Two expressions separated by a comma are evaluated left to right.

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In the following example, the order of the loop's initializers is significant: This is the only idiomatic use in elementary C programming. The most common use is to allow multiple assignment statements without using a block statement, primarily in the initialization and the increment expressions of a for loop. For embedded systems which may have limited debugging capabilities, the comma operator can be used in combination with a macro to seamlessly override a function call, to insert code just before the function call. Nevertheless, there are certain circumstances where it is commonly used, notably in for loops and in SFINAE. Further, because it is rarely used outside of specific idioms, and easily mistaken with other commas or the semicolon, it is potentially confusing and error-prone. Because it discards its first operand, it is generally only useful where the first operand has desirable side effects that must be sequenced before the second operand. The comma operator has relatively limited use cases. * Return simply gets an expression and here the expression is '(1), 2, 3'. Even though compilers will * allow for the construct return(value), the parentheses are only relative to 'value' * and have no special effect on the return keyword. This example works as it does * because return is a keyword, not a function call. */ return 1, 2, 3 /** * Returns 3, not 1, still for same reason as above. */ return a = 4, b = 5, c = 6 /** * Returns 3, not 1, for same reason as previous example. * Results: a=1, b=2, c=3, i=3 */ int a = 1, b = 2, c = 3 int i = ( a, b, c ) /** * Returns 6, not 4, since comma operator sequence points following the keyword * return are considered a single expression evaluating to rvalue of final * subexpression c=6. * Assigns value of c into i, discarding the unused a and b values. The comma operator separates expressions (which have value) in a way analogous to how the semicolon terminates statements, and sequences of expressions are enclosed in parentheses analogously to how sequences of statements are enclosed in braces: (a, b, c) is a sequence of expressions, separated by commas, which evaluates to the last expression c while /** * Commas act as separators in the first line and as an operator in the second line.

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