c - Strange behavior of Macro-expansion

Question

Here's the code:

#include <stdio.h>
#include <stdio.h>
#define VAL1(a,b)    a*b
#define VAL2(a,b)    a/b
#define VAL3(a,b)    ++a%b
int main()
{
    int a = 1;
    int b = 2;
    int c = 3;
    int d = 3;
    int e = 5;

    int result = VAL2(a,d)/VAL1(e,b)+VAL3(c,d);  // result = 1
    //int result = a/d/e*b+++c%d;                // result = 0

    printf("%d
", result);

    return 0;
}

Why aren't the results of two statements the same?

Answer 1

In one case you have + ++ and in the other case you have ++ +. + ++ and ++ + are different streams of tokens. Macro pasting doesn't change tokenization because it's tokens that are pasted.

If you punch your program into a C pre-processor, you'll get this out for that line:

int result = a/d/e*b+ ++c%d;

Notice that the preprocessor had to insert a space because one is mandatory between a + token and a ++ token.