Why printf inside if condition works in c?

Question

if(printf("Hello world")){}

The output for above code is

Hello world

My friend told me that the function printf returns the length of characters which is non zero here, so the condition is true.

But I can't understand why it executes that printf statement. Shouldn't printf be executed only when it is inside { }?

Answer 1

The expression within if(expression) is always evaluated, and in your case that's a call to printf.

The value of this expression is used to determine if the body (blank in your case) of the if is run.