18.2.4: The Comma Operator

Once in a while, you find yourself in a situation in which C expects a single expression, but you have two things you want to say. The most common (and in fact the only common) example is in a for loop, specifically the first and third controlling expressions. What if (for example) you want to have a loop in which i counts up from 0 to 10 at the same time that j is counting down from 10 to 0? You could manipulate i in the loop header and j ``by hand'':

	j = 10;
	for(i = 0; i < 10; i++)
		{
		... rest of loop ...
		j--;
		}

but here it's harder to see the parallel nature of i and j, and it also turns out that this won't work right if the loop contains a continue statement. (A continue would jump back to the top of the loop, and i would be incremented but j would not be decremented.) You could compute j in terms of i:

	for(i = 0; i < 10; i++)
		{
		j = 10 - i;
		... rest of loop ...
		}

but this also makes j needlessly subservient. The usual way to write this loop in C would be

	for(i = 0, j = 10; i < 10; i++, j--)
		{
		... rest of loop ...
		}

Here, the first (initialization) expression is

	i = 0, j = 10

The comma is the comma operator, which simply evaluates the first subexpression i = 0, then the second j = 10. The third controlling expression,

	i++, j--

also contains a comma operator, and again, performs first i++ and then j--.

Precisely stated, the meaning of the comma operator in the general expression

	e1 , e2

is ``evaluate the subexpression e1, then evaluate e2; the value of the expression is the value of e2.'' Therefore, e1 had better involve an assignment or an increment ++ or decrement -- or function call or some other kind of side effect, because otherwise it would calculate a value which would be discarded.

There's hardly any reason to use a comma operator anywhere other than in the first and third controlling expressions of a for loop, and in fact most of the commas you see in C programs are not comma operators. In particular, the commas between the arguments in a function call are not comma operators; they are just punctuation which separate several argument expressions. It's pretty easy to see that they cannot be comma operators, otherwise in a call like

	printf("Hello, %s!\n", "world");

the action would be ``evaluate the string "Hello, %s!\n", discard it, and pass only the string "world" to printf.'' This is of course not what we want; we expect both strings to be passed to printf as two separate arguments (which is, of course, what happens).

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