indira indira - 1 month ago 18
C Question

comma operator in if condition

int a = 1, b = 0;

if(a, b)
printf("success\n");
else
printf("fail\n");

if(b, a)
printf("success\n");
else
printf("fail");


This is a cpp file and I got the output in Visual Studio 2010 as

fail
success


Why this behavior? Could you please explain?

Answer

http://en.wikipedia.org/wiki/Comma_operator:

In the C and C++ programming languages, the comma operator (represented by the token ,) is a binary operator that evaluates its first operand and discards the result, and then evaluates the second operand and returns this value (and type).

In your first if:

if (a, b)

a is evaluated first and discarded, b is evaluated second and returned as 0. So this condition is false.

In your second if:

if (b, a)

b is evaluated first and discarded, a is evaluated second and returned as 1. So this condition is true.

If there are more than two operands, the last expression will be returned.

If you want both conditions to be true, you should use the && operator:

if (a && b)