While implementing a #define a bug emerged that I cannot find

Hey Guys,
I'm learning about marcros. Specifically #define with parameters. I cannot seem to get my loop to work using the code provided by the book. The Goal: print ABCD from abcd but, I'm supposed to use this code:

Code:

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putchar(TOUPPER(s[++i]));

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.

Here's the complete program:

Code:

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//ANSWER: http://knking.com/books/c2/answers/c14.html
// Problem 5
#include <stdio.h>
#include <string.h>


#define TOUPPER(c) ('a'<=(c)&&(c)<='z'?(c)-'a'+'A':(c))


int main(){
    char s[6];
    strcpy(s, "abcd");
    int len=strlen(s), i;
    for(i=0;i<len;){          // DOESN'T WORK, PRINTS " D"         
        printf(" ");
        putchar(TOUPPER(s[++i]));
    }
    /*for(i=0;i<len;i++){    // THIS CODE WORKS
        printf("%c, ",TOUPPER(s[i]));
    }*/
    return 0;
}

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Somehow the looping syntax is flawed. Please help.

> #define TOUPPER(c) ('a'<=(c)&&(c)<='z'?(c)-'a'+'A':(c))
Look what happens when you use an expression with a side effect.

putchar(TOUPPER(s[++i]));
you get
putchar( ('a'<=(s[++i]))&&(s[++i]))<='z'?(s[++i]))-'a'+'A':(s[++i]))) );

You end up with rather more ++i's than you bargained for.

It's one of the hazards of using macros.

Oh, and if you're ever confused about what a macro is doing, gcc -E is your friend.

Code:

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$ gcc -E foo.c | tail
    int len=strlen(s), i;
    for(i=0;i<len;){
        printf(" ");
        putchar(('a'<=(s[++i])&&(s[++i])<='z'?(s[++i])-'a'+'A':(s[++i])));
    }
   


    return 0;
}

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It shows you what your code looks like after all the pre-processing has happened.

Ah, gcc -E I should have tried that as it made the problem obvious. Thank you very much. I just setup my environment to allow me to use this going forward.