Using macro results in incorrect output when used as part of a larger math expression - why does this happen?

Question

This is a normal C routine program which i found out in some question bank. It is shown below:

#define CUBE(p) p*p*p

main()
{
    int k;
    k = 27 / CUBE(3         


        

Answer 1

Because macros are a textual substitution, that works out to:

k = 27 / 3 * 3 * 3;

Since multiplication and division happen left to right, that works out to:

k = ((27 / 3) * 3) * 3;

So, you want to change that in two ways:

#define CUBE(p) ((p)*(p)*(p))

The outer parentheses cause the multiplications to be done before any other operations.

The parentheses around the individual p's are for the case where you do:

CUBE(1 + 2);

Without those inner parentheses, operator precedence will trip you up.