CODEWITHSUNDEEP
objective-cvariable-assignmentinstance-variables
Manu
Manu

Reputation: 37

Setting member variable of one instance overwrites another instance's

I have a question about the following code. The output is wrong: frac1 should be printing out "2/3", but instead its being overwritten to "3/7". I can't seem to figure out why its being overwritten.

#import <Foundation/Foundation.h>

// ---- INTERFACE -----
@interface Fraction : NSObject
-(void) print;
-(void) setNumerator: (int) n;
-(void) setDenominator: (int) d;
@end


//------implementation section ------
@implementation Fraction
int numerator;
int denominator;


-(void) print{

    NSLog(@"%i/%i", numerator,denominator);
}


-(void) setNumerator:(int)n{

    numerator = n;
}

-(void) setDenominator:(int)d{

    denominator = d;
}

@end


// ---------- EXECUTION--------
int main (int argc, char * argv[])
{
@autoreleasepool {
    Fraction * frac1 = [[Fraction alloc]init];
    Fraction * frac2 = [[Fraction alloc]init];

    [frac1 setNumerator:2];
    [frac1 setDenominator:3];

    // set second fraction
   [frac2 setNumerator:3];
   [frac2 setDenominator:7];


    NSLog(@"this is the value of frac1:");
    [frac1 print];


    NSLog(@"this is the value of frac2:");
    [frac2 print];


}
    return 0;
}

Upvotes: 1

Views: 63

Answers (2)

zaph
zaph

Reputation: 112857

The problem is that numerator and denominator are being set as global variables, not instance variables. To make then instance variables enclose them in {} as part of the @implementation definition.

Fixed:

@implementation Fraction {
int numerator;
int denominator;
}

Output:

this is the value of frac1:
2/3
this is the value of frac2:
3/7

Upvotes: 3

Sinisa Drpa
Sinisa Drpa

Reputation: 917

Use this code:

@interface Fraction : NSObject
@property (assign, nonatomic) int numerator;
@property (assign, nonatomic) int denominator;
- (void)print;
@end

@implementation Fraction

- (void)setNumerator:(int)numerator {
   if (_numerator != numerator) {
      _numerator = numerator;
   }
}

- (void)setDenominator:(int)denominator {
   if (_denominator != denominator) {
      _denominator = denominator;
   }
}

-(void) print{
   NSLog(@"%i/%i", _numerator, _denominator);
}

@end

// OUTPUT

2015-01-18 18:12:58.844 fraction[29486:2733137] this is the value of frac1:
2015-01-18 18:12:58.845 fraction[29486:2733137] 2/3
2015-01-18 18:12:58.846 fraction[29486:2733137] this is the value of frac2:
2015-01-18 18:12:58.846 fraction[29486:2733137] 3/7

In your case int numerator; int denominator; are global vars, not object properties.

Upvotes: 2

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