The C comma operator

You know how semicolons are statement terminators in C; but remember how a semicolon works in sh, in which it is a statement separator — a single statement in sh does not require a semicolon, but you can use a semicolon to put two statements together. It's like an operator, and it means to do one statement and then the other.

The comma operator in C is like the semicolon operator in sh, but for expressions rather than statements. It means to evaluate one expression, then the other.

All binary operators have a "left" operand and a "right" operand. For example, in the expression "3,4", 3 is the left operand to the comma operator, and 4 is the right operand to the comma operator. (Just like left and right operands to "+" in an expression like "3+4".)

With the C comma operator, the left operand is evaluated first, and its value is discarded.

Then the right operand is evaluated, and its value is the value of the overall expression.

So if for example we want a "while" loop which continues so long as x is 3, we could write

	while (x == 3)

But if each time around the loop we need to execute a function called "computex" to compute x before we can test it, we can write

	while (computex(), x == 3)

The computex() function call occurs, its return value if any is discarded, and then we execute the expression "x==3" which is what counts as far as whether or not the while loop continues to iterate.

It matters whether computex() or x==3 is evaluated first, because computex changes the value of x. The comma operator specifies that the left operand is evaluated first.

When used to excess, this sort of construct can get very messy; but in the above case, it makes for cleaner code than writing

	computex();
	while (x == 3) {
	    ... various stuff goes here ...
	    computex();
	}