If then statement in Macro

I am trying to write a macro that returns true if its parameter is divisible by ten. False if otherwise.

This is for an "On your own learning" with a book and this problem is stumping me. it complains" expected primary expression before else"

I appreciate any help

Thank you

Code:

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#include <stdio.h>#include <string.h>
#include <stdlib.h>


/* Macro funcions*/
#define modulus(x) {if(x%10 == 0){printf("true\n"); else printf("false\n");}}
                 
           
                 


/* Declare variables*/
float centi;
float fahr;
char line[100];


int main()
{ 
    int count = 12;
    modulus(count);
   
   
   
   
   
    system("pause");                    /* the fix for the collapsing window*/
    return (0);
}

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Quote:

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Originally Posted by psppb

I am trying to write a macro that returns true if its parameter is divisible by ten. False if otherwise.

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Macros don't actually return anything. They just get replaced by some code.

So, let's examine the code:

Code:

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#define modulus(x) {if(x%10 == 0){printf("true\n"); else printf("false\n");}}

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If we expand the replacement code out nicely:

Code:

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{
    if(x%10 == 0){
        printf("true\n");
        else printf("false\n");
    }
}

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Now, it is clear that the else does not match the if because of the misplaced braces. What you should do is either write out the replacement code first and then compress it into one line, or use backslashes at the end of the line as line continuations so you can write:

Code:

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#define modulus(x) do { \
    if (x % 10 == 0) { \
        printf("true\n"); \
    } else { \
        printf("false\n"); \
    } \
} while (0)

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I've used a do while(0) so that a missing terminating semi-colon would result in an error, whereas just using braces for a code block will not, which can look weird sometimes.

May I suggest also choosing a name for the macro that is more closely related to what it does? Prefarably uppercase too.
For example DIVISIBLE_BY_10 would work.

I also suspect that they are looking for something much simpler than what you have, that does not actually involve printing anything, just giving the result. I.e. something that allows this code to work:

Code:

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if (DIVISIBLE_BY_10(count))
    printf("true\n");
else
    printf("false\n");

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Thank you both! Awesome