Question

I get this code from a book but i could not get it work. What is my problem here? As the title says

“No previous prototype for function BOOL areIntsDifferent (int thing1, int thing2 ) and NSString * boolString (BOOL yesNo)”

#import <Foundation/Foundation.h>

BOOL areIntsDifferent (int thing1, int thing2 ) {
    if ( thing1 == thing2) {
        return (NO);
    }
    else {
        return (YES);
    }
}    

NSString * boolString (BOOL yesNo) {
    if (yesNo== NO) {
        return (@"NO");
    }
    else {
        return (@"YES");
    }    
} 

int main (int argc, const char * argv[]) {
    BOOL aretheydiffrent;
    aretheydiffrent = areIntsDifferent (5,5);
    NSLog(@"are %d and %d diffrent? %@", 5, 5, boolString(aretheydiffrent));
    aretheydiffrent = areIntsDifferent(23,42);
    NSLog(@"are %d and %d diffrent? %@", 23, 42, boolString(aretheydiffrent));
    return 0;
}

Answer 1

The error message translates to:

This means that GCC found a global function definition without seeing
a prototype for the function. If a function is used in more than one
file, there should be a prototype for it in a header file somewhere.
This keeps functions and their uses from getting out of sync

Making the functions static should sort this out:

#import < Foundation/Foundation.h >

static BOOL areIntsDifferent (int thing1, int thing2 )
{
     return thing1 != thing2;
}

static NSString * boolString (BOOL yesNo)
{
    return yesNo ? @"YES" : @"NO";

}

int main (int argc, const char * argv[])

{
   BOOL aretheydiffrent;

   aretheydiffrent = areIntsDifferent (5,5);

   NSLog(@"are %d and %d diffrent? %@",
      5, 5, boolString(aretheydiffrent));

   aretheydiffrent = areIntsDifferent(23,42);

   NSLog(@"are %d and %d diffrent? %@",
      23, 42, boolString(aretheydiffrent));

   return 0;
}