C宏，带有表达式不必要的结果

Question

I am running the following program and getting a result as 9 7, I understood why 9 is the output but I can't figure out why I'm getting 7 as output.

#include<stdio.h>
#define sqr(i) (i*i)

int main()
{
    printf("%d %d", sqr(3), sqr(3+1));
    return 0;
}

For the second function that is sqrt(3+1) how the micro is getting expanded and how Im getting 7 output?

Answer 1

You can have the compiler or IDE preprocess the file and show you how the macro expanded.

In your case sqr(3+1) expands to (3+1*3+1) . Now the precedence of C operators means that the multiplication is done before the addition. So (3+1*3+1) -> (3+3+1) -> (7) .

You can fix this by defining your macro this way, with parentheses around the argument:

#define sqr(i) ((i)*(i))