1    Badness     This  however is OK      Derived amp  baseInstance  m fooBar   1    OK

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COMEFROM is the weirdest  and probably most useless  language feature I ever saw   Runner-up would be the ternary operator  because it violates rule  1 of optimization  And it does more harm than it solves problems  It does more harm because it makes code less readable   Not really a language feature  but interesting awesome use of features is Duff s device

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In C   the following code generates compiler error  Cannot convert from method group to Delegate   Though the logic behind is reasonable  it still feels strange to me   control BeginInvoke delegate   DoSomething

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Inheriting from random class in Ruby   class RandomSubclass  lt   Array  Hash  String  Fixnum  Float  TrueClass  sample        end    first seen at Hidden features of Ruby

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As an NHibernate enthusiast  I was thrilled when I heard about become from Smalltalk    e g   a become  b   it literally changes the a object into b  which makes it trivial to write lazy-initialized proxies because all references to a will now reference b  Pretty neat   I think it qualifies as a strange language feature in that no other language has this ability to my knowledge

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In c    const char  wtf         char buf 100       return buf    string s         wtf            creates interesting values in s  Partly string  partly stack contents  mixed with zeroes so that s length    strlen s c str     The strangest thing is that compiler has absolutely no problems with returning pointers to stack - compiler programmers s hand would probably fall off if he would have added a warning there

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here is my 2 cents  In c     int  t   new int 15   delete t

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C     void f int bitand i     WTF     i      int main        int i   0      f i       cout  lt  lt  i  lt  lt  endl    1     return 0

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I can t believe this one isn t on here yet  JSF property access   In a JSF UI tag  you would put the value of a property from the server-side object into the interface by referencing it thusly    lt h inputText value    myObject property  gt  lt  h inputText gt    The thing is that Java doesn t support properties  so you have to write methods starting with get and set in order to link the UI object to the  property  on the server   public void setProperty String property       public String getProperty          This confused me when I first learned JSF and I still consider it WTF-worthy    even though there s really no other way to do it until Java implements support for C -style properties

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Perl s     deprecated   this was mentioned in another earlier post about generic perl variables  but it deserves specific mention with better explanation  Changes to    are limited to current scope  More information and a quick writeup of how you can use this and its implications    can be found in    is under respected at http   www perlmonks org index pl  node id 480333

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Java s Integer class s base-conversion static methods   P Very few languages have this functionality built right in  it seems

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Best of show entry in the Perl Journal s Obfuscated Perl Contest in 2000         -    -   -     -   0-   -      -     -      open Q  0  while  lt Q gt   if            for split  -   1    q 0 for split  s         xg s      g  Q    length     q   q  Q  Q 20  print chr  q     print  n           -     -      -      -    -       -        Code fully explained by its author at http   mysite verizon net les peters id2 html

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In C   warning C4013   myfunc  undefined  assuming extern returning int   I remember for some reason not seeing warnings  too much of them in some legacy code   and puzzling over why conversion from int causes compiler error where non int-returning function is used   Compiler assuming such stuff was quite unexpected

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Why does C  s List lt T gt  AddRange   not let me Add elements of a subtype of T  List lt T gt  Add   does  All it would take would be ONE extra line of code on Microsoft s part   public void AddRange lt S gt       IEnumerable lt S gt  collection   where S   T

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In JavaScript the result of a method can depend upon the style braces are placed  This is the K amp R style  where braces are placed right after the method signature and after a return statement   var foo   function       return       key   value          foo      returns an object here   Now  if I format this code to the Allman style  where braces are always placed on a new line  the result is different   var foo   function       return         key   value          foo      returns undefined here   How come  In JavaScript the language places automatically semicolons at the end of each line if you won t do it yourself  So what really happened in the last code fragment was this   var foo   function       return     here s actually a semicolon  automatically set by JavaScript          key   value           So if you d call foo    the first statement in the method would be a return statement  which would return undefined and would not execute other following statements

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x   x   1  This was very difficult to digest when I was a beginner and now functional languages don t use it  which is even more difficult   If you don t see how this is strange  Consider the equals sign as a statement of assertion instead of an assignment action  as you used to do in basic algebra  then this is the equivalent of saying  zero equals one

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For me it s definitely the PLEASE modifier in INTERCAL  If PLEASE does not appear often enough  the program is considered insufficiently polite  and the error message says this  if too often  the program could be rejected as excessively polite

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Delphi don t care about typecast like  word  and will read outside of array arr 0  65535  where pos   65535  arr word pos   10

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In Scala  there are no operators  just methods  So a   b - c is actually the same as a   b  - c   In this  it is equal to Smalltalk  However  unlike Smalltalk  precedence is taken into account  The rules are based on the first character  so an hypothetical method called    would have precedence over one called     An exception is made so that any method ending in   will have the same precedence as    -- meaning    and     non-hypothetical methods  have different precedence   All ASCII letters have the lowest precedence  but all non-ASCII  unicode  characters have the highest precedence  So if you wrote a method is comparing two ints  then 2   2 is 1   3 would compile and be true  Were you to write it in portuguese      then 2   2    1   3 would result in error  as it would see that as 2    2    1    3   And  just to top off the WTF of operators in Scala  all methods ending in   are right-associative instead of left-associative  That means that 1    2    Nil is equivalent to Nil    2     1  instead of 1    2     Nil

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I would not dare to claim that XML is a programming language  but isn t it close to our heart   -   The strangest feature  to my mind  in XML is that the following is a well-formed document    lt       gt   lt        gt    Here is the the lexical definition of NT-Name that allows consecutive dots

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Ruby Flip-Flops        and      in conditional statements are not always range operators    0  20  each do  x    if   x 10     5     x 10     5      print    x      end end   0  20  each do  x    if   x 10     5      x 10     5      print    x      end end   This will output   5 15 5 6 7 8 9 10 11 12 13 14 15      checks both statements on each pass      only checks the  on  or  off  statement in each pass  depending on the flip-flop state   They are stolen from awk and sed   Matz writes in  The Ruby Programming Language    Flip-flops are a fairly obscure feature of Ruby and probably best avoided

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Quoting Neil Fraser  look at the end of that page    try       return true    finally       return false       in Java  but behaviour is apparently the same in JavaScript and Python   The result is left as an exercise to the reader   EDITED  As long as we are on the subject consider also this   try       throw new AssertionError      finally       return false

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APL  other than ALL of it   the ability to write any program in just one line   e g  Conway s Game of Life in one line in APL   alt text http   catpad net michael APLLife gif  If that line isn t WTF  then nothing is   And here is a video

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In Javascript  I believe the following are equivalent   a  title      Syntactic sugar is good for yr teeth    a title    Syntactic sugar is good for yr teeth

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In Python  at least for me  this was very wft  the first time I saw it    gt  gt  gt   ja     5  ja ja ja ja ja     You can multiply strings  WTF    PS  I think this is because x   n means  n times x so  5 times  ja   is  ja   ja   ja   ja   ja   and because you can concatenate strings like this    gt  gt  gt   ja   ja   ja   ja   ja    ja ja ja ja ja     That two codes have the same result  and maybe are just the same

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The entirety of the Malbolge programming language  http   en wikipedia org wiki Malbolge

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To alternate between things in many languages   boolean b   true  for int i   0  i  lt  10  i      if b    b      print i    on first glance  how can b really not be equal to itself   This acctually would print odd numbers only

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I once wrote a programming language that had a  strfry  operator    hello world       gt   wdo rlholle    Useful  eh

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In Lisp you can copy a list  and you can copy a vector  and you can copy a struct  and you can copy a CLOS object         but you cannot copy an array or a hash table

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Here s some messing around in the Perl debugger     DB lt 1 gt  sub foo     1  20       DB lt 2 gt   bar   foo      list of 1  2  3  4   20   DB lt 3 gt  x scalar  bar   size of list 0  20   DB lt 4 gt  x scalar foo    0       That s right  When you call the method like that  the scalar context from scalar propagates down into the subroutine call  turning the innocuous-looking    into an entirely different operator   That s the  flip-flop  operator  instead of the range operator

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In JavaScript   2     2      Even stranger 2       2        And down-right nutty var a      abc    1    a     abc          a  abc       this is also true  Luckily the kind folks at stackoverflow com explained the whole thing to me  http  stackoverflow com questions 1724255 why-does-2-2-in-javascript

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Perl  my  h1   map        gt  1   qw foo bar baz         construct an  is member  type lookup table my  h2   map          gt  1   qw foo bar baz     the second line is a syntax error even though to even an experienced perl programmer it looks like it would be identical   The downside to perl always trying to do what you mean  not what you said

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Some 20 years ago  when I last dabbled in MUMPS  the implementations had some curious limitations  While hosts MUMPS was becoming ever more popular  MUMPS was traditionally a self-hosted language  computer language  operating system and database in a single package   MUMPS was essentially about its database  Essentially  a huge multidimensional hash table  supported by a B  tree that made for very fast access  There wasn t any barrier between the language and the database either  if you wanted something to be stored there  you just prefixed the variable with a symbol indicating it was to be persisted to the backing store   On the other hand  a filesystem was almost non-existent  and support for it even less so  About the only thing one could do was to load a program into memory from a file  and send whatever was in memory back to a file  And one had better clear the buffer before loading  otherwise it would get mixed with whatever was there first   So  considering its self-hosting nature and the extremely hostile file system  one could wonder how these programs were edited  The editors  as a matter of fact  were written in MUMPS itself -- so how could the editor store the program in memory without written over itself   Well  the trick was the ability to execute the contents of a variable as source code  An editor  then  loaded itself into variables  executed itself in them  cleared the memory  and then loaded  saved and edited files in memory  all the time executing from variables   Add to that the fact that all commands could be shortened to their first letters  except the Z commands  shortened to two letters  that mostly handled the filesystem   and curiosities like the fact that IF  I  set a variable which was then consulted by ELSE  E  -- and  of course  could be overridden by any intervening I  or by the program itself  On second thought  I think the whole language was a WTF  And  yet  it had a strange attraction

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Let s have a vote for all languages  such as PL I  that tried to do away with reserved words    Where else could you legally write such amusing expressions as   IF IF THEN THEN   ELSE ELSE ELSE   THEN    IF  THEN  ELSE are variable names   or  IF IF THEN THEN ELSE ELSE    IF is a variable  THEN and ELSE are subroutines

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VBScript s date time literals  why is this still so rare     mydate    1 2 2010 5 23 PM   If mydate  gt   1 1 2010 17 00  Then         Edit  Date literals are relative  are they technically literals  then     mydate    Jan 3    Jan 3 of the current year   VB NET  since it is compiled  does not support relative date literals  Date only or time only literals are supported  but the missing time or date are assumed to be zero   Edit 2   Of course  there are some bizarre corner cases that come up with relative dates     mydate    Feb 29    executed on 2010-01-05  yields 2 1 2029

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Algol pass by name  illustrated using C syntax    int a 3      1  2  3    int i   1   void f int j        int k      k   j      k   2     i   0      k   j      k   1              int main         f a i

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Java caches Integer object instances in the range from -128 to 127  If you don t know this the following might be somewhat unexpected   Integer valueOf 127     Integer valueOf 127      true  same instance Integer valueOf 128     Integer valueOf 128      false  two different instances

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In SQL  NULL is not equal to NULL  So you can t do   WHERE myValue    NULL   This will always return false    NULL    NULL

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In Python    gt  gt  gt  a 0     hello  NameError  name  a  is not defined  gt  gt  gt  a 0      hello  NameError  name  a  is not defined  gt  gt  gt  a       gt  gt  gt  a 0     hello  IndexError  list assignment index out of range  gt  gt  gt  a 0      hello   gt  gt  gt  a   h    e    l    l    o     These slice assignments also give the same results   a       hello  a 42      hello  a  33     hello

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Here s a good bunch of strange C features  http   www steike com code useless evil-c

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In Java  there is some inconsistency as to how Strings handle the    operator depending on how it was constructed    String a    Hello   String b    Hello   System out println a    b       prints true  String c   new String  Hello    String d   new String  Hello     System out println c    d       prints false

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COMEFROM anyone

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Labeled break and continue statements in Java     They allow you to break out of multiple control-blocks with a single break

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A Java source file can end with the character  u001a  control-Z

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I ve always wondered about the purpose of this function in the Math class of the Java Core library   static double expm1 double x       Returns e x - 1

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Other weird things   In C   overriding a virtual method hides all other overloads of that method  In Java this does not happen  This is very annoying  Example  http   codepad org uhvl1nJp  In C   if a base class has a public virtual method foo   and a subclass has a private method foo    this private method overrides the other one  This way you can call what is a private method outside of the class just by casting the subclass object pointer to a superclass object pointer  This shouldn t be possible  it s a violation of encapsulation  The new method should not be treated as an override of the old one  Example  http   codepad org LUGSNPdh  In PHP you can define functions to accept typed parameters  e g  objects that are subclasses of a certain interface class   the annoying thing is that this way you cannot use NULL as the actual parameter value in this case  Example  http   codepad org FphVRZ3S

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One unexpected feature was the trailing commas in enum def lists and array initialization lists in C  C   Ruby  etc     string   foods      tofu    grits    cabbage      public enum ArtPeriod     Modern    Romantic    Dada

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In JavaScript  2 0 - 1 1   0 8999999999999999   This is a result of the implementation of floats in the specification  so it will always be like this

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In JavaScript this   var something   12   function nicelyCraftedFunction       something   13           some other code          and in Galaxy far  far away this    if  false      so the block never executes           var something         nicelyCraftedFunction       call of the function   Normally you would expect that something variable will get value of 13  But not in JavaScript - variables there have function scope so later declaration affects everything up-stream    In languages that use C C   Java notation  like JS  you would expect variables having block scope  not like this       So dead block of code that compiler can even remove from final generated bytecode still have side effects in the rest of code that executes normally   Therefore something will be still 12 - not change after invocation of the function

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In Matlab  the following may be surprising  especially if you are used to Python    gt  gt  not true  ans         0     0     0     0  gt  gt  not false  ans         0     0     0     0     0   There are two weird features here  The first one is that a b is interpreted as a  b    so not true is interpreted as not  true    The second weird feature is that not of any character returns 0  presumably because there is no false or true in matlab  only 0 or 1

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PHP  PHP has inconsistent handling of overloading for instance variables and methods  Consider   class Foo       private  var    avalue        private function doStuff                 return  Stuff              public function   get  var                return  this- gt  var             public function   call  func  array  args   array                  return call user func array array  this   func    args             foo   new Foo  var dump  foo- gt var   var dump  foo- gt doStuff       The dump of  var works  Even though  var is private    get   is invoked for any member which doesn   t exist or is inaccessable  and it returns the correct value  This is not the case for doStuff    which fails with   Fatal error  Call to private method Foo  doStuff   from context           I think a lot of these work in C-style languages  but I   m not sure    Pass a here document as a function argument   function foo  message        echo  message     n      foo  lt  lt  lt EOF     Lorem ipsum dolor sit amet  consectetur adipiscing elit  Nunc     blandit sem eleifend libero rhoncus iaculis  Nullam eget nisi at     purus vestibulum tristique eu sit amet lorem  EOF         You can assign a variable in an argument list   foo  message    Hello    echo  message    This works because an assignment is an expression which returns the assigned value  It   s the cause of one of the most common C-style bugs  performing an assignment instead of a comparison    Python  In Python  mutable default function arguments cause unexpected results   def append thing  collection          collection append thing      return collection  print append  foo     - gt    foo   print append  bar     - gt    foo    bar   print append  baz         - gt    baz   print append  quux     - gt    foo    bar    quux     The empty list is initialized at function definition time  not call time  so any changes to it persist across function invocations   MySQL Case Sensitivity  MySQL has really unusual case sensitivity rules  Tables are case sensitive  column names     and string values aren t   mysql gt  CREATE TEMPORARY TABLE Foo  name varchar 128  NOT NULL   DESCRIBE foo  ERROR 1146  42S02   Table  foo  doesn t exist mysql gt  DESCRIBE Foo   ------- -------------- ------ ----- --------- -------    Field   Type           Null   Key   Default   Extra    ------- -------------- ------ ----- --------- -------    name    varchar 128    NO           NULL               ------- -------------- ------ ----- --------- -------  1 row in set  0 06 sec  mysql gt  INSERT INTO Foo   name   VALUES   bar      baz    Query OK  2 row affected  0 05 sec   mysql gt  SELECT   FROM Foo WHERE name    BAR    ------    name    ------    bar     ------  1 row in set  0 12 sec   mysql gt  SELECT   FROM Foo WHERE name    bAr    ------    name    ------    bar     ------  1 row in set  0 05 sec

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In ruby python c  you can concatenate strings just like this   a    foo   bar  print a     gt   foobar

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Here is one about python    gt  gt  gt  print 07 7  gt  gt  gt  print 08   File   lt stdin gt    line 1     print 08              SyntaxError  invalid token   Ain t that a beauty   Especially unthoughtful when you think of how human write dates  which has the following effect   datetime date 2010 02 07    ok datetime date 2010 02 08    error     the reason is that 0x is interpreted as octal  so print 010 prints 8

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This old PHP favorite isn t all that WTFish on its own  but a scope resolution error is one of those things that gets seen by so many developers that it s worth giving some WTF love    class   new StdClass     class  test     PHP Parse error   syntax error  unexpected T PAAMAYIM NEKUDOTAYIM on line 3

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C  has a feature called  extension methods   which are roughly analogous to Ruby mix-ins - Essentially  you can add a method to any pre-existing class definition  for instance  you oould add  reverse    to String if you were so inclined    That alone is fine-  The  Weird  part is that you can add these extension methods  with a method body and everything  to an interface   On the one hand  this can be handy as a way to add a single method to a whole swath of classes which aren t part of the same inheritance tree   On the other  you re adding fleshed out methods to interfaces  essentially breaking the very definition of an interface

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In PHP  var export  false     0            true  var export  false     true         true  var export  false     false        false   EDIT  As  Kobi mentioned  it could happen because language interpret any value as  TRUE  except  FALSE   but not in case of PHP  where things are even more strange than you thought   This case is fully documented in chapter  String conversion to numbers  of a PHP manual  which says       If the string starts with valid   numeric data  this will be the value   used  Otherwise  the value will be 0    zero     Here is example   print  int   zero         0 print  int   false        0    but print  int   1 - one      1   P S  I see more harm than usefulness of such implicit type conversions

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I m surprised no one mentioned the REALLY ugly switch-case implementation in most C-like languages  switch  someInt        case 1      case 2  System out println  Forgot a break  idiot         case 3  System out println  Now you re doing the wrong thing and maybe need hours to find the missing break muahahahaha                break      default  System out println  This should never happen - -                The good thing is newer languages got it implemented right

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In retrospect  FORTRAN s computed goto is pretty odd   Wikipedia tells me some BASICs outdo it   Another famous favourite is Algol 60 s call by name parameter passing

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In PHP  true    false  and  null  are constants which normally cannot be overridden  However  with the introduction of namespaces in PHP   5 3  one can now redefine these constants within any namespace but the global namespace  Which can lead to the following behaviour    namespace       define  test true   42       define  test false   42       define  test null   42      namespace test       var dump true     false  amp  amp  false     null      is  bool  true     Of course if you want your trues to be true  you can always import true from the global namespace  namespace test       var dump  true      false      is  bool  false

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Python 2 x demonstrates a poor list comprehension realization   z   4 s    z z for z in range 255   print z   This code returns 254  The list comprehension s variable collides with an upper defined   Python 3 x had disposed of this feature  but closures are still using dynamic linking for external variables and brings many WTFs in the functional style python programmer  def mapper x       return x x continuations    lambda  mapper x  for x in range 5   print   c   for c in continuations     This code returns obviously  16 16 16 16 16

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An amusing side effect of Python s everything-is-really-a-reference    gt  gt  gt  a     1     7  gt  gt  gt  a   1    1    1    1    1    1    1    gt  gt  gt  a 0  0    2  gt  gt  gt  a   2    2    2    2    2    2    2

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dynamic  in C    Ruins the day for everyone who has to work together with a RAD or python victim because Intellisense  type safety and determinism die instantly with the first use of the  dynamic  keyword

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C C     The Fast Inverse Square Root algorithm takes advantage of the IEEE floating-point representation  code copied from Wikipedia    float InvSqrt float x        union           float f          int i        tmp      tmp f   x      tmp i   0x5f3759df -  tmp i  gt  gt  1       float y   tmp f      return y    1 5f - 0 5f   x   y   y

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I struggled a bit about this   1    In perl  modules need to return something true

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A very tiny thing that annoyed me in COBOL was that there was no dedicated modulo operation  Instead you could do a division specifying that you only wanted whole number results and store the rest in a different variable  Since COBOL is very sensitive when it comes to variables that means that you ended up with a variable you didn t really need  i e  the actual result of the division  This is the story of how I once named a variable  USELESS  - that was the most appropriate name I could think of

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Atari BASIC   You can fill a string with a character without writing a loop   10 DIM A  100  20 A  1      A  100      A  2  A

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Java has a whole freakin book about them   book http   www javapuzzlers com lg-puzzlers-cropped jpg  Java Puzzlers

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In ColdFusion arrays start at 1

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Don t know if it can be considered a language feature  but  in C   almost any compiler error related to templates delivers a fair amount of WTF to many C   programmers around the world on daily basis

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Java Generics Are a WTF    List lt String gt  ls   new ArrayList lt String gt       1 List lt Object gt  lo   ls    2   2  Is illegal       this is puzzling but you have to think what could happen next   lo add new Object     String s   ls get 0     We would be assigning an Object to a String reference  oh noes  And like this there a lots of gotchas around them

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About 20 years ago I worked with a compiler for a language called Coral which allowed me to declare writeonly variables   It made sense  though  as they were global and used as a signalling mechanism  One process would write a value and another would read it

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In Java   String s   null  System out println s    hello      This outputs  nullhello

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In Unity   GameObject Find  MyObject     will return your object normally  However  if you do something like this   GameObject Find  MyObject   active   false    do other stuff if  GameObject Find  MyObject   active          do stuff     Then you will get a null reference  In Unity iPhone  this code will often work fine in the editor but will cause a SIGBUS when running from the iPhone  The problem is that GameObject Find   will only locate active objects  so even if you re just checking to see if it s active  you are effectively calling if  null active     To make it work right  you ve got to store it prior to making it inactive   GameObject obj   GameObject Find  MyObject    obj active   false    do other stuff if  obj active          do stuff     Arguably that s better practice anyway  but the way Unity treats inactive objects in general is quite weird  It appears to unload a large portion of the inactive object  textures  etc   but not all of it  so inactive objects can still eat up a lot of memory

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Not technically a language WTF  but an architecture one   http   www 6502 org tutorials 6502opcodes html JMP  6502 assembly  indirect JMP      Note that there is no carry associated with the indirect jump so       AN INDIRECT JUMP MUST NEVER USE A      VECTOR BEGINNING ON THE LAST BYTE      OF A PAGE      For example if address  3000 contains  40   30FF contains  80  and  3100 contains  50  the result of JMP   30FF  will be a transfer of control to  4080 rather than  5080 as you intended i e  the 6502 took the low byte of the address from  30FF and the high byte from  3000     So adding a single byte to your code could make your indirect jump go wildly off target

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In php   easter date     Get Unix timestamp for midnight on Easter of a given year  int easter date     int  year

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I m surprised  that no one has mentioned Visual Basic s 7 loop constructs   For i As Integer   1 to 10     Next While True     End While Do While True     Loop Do Until True     Loop Do     Loop While True Do     Loop Until True While True     Wend   Because sticking an   in front of your conditional is way too complicated

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C  1x Lambda s       int x    std  cout  lt  lt  x  lt  lt  std  endl          These can be abused for some odd syntax                             This is completely valid C  1x

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Here s one I thought was weird   In C C   you can have as many semicolons as you want at least in the MS C     int main void     cout lt  lt  Hello World                                      return 0

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One of my favorites in C   is the  public abstract concrete inline destructor    class AbstractBase   public      virtual  AbstractBase     0        PACID       virtual void someFunc     0      virtual void anotherFunc     0       I stole this from Scott Meyers in Effective C     It looks a bit weird to see a method that s both pure virtual  which generally means  abstract   and implemented inline  but it s the best and most concise way I ve found to ensure that an object is polymorphically destructed

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Some early dynamic languages  including  if I remember correctly  early versions of Perl  hadn t figured out what was good dynamism and what was bad dynamism   So some of them allowed this   1   2    After that statement  the following would be true   if 1   1    4

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The many name spaces of C   typedef int i   void foo         struct i  i i   i      i  i i   3      printf    i n   i i       Or with characters   typedef char c   void foo         struct c  c c   c      c  c c    c       printf    c n   c c

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Smalltalk    Have a class method in a class Test  that returns a constant string    method1        niko    You should expect that this method constantly returns the string  niko  whatever happens  But that is not the case    s    Test method1     Set s to  niko     s at  4 put   i     Set s to  niki     s    Test method1    Set s to  niki  again    So  what happens is that the second line of code permanently changed method1 to return  niki  rather than  niko   even though the source code of the method was not updated

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In C   you can call static methods from null pointers - behold   class Foo     public      static void bar           std  cout  lt  lt   WTF     lt  lt  std  endl            int main void      Foo   foo   NULL    foo- gt bar        gt  WTF     return 0     Ok      That one caught me by surprise

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In C  you can use the new operator on an interface

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Not sure whether someone mentioned it   In Java  in finally block it can return a value  It will stop the propagation of an exception and override the normal return statement

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Being able to cast out of range ints to enums in C  is quite weird in my opinion  Imagine this enum   enum Colour       Red   1      Green   2      Blue   3     Now  if you write   Colour eco  eco    Colour 17    The compiler thinks that   s fine  And the runtime  too   See here for more details

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In my opinion this should not be allowed in C     class A   public    virtual string foo   return  A  foo        class B   public A   public    virtual string foo   return  B  foo        int main        B  b   new B         In my opinion the following should not be allowed   cout  lt  lt  b- gt A  foo    lt  lt  endl      Will print  A  foo      This may seem right  but this means that you cannot override a method without allowing users of the subclass to call the original method instead of the new one   Just think about a subclass of a collection where you want to increment the number of elements when adding an element to the collection itself   A logical solution would be to override the add   method to increase the counter before adding the element  but a user of the new collection could add an element to it using the old method so bypassing your increment and resulting in your elements-counter disagree with the actual number of elements of the collection   This is not possible in Java

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In PHP one can do   System out print  hello

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Variable assignment in JavaScript can create global variables  If a variable is a assigned a value within a function and it is not declared as var in the same scope it is implicitly declared global   function foo       x    juhu       creates a global variable x    var y    kinners     foo    alert x      alerts  juhu  alert y      alerts undefined   Note that the var statement can also be used after a value has been assigned to the variable   function foo       x   12    var x     x is now local   return x     alert foo        will alert 12  alert x      will alert undefined

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In C  the sizeof operator does not evaluate its argument   This allows one to write code that looks wrong but is correct   For example  an idiomatic way to call malloc    given a type T is    include  lt stdlib h gt   T  data   NULL  data   malloc sizeof  data     Here   data is not evaluated when in the sizeof operator  data is NULL  so if it were evaluated  Bad Things would happen     This allows one to write surprising code  to newcomers anyway   Note that no one in their right minds would actually do this    include  lt stdio h gt   int main            int x   1      size t sz   sizeof x         printf   d n   x       return 0         This prints 1  not 2  because x never gets incremented   For some real fun confusion with sizeof    include  lt stdio h gt  int main void        char a      Hello       size t s1   sizeof a      size t s2   sizeof   Hi   a       printf   zu  zu n   s1  s2       return 0       The confusion is only if one is confused about arrays  pointers  and operators

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The concatenation in Tcl is adding two strings it become one string   set s1  prime  set s2  number  set s3  s1 s2 puts s3   The output is      primenumber

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The BrainFuck language  It really is

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powerbasic  www powerbasic com  includes the compiler directive     BLOAT  bloatsize    this increases the size of the compiled executable by  lt bloatsize gt  bytes  this was put in the compiler in case people creating the executable don t like the small size of the generated executable  it makes the EXE seem bigger to compete with bloated programming languages

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The JavaScript octal conversion  feature  is a good one to know about   parseInt  06      6 parseInt  07      7 parseInt  08      0 parseInt  09      0 parseInt  10      10   More details here

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In C    the ability to create a protected abstract virtual base pure virtual private destructor   This is a pure virtual private destructor that is inherited from a protected abstract virtual base    IOW  a destructor that can only be called by members or friends of the class  private   and which is assigned a 0  pure virtual  in the base class  abstract base  that declares it  and which will be defined later overriden in a derived class that shares the multiple-inherited base  virtual base  in a protected way

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I don t know if it s still true  but we discovered by accident that VS FORTRAN 66 or 77  will not support recursion  The recursion was accidental and our default F77 supported it beautifully  but when we took the source to an IBM - Whatta Mess

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Perl   print  Foo n  unless  foo

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I like sneaking-in octal values in C   int values 8      123  154  103  310  046  806  002  970

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Unary operators in INTERCAL  AND  OR and XOR

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PHP s list construct    array   array 0 1 2   list     x     array   x    2     true

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In ColdFusion  Struct  aka Java HashMap  is passed by reference     You d have thought other data type behaves like Java     Array is passed by value  wtf   List is just a plain old comma-separated string

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In C    you can do   std  string my str  std  string my str concat   my str    foo     But you can t do   std  string my str concat    foo    my str    Operator overloading is generally subject to WTF

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In Python   i   1   i print i   prints  1   The line    i  evaluates to    i   Python doesn t support increment operators

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NSIS  the Nullsoft Scriptable Install System  has the StrCmp instruction    StrCmp str1 str2 jump if equal  jump if not equal        Compares  case insensitively  str1 to str2  If str1 and str2 are equal  Gotos jump if equal  otherwise Gotos jump if not equal   StrCmp  0  a string  0  3  DetailPrint    0     a string    Goto  2  DetailPrint    0     a string      The icing on the cake  jump if equal and jump if not equal can be negative  too  But I guess you already figured that out from the   symbol in front of positive numbers  I don t remember whether it s mandatory  or just a horrible convention   This basically combines the worst of BASIC and the worst of Assembler

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My biggest most hated feature is any configuration file syntax which includes conditional logic  This sort of thing is rife in the Java world  Ant  Maven  etc  You know who you are     You just end up programming in a c  p language  with limited debugging and limited editor support   If you need logic in your configuration the  Pythonic  approach of coding the configuration in a real language is much much better

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In Perl you can do   my  test    Hello World   substr  test  0  5     Goodbye    print  test    Is this possible in other languages

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in Ruby      i true while i     i false    a 2 end puts defined  a     returns true

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JavaScript is object oriented  right  So running methods on literal strings and numbers should work  Like  hello  toUpperCase   and 3 toString    Turns out that second one is a syntax error  why  Because the parser expects a number followed by a dot to be a floating point literal  That s not the WTF  the WTF is that you only have to add another dot to make it work    3  toString     The reason is that the literal 3  is interpreted as 3 0  and 3 0 toString   works fine

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Fun with auto boxing and the integer cache in Java   Integer foo   1000  Integer bar   1000   foo  lt   bar     true foo  gt   bar     true foo    bar     false    However  if the values of foo and bar are between 127 and -128  inclusive    the behaviour changes   Integer foo   42  Integer bar   42   foo  lt   bar     true foo  gt   bar     true foo    bar     true     Explanation  A quick peek at the Java source code will turn up the following          Returns a  lt tt gt Integer lt  tt gt  instance representing the specified     lt tt gt int lt  tt gt  value     If a new  lt tt gt Integer lt  tt gt  instance is not required  this method    should generally be used in preference to the constructor      link  Integer int    as this method is likely to yield    significantly better space and time performance by caching    frequently requested values         param  i an  lt code gt int lt  code gt  value      return a  lt tt gt Integer lt  tt gt  instance representing  lt tt gt i lt  tt gt       since  1 5     public static Integer valueOf int i        if  i  gt   -128  amp  amp  i  lt   IntegerCache high          return IntegerCache cache i   128       else         return new Integer i       Note  IntegerCache high defaults to 127 unless set by a property   What happens with auto boxing is that both foo and bar the same integer object retrieved from the cache unless explicitly created  e g  foo   new Integer 42   thus when comparing reference equality  they will be true rather than false   The proper way of comparing Integer value is using  equals

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In PowerShell  you can rename variables    gt   a    some value   gt   b    a   gt   c    d   gt  Rename-Item variable  b  c  gt   d some value   Indirect indirection  Take that  PHP   Literals work  too    gt  Rename-Item variable d e  gt   e some value

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By far the strangest feature I ve ever encountered was a  RETURN n  statement in a dialect of BASIC  don t remember which one  this was about 28 years ago    n  was optional and defaulted to 1  It could be a positive or negative number that indicated which line relative to the invoking GOSUB is the next to get executed   For example the following would output  30    10 GOSUB 200 20 PRINT  20  30 PRINT  30  100 END 200 RETURN  2   I encountered this when I had to translate a program written in this bizarre BASIC to FORTRAN  The BASIC program used this feature quite a bit to return to different statements based on various conditions and it took me a while to understand the logic flow  Once I understood it  I was able to write a much simpler version of the program  Needless to say  the simpler FORTRAN version had fewer bugs than the original BASIC program

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In FoxPro  if I remember correctly  every command can be abbreviated to 4 characters and everything else is ignored  so READ  READY  READINESS is all the same - whatever is after the first 4 characters is ignored  The guy who explained it to me liked that feature  but I thought it was creepy

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FORTRAN isn t a really WTF moment but rather it s more a  Why do I need to type all this garbage moment   IF 12  gt  11  THEN     Do some magic ENDIF   The   gt   threw me off when I was playing with the language for a bit until I realized it was the     symbol  Oh how I love not being a biology major and having to dabble in this crap day to day

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Java s access modifiers are a recent WTF to me  as I had to learn a bit of it    Apparently packages are more intimate than class hierarchies  I can t define methods and attributes that are visible to sub-classes but not to other classes in the package  And why would I want to share the insides of a class to other classes   But I can define attributes and methods that are visible to every class inside the package  but not to subclasses outside the package   No matter how hard I think about this  I still can t see the logic  Switch over the access modifiers and make protected act like it works in C   and keep the package private modifier as it is and it would make sense  Now it doesn t

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Reading a line from a text file in Java   BufferedReader in   null  try      in   new BufferedReader new FileReader  filename        String str     str   in readLine       if  str    null                      catch  IOException e             finally      try         if  in    null             in close                 catch  IOException e         Ugh  Although I admit it is not strange   just evil   -   A shorter  more idiomatic version   try      BufferedReader in   new BufferedReader new FileReader  filename        try          String str   in readLine           while  str    null              str   in readLine                    finally           in close            catch  IOException e        e printStackTrace

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I think this one isn t actually a  language feature   C  and I m quite possibly being widely ignorant in posting it  but I couldn t figure why this happens  so I ll ask  If it turns out to be related to some odd language feature   well  it really made me  WTF   so it s worth this place   int a   0  int  p    amp a   printf   d   d   d  n    p    p      p      Outputs  1  0  0  n  on MinGW s GCC 4 4 1   Why   -- edit  Just got it  and it s not big deal  I can sense the C   gurus laughing at me now  I guess the order in which function parameters are evaluated is unspecified  so compilers are free to call them as they wish  and I think I ve read that one somewhere in boost s documentation   In this case  the argument statements were evaluated backwards  probably reflecting the calling convention of the function

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I would say the whole whitespace thing of Python is my greatest WTF feature  True  you more-or-less get used to it after a while and modern editors make it easy to deal with  but even after mostly full time python development for the past year I m still convinced it was a Bad Idea   I ve read all the reasoning behind it but honestly  it gets in the way of my productivity  Not by much  but it s still a burr under the saddle   edit  judging by the comments  some people seem to think I don t like to indent my code  That is an incorrect assessment  I ve always indented my code no matter what the language and whether I m forced to or not  What I don t like is that it is the indentation that defines what block a line of code is in  I prefer explicit delimiters for that  Among other reasons  I find explicit delimiters makes it easier to cut and paste code    For example  if I have a block indented 4 spaces and paste it at the end of a block that is indented 8 spaces  my editor  all editors   have no idea if the pasted code belongs to the 8-space block or the outer block  OTOH  if I have explicit delimiters it s obvious which block the code belongs to and how it should be  re- indented -- it does so by intelligently looking for block delimiters    edit 2  some people who provide comments seem to think this is a feature I hate or that I think makes python a poor language  Again  not true  While I don t like it all that much  that s beside the point  The question is about the strangest language feature  and I think this is strange  by virtue of it being something very  very few  but  0  languages use

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C  s default inheritance model wins my vote   public class Animal       public string Speak     return  unknown sound         public class Dog   Animal       public string Speak     return  Woof          class Program       static void Main  string   args                 Dog aDog   new Dog             Animal anAnimal    Animal  aDog            Console WriteLine   Dog sez   0      aDog Speak               Console WriteLine   Animal sez   0      anAnimal Speak                return             Running the program give the following as a result      Dog says  Woof     Animal says  unknown sound    Getting that sort of behavior should require the programmer to go out of the programmer s way  The subclass instance doesn t stop being what it is because it s been upcast to its supertype  Instead you have to explicitly request the expected  and almost always desired  result   public class Animal       public virtual string Speak     return  unknown sound         public class Dog   Animal       public override string Speak     return  Woof

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What datatype is foo   SELECT TOP 1    NULL AS foo INTO    dbo bar FROM    sys columns --trivial   Why does everything go to zero   SELECT CAST    AS int   CAST    AS datetime   CAST    AS float       except this  SELECT CAST    AS decimal

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In JavaScript   1   0     Infinity 1   -0     -Infinity

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In SQL Server you may end up with a nasty surprise if you use select   in your production code  Using select   is not considered as a good practice anyway but it is good to know of some of the interesting behaviour    See question    select   from table    vs    select colA colB etc from table    interesting behaviour in SqlServer2005 for more details

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In Python  every string contains the empty string     answer   input  Enter  Y es  or N o     if answer in  YyNn            verify input     process answer     Just hitting return at the above query will set answer to the null string  pass the if answer in     test  and be processed as a correct answer  To put it more succinctly     gt  gt  gt   ABCDEFGHIJ    contains       True   As usual  Python s behavior here is mathematically and logically impeccable  As I recall from a long ago class in set theory  The empty set is a member of every set     It s still surprising on the few occasions when I ve been bitten by it  but I wouldn t have it any other way

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Modula-2 doesn t have elseif or else if  it has elsif

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For those who don t know  bc is an  arbitrary precision calculator language   and I use it quite often for quick calculations  particularly when the numbers involved are large    is the prompt      bc -lq 12 345 20774466823273785598434446955827049735727869127052322369317059031795  19704325276892191015329301807037794598378537132233994613616420526484  93077727371807711237016056649272805971389591721704273857856298577322  13812114239610682963085721433938547031679267799296826048444696211521  30457090778409728703018428147734622401526422774317612081074841839507  864189781700150115308454681772032   bc has been a standard Unix command for a long time   Now for the  WTF feature    This is from man bc  emphasis mine       quit  When the quit statement is read  the bc processor is terminated    regardless  of  where the quit statement is found    For example   if  0    1  quit  will cause bc to terminate       halt  The  halt statement  an extension  is an executed statement that   causes the bc processor to quit only when it is  executed    For example    if   0      1   halt   will not cause bc to terminate because the halt is not executed

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Objective-C s use of   for strings  Example    This is a string

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The rest of these have nothing on the astounding Ruby Flip-Flop Operator   p   proc   a b  a  b    yes     no     p false false       gt   no  p true false        gt   yes  p false false       gt   yes          p false true        gt   yes  p false false       gt   no    Yes  program state stored in the interpreter s parse tree  Things like this are why it takes forever to make a compliant Ruby implementation  But I forgive you  Ruby  lt 3

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The following is similar to this answer which is about arrays   In Powershell  like other dynamic languages  strings and numbers are somewhat interchangeable  However  Powershell can t make up its mind    PS gt   a    4       string PS gt   a   3        Python can do this  too 444 PS gt  3    a        Python doesn t do it this way  string repetition is commutative 12 PS gt   a   3        Python gives a mismatched types error 43 PS gt  3    a        Python would give an error here  too 7   If the variable is an integer instead of a string  then the operations are commutative   PS gt   a   4        integer PS gt   a   3 12 PS gt  3    a 12 PS gt   a   3 7 PS gt  3    a 7   When in doubt  do a cast   PS gt   a    4  PS gt   b   3 PS gt   int   a    int   b 12   You could also use  float

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JavaScript dates are full of WTF   var d   new Date  1 1 2001     var wtfyear   d getYear       101  the year - 1900     to get the  actual  year  use d getFullYear    var wtfmonth   d getMonth       0    months are 0-based

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PHP as an entire language is mostly WTF   The langauge definition is defined  see www php org  not by a grammar  or a standard  but by a bunch of  you can write this example  sections  can you write anything else  sure  just guess at the generalization   with honest-to-god user contributions saying  but it does this wacko thing         I periodically encounter glitches with a PHP parser we built   Here s the latest     abc A define def    Now  PHP is a  truly bad  copy of PERL  and so it allows strings to be constructed with implicit substition of variables    X in the string says  plug the value of  X into the string   equivalent to  abc     X    def  where     is PHP s string-concatenate operator    A 7  in a string says   plug the value of the seventh slot of array  A into the string  equivalent to  abc     A 7     def    Now  the language  website  clearly says   define  is a keyword  and you can t use it whereever you d find an expression    So the above gem containing  define  does what   Throw a syntax error   Nah  that would make sense   No  what it actually means is     abc     A  define      def    It does this ONLY if you write an thing that looks like an identifier  keyword or not   in an simple array access in a string   Nowhere else in the language does this behaviour occur  What  writing  abc A  define  def  was unreasonable so the PHP inventors had to throw this in  Give me a break    To compound the felony  there s  complex array access in a string  and of course it works differently   Check out  abc  A define  def   that is illegal according to the PHP website    Turns out PHP arrays are associate hashes  so looking up an array  well  hash table  member by name isn t a terrible idea    The language is full of gotchas like this   If you like  gee  look what squirmy thing I found under my subroutine today   you should switch to PHP

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When I was in college  I did a little bit of work in a language called SNOBOL   The entire language  while cool  is one big WTF     It has the weirdest syntax I ve ever seen   Instead of GoTo  you use   label    And who needs if s when you have  S label   goto label on success true  and  F label   goto label on failure false  and you use those functions on the line checking some condition or reading a file   So the statement   H   INPUT  F end    will read the next line from a file or the console and will go to the label  end  if the read fails  because EOF is reached or any other reason      Then there is the   sign operator   That will use the value in a variable as a variable name   So   ANIMAL    DOG  DOG    BARK  output    ANIMAL   will put the value  BARK  on teh console   And because that isn t weird enough    DOG    SOUND    will create variable named BARK  see the value assigned to DOG above  and give it a value of  SOUND      The more you look at it  the worse it gets   The best statement I ever found about SNOBOL  from link text  is  the power of the language and its rather idiomatic control flow features make SNOBOL4 code almost impossible to read and understand after writing it

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In C   a   cond   b   c  If  b   amp   c  are  assign incompatible   you ll never get result  even if  a  is object  It s frequently used and most idiotically implemented operator from MS  For comparison see implementation in D language  note on type inference

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My favorite weirdness in C is 5  Hello World    but since that was already posted  my next-favorite weirdness is the Windows versioned-structure initialization hack   void someWindowsFunction         BITMAPINFOHEADER header    sizeof header           do stuff with header        That one  subtle line accomplishes the following    Declares a BITMAPINFOHEADER structure Concisely sets the  size  member of the structure  without hardcoding a size constant  since many Window structures  including BITMAPINFOHEADER  follow the convention of specifying the size of the structure as the first member  Declares the version of the structure  since many Windows structures  including BITMAPINFOHEADER  identify their version by the declared size  following the convention that structures definitions are append-only  Clears all other members of the structure  a C standard behavior when a structure is incompletely initialized

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Perl is full of odd but neat features   if may be used before or after the statement like this   print  Hello World  if  a  gt  1      if   a  gt  1    print  Hello World       The same is true for foreach   print  Hello     n  foreach qw world Dolly nurse

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