The Comma Operator in C: Sequencing Expressions and When It Actually Applies

The comma operator evaluates its left operand, discards the result, evaluates its right operand, and returns that. a = (1, 2, 3) assigns 3 to a. The 1 and 2 are evaluated and thrown away. This is not the same as the commas in a variable declaration or a function argument list. Those are separators, not operators. The comma operator appears in for loop headers, in macro definitions, and in code that is trying too hard to be clever.

In the for header, i = 0, j = 10 is two declarations separated by a comma separator, not the comma operator. But i++, j-- in the increment slot is the comma operator: it evaluates i++, discards the result, evaluates j--, and the whole expression's value is discarded anyway. The for loop is the one place where the comma operator is idiomatic and readable.

#Comma vs Semicolon#link

The semicolon separates statements. The comma operator sequences expressions within a single statement. You cannot put a semicolon inside a for header's increment slot. You can put a comma. That is the practical reason the comma operator exists: it lets you do two things in a context that syntactically requires one expression.

The comma operator is rarely needed outside of for loops. When you see it elsewhere, read carefully. It is probably doing something subtle, or it is a bug where someone meant to write a semicolon. Next: lvalues and rvalues, the distinction that determines what can appear on the left side of an assignment.