What is the -- operator in C C

Question

After reading Hidden Features and Dark Corners of C   STL on comp lang c   moderated  I was completely surprised that the following snippet compiled and worked in both Visual Studio 2008 and G   4 4  Here s the code   include  lt stdio h gt  int main         int x   10      while  x -- gt  0     x goes to 0               printf  quot  d  quot   x            Output  9 8 7 6 5 4 3 2 1 0  I d assume this is C  since it works in GCC as well  Where is this defined in the standard  and where has it come from

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Utterly geek  but I will be using this    define as  while  int main int argc  char  argv          int n   atoi argv 1        do printf  n is  d n   n  as   n -- gt  0       return 0

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Here -- is the unary post decrement operator   while  x--  gt  0     x goes to 0         printf  quot  d  quot   x        In the beginning  the condition will evaluate as  x  gt  0     10  gt  0 Now because the condition is true  it will go into the loop with a decremented value x--    x   9 That s why the first printed value is 9 And so on  In the last loop x 1  so the condition is true  As per the unary operator  the value changed to x   0 at the time of print  Now  x   0  which evaluates the condition  x  gt  0   as false and the while loop exits

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-- gt  is not an operator  It is in fact two separate operators  -- and  gt    The conditional s code decrements x  while returning x s original  not decremented  value  and then compares the original value with 0 using the  gt  operator   To better understand  the statement could be written as follows   while   x--   gt  0

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This code first compares x and 0 and then decrements x   Also said in the first answer  You re post-decrementing x and then comparing x and 0 with the  gt  operator   See the output of this code   9 8 7 6 5 4 3 2 1 0   We now first compare and then decrement by seeing 0 in the output   If we want to first decrement and then compare  use this code    include  lt stdio h gt  int main void        int x   10       while  --x gt  0      x goes to 0               printf   d    x             return 0      That output is   9 8 7 6 5 4 3 2 1

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This is exactly the same as  while  x--       printf   d    x       for non-negative numbers

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The usage of -- gt  has historical relevance  Decrementing was  and still is in some cases   faster than incrementing on the x86 architecture  Using -- gt  suggests that x is going to 0  and appeals to those with mathematical backgrounds

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It s a combination of two operators  First -- is for decrementing the value  and  gt  is for checking whether the value is greater than the right-hand operand    include lt stdio h gt   int main         int x   10       while  x--  gt  0          printf   d   x        return 0      The output will be   9 8 7 6 5 4 3 2 1 0

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It s equivalent to  while  x--  gt  0    x--  post decrement  is equivalent to x   x-1 so  the code transforms to   while x  gt  0        x   x-1         logic   x--       The post decrement done when x  lt   0

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char sep     n    1    int i   68     1      while  i  ---      1                                                   1 1 1                                1                                                               1                                                               1                                                               1                                                               1                                          1                                          1                                           1                                            1                               1             1                                 1           1                                    1         1         1 1 gt  0  std  cout                                  lt  lt i lt  lt                                sep   For larger numbers  C  20 introduces some more advanced looping features  First to catch i we can build an inverse loop-de-loop and deflect it onto the std  ostream  However  the speed of i is implementation-defined  so we can use the new C  20 speed operator  lt  lt i lt  lt  to speed it up  We must also catch it by building wall  if we don t  i leaves the scope and de referencing it causes undefined behavior  To specify the separator  we can use   std  cout              sep  and there we have a for loop from 67 to 1

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x can go to zero even faster in the opposite direction   int x   10   while  0  lt ---- x        printf   d    x       8 6 4 2  You can control speed with an arrow   int x   100   while  0  lt -------------------- x        printf   d    x       90 80 70 60 50 40 30 20 10

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x -- gt  0  means  x--  gt  0    You can use  x -- gt   Output  9 8 7 6 5 4 3 2 1 0     You can use  -- x  gt  0   It s mean  --x  gt  0  Output  9 8 7 6 5 4 3 2 1     You can use   --             x  gt  0   Output   9 8 7 6 5 4 3 2 1  You can use            x -- gt  0   Output  9 8 7 6 5 4 3 2 1 0  You can use            x -- gt  0                                         Output  9 8 7 6 5 4 3 2 1 0  You can use also     x    -- gt         0           Output  9 8 7 6 5 4 3 2 1 0 Likewise  you can try lot of methods to execute this command successfully

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This -- gt  is not an operator at all  We have an operator like - gt   but not like -- gt   It is just a wrong interpretation of while x--  gt 0  which simply means x has the post decrement operator and this loop will run till it is greater than zero  Another simple way of writing this code would be while x--   The  while loop will stop whenever it gets a false condition and here there is only one case  i e   0  So it will stop when the x value is decremented to zero

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That s a very complicated operator  so even ISO IEC JTC1  Joint Technical Committee 1  placed its description in two different parts of the C   Standard   Joking aside  they are two different operators  -- and  gt  described respectively in  sect 5 2 6 2 and  sect 5 9 of the C  03 Standard

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My compiler will print out 9876543210 when I run this code    include  lt iostream gt  int main         int x   10       while  x -- gt  0      x goes to 0               std  cout  lt  lt  x            As expected  The while  x--  gt  0   actually means while  x  gt  0   The x-- post decrements x   while  x  gt  0          x--      std  cout  lt  lt  x      is a different way of writing the same thing   It is nice that the original looks like  while x goes to 0  though

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Conventional way we define condition in while loop parenthesis quot    quot  and terminating condition inside the braces quot    quot   but this --  amp   gt  is a way one defines all at once  For example  int abc        int a   5     while  a--   gt  0      Decrement and comparison both at once            Code          It says  decrement a and run the loop till the time a is greater than 0 Other way it should have been like  int abc         int a   5      while a  gt  0            a   a -1    Decrement inside loop            Code          Both ways  we do the same thing and achieve the same goals

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There is a space missing between -- and  gt   x is post decremented  that is  decremented after checking the condition x gt 0

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Why all the complication  The simple answer to the original question is just   include  lt stdio h gt   int main         int x   10      while  x  gt  0                printf  quot  d  quot   x           x   x-1           It does the same thing  I am not saying you should do it like this  but it does the same thing and would have answered the question in one post  The x-- is just shorthand for the above  and  gt  is just a normal greater-than operator  No big mystery  There are too many people making simple things complicated nowadays

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Anyway  we have a  goes to  operator now   -- gt   is easy to be remembered as a direction  and  while x goes to zero  is meaning-straight   Furthermore  it is a little more efficient than  for  x   10  x  gt  0  x --   on some platforms

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Actually  x is post-decrementing and with that condition is being checked  It s not -- gt   it s  x--   gt  0  Note  value of x is changed after the condition is checked  because it post-decrementing  Some similar cases can also occur  for example   -- gt     x-- gt 0    gt     x   gt 0 -- gt     x-- gt  0    gt     x   gt  0

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Or for something completely different    x slides to 0   while  x --                                                               gt  0       printf   d    x     Not so mathematical  but    every picture paints a thousand words

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One book I read  I don t remember correctly which book  stated  Compilers try to parse expressions to the biggest token by using the left right rule   In this case  the expression   x-- gt 0   Parses to biggest tokens    token 1  x token 2  -- token 3   gt  token 4  0 conclude  x--  gt  0   The same rule applies to this expression   a-----b   After parse   token 1  a token 2  -- token 3  -- token 4  - token 5  b conclude   a-- -- - b   I hope this helps to understand the complicated expression

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-- is the decrement operator and  gt  is the greater-than operator   The two operators are applied as a single one like -- gt

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while  x--  gt  0     is how that s parsed

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It s  include  lt stdio h gt   int main void      int x   10    while  x--  gt  0       x goes to 0     printf  quot  d  quot   x         return 0     Just the space makes the things look funny  -- decrements and  gt  compares

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C and C   obey the  maximum munch  rule  The same way a---b is translated to  a--  - b  in your case  x-- gt 0 translates to  x--  gt 0   What the rule says essentially is that going left to right  expressions are formed by taking the maximum of characters which will form an valid expression

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