iOS 练习题报错：EXC_BAD_ACCESS?报错

一个分数相加的列子，代码如下：

//

//  Fraction.h

//  FractionTest

//

//  Created by Leven on 15-5-7.

//  Copyright (c) 2015年 Leven. All rights reserved.

//

#import <Foundation/Foundation.h>

//Fraction类

@interface Fraction : NSObject

@property int numerator, denominator;

-(void) print: (BOOL) b;

-(double) convertToNum;

-(void) setTo: (int) n over: (int) d;

-(Fraction *) reduce;

-(Fraction *) add: (Fraction *) f;

@end

//

//  Fraction.m

//  FractionTest

//

//  Created by Leven on 15-5-7.

//  Copyright (c) 2015年 Leven. All rights reserved.

//

#import "Fraction.h"

@implementation Fraction

@synthesize numerator, denominator;

-(void) print: (BOOL) b

{

    if (b == YES) {

        [[self reduce] print: YES];

    } else {

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

    }

}

-(double) convertToNum

{

    if (denominator != 0) {

        return (double) numerator / denominator;

    } else {

        return NAN;

    }

}

-(void) setTo:(int)n over:(int)d

{

    numerator = n;

    denominator = d;

}

-(Fraction *) reduce

{

    Fraction *result = [Fraction new];

    int u = numerator;

    int v = denominator;

    int temp;

    while (v != 0) {

        temp = u % v;

        u = v;

        v = temp;

    }

    int n = numerator;

    int d = denominator;

    n /= u;

    d /= u;

    [result setTo: n over: d];

    return result;

}

-(Fraction *) add:(Fraction *)f

{

    Fraction *result = [Fraction new];

    //相加两个分数

    //a/b + c/d = (a * d + b * c) / (b * d)

    result.numerator = numerator * f.denominator + denominator * f.numerator;

    result.denominator = denominator * f.denominator;

    return result;

}

@end

//

//  main.m

//  FractionTest

//

//  Created by Leven on 15-5-7.

//  Copyright (c) 2015年 Leven. All rights reserved.

//

#import <Foundation/Foundation.h>

#import "Fraction.h"

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

    @autoreleasepool {        

        Fraction *aFraction = [[Fraction alloc] init];

        Fraction *bFraction = [[Fraction alloc] init];

        Fraction *resultFraction;

        [aFraction setTo: 1 over: 4];

        [bFraction setTo: 1 over: 2];

        //打印结果

        [aFraction print: NO];

        NSLog(@"+");

        [bFraction print: NO];

        NSLog(@"=");

        resultFraction= [aFraction add: bFraction];

        [resultFraction print: YES];

        [resultFraction print: NO];

    }

    return 0;

}

main测试的时候报错了，在reduce方法的第一行：