What is the comma operator in C?

The comma operator evaluates its left and right expressions in order and yields the value of the rightmost one — letting you cram several side effects into one expression slot.

int a, b, c;

// Comma in for loop (common use):
for (i = 0, j = 10; i < j; i++, j--) {
    // i starts at 0 going up, j starts at 10 going down
}

// Comma as operator (returns last value):
// c = 3
c = (a = 1, b = 2, a + b);
// Evaluates: a=1, then b=2, then returns a+b

// Different from comma in declarations:
// This is NOT the comma operator
int x = 1, y = 2;

Why it exists:

• Initialize multiple loop variables
• Execute multiple side effects
• Macros that need to do multiple things

Warning: The comma operator has the lowest precedence:

// Means: (c = a), b;  -- NOT c = (a, b)
c = a, b;
// Means: c = b (after evaluating a)
c = (a, b);