Are while true loops so bad

Question

I ve been programming in Java for several years now  but I just recently returned to school to get a formal degree  I was quite surprised to learn that  on my last assignment  I lost points for using a loop like the one below   do         get some input         if the input meets my conditions  break         Otherwise ask again    while true    Now for my test I m just scanning for some console input  but I was told that this kind of loop is discouraged because using break is akin to goto  we just don t do it   I understand fully the pitfalls of goto and its Java cousin break label  and I have the good sense not to use them  I also realize that a more complete program would provide some other means of escape  say for instance to just end the program  but that wasn t a reason my professor cited  so     What s wrong with do-while true

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It s not such a terrible thing  but you need to take into consideration other developers when coding  Even in school   Your fellow developers should be able to see the exit clause for your loop  at the loop declaration  You didn t do that  You hid the exit clause in the middle of the loop  making more work for someone else who comes along and tries to understand your code  This is the same reason that things like  break  are avoided   That being said  you ll still see things like this in a LOT of code out there in the real world

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Maybe I am unlucky  Or maybe I just lack an experience  But every time I recall dealing with while true  having break inside  it was possible to improve the code applying Extract Method to while-block  which kept the while true  but  by coincidence   transformed all the breaks into returns   In my experience while true  without breaks  ie with returns or throws  are quite comfortable and easy to understand       void handleInput           while  true              final Input input   getSomeInput              if  input    null                  throw new BadInputException  can t handle null input                          if  input isPoisonPill                    return                        doSomething input

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You might just use a Boolean flag to indicate when to end the while loop  Break and go to were reasons for software being to hard to maintain - the software-crisis tm  - and should be avoided  and easily can be too   It s a question if you are pragmatic or not  Pragmatic coders might just use break in that simple situation    But it s good to get the habbit of not using them  else you might use them out of habbit in unsuitable situations  like in complicated nested loops where readability and maintainability of your code becomes harder by using break

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To me  the problem is readability   A while statement with a true condition tells you nothing about the loop  It makes the job of understanding it much more difficult   What would be easier to understand out of these two snippets   do        Imagine a nice chunk of code here   while true    do        Imagine a nice chunk of code here   while price  lt  priceAllowedForDiscount

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I would say that generally the reason it s not considered a good idea is that you are not using the construct to it s full potential  Also  I tend to think that a lot of programming instructors don t like it when their students come in with  baggage   By that I mean I think they like to be the primary influence on their students programming style  So perhaps that s just a pet peeve of the instructor s

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I use something similar  but with opposite logic  in a lot of my functions   DWORD dwError   ERROR SUCCESS   do       if    dwError   SomeFunction       ERROR SUCCESS                     handle error             continue             if    dwError   SomeOtherFunction       ERROR SUCCESS                     handle error             continue          while   0     if   dwError    ERROR SUCCESS            resource cleanup

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There s no major problem with while true  with break statements  however some may think its slightly lowers the code readability   Try to give variables meaningful names  evaluate expressions in the proper place   For your example  it seems much clearer to do something like   do      input   get input       valid   check input validity input         while   valid    This is especially true if the do while loop gets long -- you know exactly where the check to see if there s an extra iteration is occurring   All variables functions have appropriate names at the level of abstraction   The while true  statement does is tell you that processing isn t in the place you thought   Maybe you want different output on the second time through the loop   Something like   input   get input    while input is not valid input         disp msg invalid input        input   get input        seems more readable to me then  do       input   get input        if  input is valid input             break            disp msg invalid input      while true     Again  with a trivial example both are quite readable  but if the loop became very large or deeply nested  which means you probably should already have refactored   the first style may be a bit clearer

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Back in 1967  Edgar Dijkstra wrote an article in a trade magazine about why goto should be eliminated from high level languages to improve code quality  A whole programming paradigm called  structured programming  came out of this  though certainly not everyone agrees that goto automatically means bad code    The crux of structured programming is essentially that the structure of the code should determine its flow rather than having gotos or breaks or continues to determine flow  wherever possible  Similiarly  having multiple entry and exit points to a loop or function are also discouraged in that paradigm    Obviously this is not the only programming paradigm  but often it can be easily applied to other paradigms like object oriented programming  ala Java     Your teachers has probably been taught  and is trying to teach your class that we would best avoid  spaghetti code  by making sure our code is structured  and following the implied rules of structured programming    While there is nothing inherently  wrong  with an implementation that uses break  some consider it significantly easier to read code where the condition for the loop is explicitly specified within the while   condition  and eliminates some possibilities of being overly tricky  There are definitely pitfalls to using a while true  condition that seem to pop up frequently in code by novice programmers  such as the risk of accidentally creating an infinite loop  or making code that is hard to read or unnecessarily confusing    Ironically  exception handling is an area where deviation from structured programming will certainly come up and be expected as you get further into programming in Java   It is also possible your instructor may have expected you to demonstrate your ability to use a particular loop structure or syntax being taught in that chapter or lesson of your text  and while the code you wrote is functionally equivalent  you may not have been demonstrating the particular skill you were supposed to be learning in that lesson

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AFAIK nothing  really  Teachers are just allergic to goto  because they heard somewhere it s really bad  Otherwise you would just write   bool guard   true  do      getInput       if  something       guard   false    while  guard    Which is almost the same thing   Maybe this is cleaner  because all the looping info is contained at the top of the block    for  bool endLoop   false   endLoop

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1  Nothing is wrong with a do -while true   2  Your teacher is wrong   NSFS     3  Most teachers are teachers and not programmers

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Douglas Crockford had a remark about how he wished JavaScript contained a loop structure   loop        code        And I don t think Java would be any worse for having a loop structure either   There s nothing inherently wrong with while true  loops  but there is a tendency for teachers to discourage them  From the teaching perspective  it s very easy to have students create endless loops and not understand why the loop isn t ever escaped   But what they rarely mention is that all looping mechanisms can be replicated with while true  loops   while  a         fn        is the same as  loop     if    a     break    fn        and  do     fn      while  a        is the same as   loop     fn      if    a     break      and  for   a    b    c         fn        is the same as   a    loop     if    b     break    fn      c        As long as you can set up your loops in a way that works the construct that you choose to use is unimportant  If it happens to fit in a for loop  use a for loop   One last part  keep your loops simple  If there s a lot of functionality that needs to happen on every iteration  put it in a function  You can always optimize it after you ve got it working

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I wouldn t say it s bad - but equally I would normally at least look for an alternative   In situations where it s the first thing I write  I almost always at least try to refactor it into something clearer  Sometimes it can t be helped  or the alternative is to have a bool variable which does nothing meaningful except indicate the end of the loop  less clearly than a break statement  but it s worth at least trying   As an example of where it s clearer to use break than a flag  consider   while  true        doStuffNeededAtStartOfLoop        int input   getSomeInput        if  testCondition input                 break            actOnInput input       Now let s force it to use a flag   boolean running   true  while  running        doStuffNeededAtStartOfLoop        int input   getSomeInput        if  testCondition input                 running   false            else               actOnInput input             I view the latter as more complicated to read  it s got an extra else block  the actOnInput is more indented  and if you re trying to work out what happens when testCondition returns true  you need to look carefully through the rest of the block to check that there isn t something after the else block which would occur whether running has been set to false or not   The break statement communicates the intent more clearly  and lets the rest of the block get on with what it needs to do without worrying about earlier conditions   Note that this is exactly the same sort of argument that people have about multiple return statements in a method  For example  if I can work out the result of a method within the first few lines  e g  because some input is null  or empty  or zero  I find it clearer to return that answer directly than to have a variable to store the result  then a whole block of other code  and finally a return statement

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I think that yes it is pretty bad    or at least  for many developers  It is symptomatic of developers that don t think about their loop conditions  Consequently there is error prone

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The ususal Java convention for reading input is   import java io    BufferedReader br   new BufferedReader new InputStreamReader System in    String strLine   while   strLine   br readLine       null         do something with the line     And the usual C   convention for reading input is    include  lt iostream gt   include  lt string gt  std  string data  while std  readline std  cin  data          do something with the line     And in C  it s   include  lt stdio h gt  char  buffer   NULL  size t buffer size  size t size read  while   size read   getline  amp buffer   amp buffer size  stdin      -1         do something with the line   free buffer     or if you re convinced you know how long the longest line of text in your file is  you can do   include  lt stdio h gt  char buffer BUF SIZE   while  fgets buffer  BUF SIZE  stdin         do something with the line     If you re testing to see whether your user entered a quit command  it s easy to extend any of these 3 loop structures  I ll do it in Java for you   import java io    BufferedReader br   new BufferedReader new InputStreamReader System in    String line   while   line   br readLine       null   amp  amp   line equals  quit            do something with the line     So  while there certainly are cases where break or goto is justified  if all you re doing is reading from a file or the console line by line  then you shouldn t need a while  true  loop to accomplish it -- your programming language has already supplied you with an appropriate idiom for using the input command as the loop condition

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It s your gun  your bullet and your foot     It s bad because you are asking for trouble  It won t be you or any of the other posters on this page who have examples of short simple while loops   The trouble will start at some very random time in the future  It might be caused by another programmer  It might be the person installing the software  It might be the end user   Why  I had to find out why a 700K LOC app would gradually start burning 100  of the CPU time until every CPU was saturated  It was an amazing while  true  loop  It was big and nasty but it boiled down to   x   read value from database   while  true    if  x    1          break   else if  x   2          break  and lots more else if conditions     There was no final else branch  If the value did not match an if condition the loop kept running until the end of time   Of course  the programmer blamed the end users for not picking a value the programmer expected   I then eliminated all instances of while true  in the code    IMHO it is not good defensive programming to use constructs like while true   It will come back to haunt you    But I do recall professors grading down if we did not comment every line  even for i

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It might be bad if your loop runs on a background thread  so when you close your application by terminating a UI thread  that piece of code will continue to execute  As others already said  you should always use some kind of check to provide a way for cancellation

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I guess using break to your teacher is like breaking a branch of tree to get the fruit use some other tricks  bow the branch  so that you get the fruit and the branch is still alive

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While not necessarily an answer as to why not to use while  true   I ve always found this comic and accompanying author s statement a succinct explanation as to why to do while instead of do-while   With regards to your question  There is no inherent problem with   while true       do stuff       if exit time          break               if you know what you re doing and making sure that exit time will at some point evaluate to true    Teachers discourage you from using while true  because until and unless you re at the point that you know exactly what you re doing  it s an easy way to make a critical mistake

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It s bad in the sense that structured programming constructs are preferred to  the somewhat unstructured  break and continue statements  They are  by comparison  preferred to  goto  according to this principle   I d always recommend making your code as structured as possible    although  as Jon Skeet points out  don t make it more structured than that

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It s more of an aesthetics thing  much easier to read code where you explicitly know why the loop will stop right in the declaration of the loop

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According to my experience in most cases loops have the  main  condition to continue  This is the condition that should be written into while   operator itself  All other conditions that may break the loop are secondary  not so important etc  They can be written as additional if    break  statements    while true  is often confusing and is less readable    I think that these rules do not cover 100  of cases but probably only 98  of them

[java] Are "while(true)" loops so bad?

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