Simple calculator app crashes when a third number key is punched - iphone

Question

I am a newbie to the iphone app world. So I thought I try my luck with a calculator app.
Unfortunately I am running into an issue where if I press a third key in the calculator the app crashes. Sometimes I get this error EXC_BAD_ACCESS. Here is a code in my CalculatorViewController.m file.
#import "CalculatorViewController.h"
#implementation CalculatorViewController
#synthesize screenText;
- (IBAction)buttonPressed:(id)sender {
NSString *title = [sender titleForState:UIControlStateNormal];
[self collect:title];
}
- (void)collect:(NSString *)digitz {
NSString * newText = nil;
if ([digitz isEqualToString:#"+"]) {
[self add:result];
big_digit = nil;
}
else if ([digitz isEqualToString:#"+"]) {
[self sub:result];
}
else if ([digitz isEqualToString:#"x"]) {
[self multiply:result];
}
else if ([digitz isEqualToString:#"="]) {
[self equate:result];
}
else {
if (big_digit != nil && [big_digit isEqualToString:#"0"] == FALSE)
big_digit = [big_digit stringByAppendingFormat:#"%#",digitz];
else
big_digit = (NSMutableString *) digitz;
result = (int) big_digit;
newText = [[NSString alloc] initWithFormat:
#"%#",big_digit];
}
screenText.text = newText;
[newText release];
}
- (void)add:(int)res {
NSString * newText = nil;
ans = ans + res;
newText = [[NSString alloc] initWithFormat:
#"%#",ans];
screenText.text = newText;
[newText release];
}
Can anyone spot an obvious issue here. Here is the respective header file too.
#import <UIKit/UIKit.h>
#interface CalculatorViewController : UIViewController {
UILabel *screenText;
int number;
int result;
int ans;
//NSString *big_digit;
NSMutableString * big_digit ;
}
#property (nonatomic, retain) IBOutlet UILabel *screenText;
- (IBAction)buttonPressed:(id)sender;
- (void)collect:(NSString *)digitz;
- (void)add:(int)num;
- (void)sub:(int)num;
- (void)multiply:(int)num;
- (void)equate:(int)num;
#end

Answer 1

Well, you probably don't want to just cast a string to an integer (ala (int)big_digit). Instead you want to use [big_digit integerValue];
I think what is happening is that your big_digit property is not retained. In this line, you just assign a string to it that is autoreleased:
big_digit = [big_digit stringByAppendingFormat:#"%#",digitz];
On the next pass through, big_digit is != nil, but [big_digit isEqualToString:#"0"] == FALSE fails because big_digit now points to an invalid memory location.
What you want to do is make big_digit a property in your interface, like so...
#property (nonatomic, retain) NSMutableString *big_digit;
I know reading docs sucks, but looking at your code I think you would really find reading through this useful. Memory management in objective c is quite a bit different from regular old C.
http://developer.apple.com/iphone/library/documentation/cocoa/conceptual/memorymgmt/Articles/mmPractical.html#//apple_ref/doc/uid/TP40004447-SW1

Answer 2

In your format strings, you're doing this:
newText = [[NSString alloc] initWithFormat:#"%#", ans];
But according to your #interface, ans is an integer. So that line should read:
newText = [[NSString alloc] initWithFormat:#"%d", ans];
since %d is the format specifier for an integer.