Assigning value in while loop condition

Question

I found this piece of code on Wikipedia.

#include <stdio.h>

int main(void)
{
  int c;

  while (c = getchar(), c != EOF && c != 'x')
  {
    switch (c)
      {
      case '\n':
      case '\r':
        printf ("Newline\n");
        break;
      default:
        printf ("%c",c);
      }
  }
  return 0;
}

I'm curious about expression used as condition for while loop:

while (c = getchar(), c != EOF && c != 'x')

It's quite obvious what it does, but I've never seen this construction before. Is this specific to while loop? If not, how does parser/compiler determine which side of comma-separated expression returns boolean value for while loop?

Answer 1

The comma operator is a binary operator that evaluates its first operand and discards the result, it then evaluates the second operand and returns this value.

It is also a "sequence point", which means that all side effects will be calculated before the next part of the code is executed.

Answer 2

The comma operator is a weird beastie until you get to understand it, and it's not specific to while.

The expression:

exp1, exp2

evaluates exp1 then evaluates exp2 and returns exp2.

You see it frequently, though you may not realize it:

for (i = j = 0; i < 100; i++, j += 2)

You're not actually using the return value from "i++, j += 2" but it's there nonetheless. The comma operator evaluates both bits to modify both i and j.

You can pretty well use it anywhere a normal expression can be used (that comma inside your function calls is not a comma operator, for example) and it's very useful in writing compact source code, if that's what you like. In that way, it's part of the family that allows things like:

while ((c= getchar()) != EOF) {...}
i = j = k = 0;

and so on.

For your specific example:

while (c = getchar(), c != EOF && c != 'x')

the following occurs:

• c = getchar() is executed fully (the comma operator is a sequence point).
• c != EOF && c != 'x' is executed.
• the comma operator throws away the first value (c) and "returns" the second.
• the while uses that return value to control the loop.