Adding items to NSMutableArray doesn't work

Question

I'm taking the lectures of Stanford's iPhone application development and the first assignment is to build a RPN calculator.

I'm having an issue that is probably something simple that I'm missing out.

There is a model class, called CalculatorBrain:

.h

#import <Foundation/Foundation.h>

@interface CalculatorBrain : NSObject

- (void)pushOperand:(double)operand;
- (double)performOperation:(NSString *)operation;

@property (nonatomic, strong) NSMutableArray *operandStack;

@end

.m

#import "CalculatorBrain.h"

@interface CalculatorBrain()

@end

@implementation CalculatorBrain

@synthesize operandStack = _operandStack;

- (NSMutableArray *)operandStack
{
    if (!_operandStack) {
        _operandStack = [[NSMutableArray alloc] init];
    }

    return _operandStack;
}

- (void)pushOperand:(double)operand
{
    NSNumber *operandObject = [NSNumber numberWithDouble:operand];
    [self.operandStack addObject:operandObject];
}

- (double)popOperand
{
    NSNumber *operandObject = [self.operandStack lastObject];
    if (operandObject) [self.operandStack removeLastObject];
    return [operandObject doubleValue];
}

@end

And there's the controller class, CalculatorViewController: .h

#import <UIKit/UIKit.h>

@interface CalculatorViewController : UIViewController
@property (weak, nonatomic) IBOutlet UILabel *display;

@end

.m

#import "CalculatorViewController.h"
#import "CalculatorBrain.h"

@interface CalculatorViewController ()
@property (nonatomic) BOOL userIsInTheMiddleOfEnteringANumber;
@property (nonatomic, strong) CalculatorBrain *brain;
@end

@implementation CalculatorViewController

- (IBAction)digitPressed:(UIButton *)sender {
    NSString *digit = [sender currentTitle];
    NSString *currentDisplayText = self.display.text;

    if (self.userIsInTheMiddleOfEnteringANumber) {
        self.display.text = [currentDisplayText stringByAppendingString:digit];
    }
    else {
        self.display.text = digit;
        self.userIsInTheMiddleOfEnteringANumber = YES;
    }
}

- (IBAction)enterPressed {
    [self.brain pushOperand:[self.display.text doubleValue]];

    NSLog(@"count: %lu", (unsigned long) [self.brain.operandStack count]);
    NSLog(@"index 0: %@", [self.brain.operandStack objectAtIndex:0]);
    self.userIsInTheMiddleOfEnteringANumber = NO;
}

@end

The problem is in the enterPressed method. As you can see, I put 2 NSLog lines to see if the numbers are being pushed into the array, but they're not. The two lines return me 0 and null. I know I don't need to use @synthesize anymore(in the CalculatorBrain class) but I've tried with and without it, and the items are not being added anyway.

Can anybody help me?

Thank you.

The lazy creation of the NSMutableArray in that getter is quite non-standard. Better to create it in your initializer, which also serves to eliminate the custom getter/setter entirely. — bbum, Jun 05 '13 at 19:23

score 3 · Accepted Answer · edited Jun 05 '13 at 18:08

3

@synthesize only creates the setters and getters for the property. They are default in modern Objective-C. But @syntehsize doesn't initialize the object and assign it by itself. So you still have to initialize brain. The default value is nil. So unless you assign a new object to it, it will stay nil. That's why you were getting these values.

You can do that in - viewDidLoad

- (void) viewDidLoad {
    self.brain = [[CalculatorBrain alloc] init];
}

edited Jun 05 '13 at 18:08

pasawaya

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answered Jun 05 '13 at 17:54

Mohannad A. Hassan

1,650
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1

The initialization was in the walkthrough document for assignment 1. Page 199. They have you lazy initializing. – stephen.vakil Jun 05 '13 at 18:15
Woops, guess I missed that for some reason, thanks (: – gabsferreira Jun 05 '13 at 18:46

score 2 · Answer 2 · answered Jun 05 '13 at 17:53

2

Almost for sure self.brain is nil - try logging it too.

answered Jun 05 '13 at 17:53

David H

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Adding items to NSMutableArray doesn't work

2 Answers2