Calculator app won't add operand to array - ios

Question

I am trying to make a calculator app, but when I press enter nothing is pushed into the array. I have a class called CaculatorBrain where the pushElement method is defined, however (for now) I defined and implemented pushElement method in the view controller.
When I log the operand object as it is typed in the console when enter is pressed the contents of array is nil! Why is that?
#import "CalculatorViewController.h"
#import "CalculatorBrain.h"
#interface CalculatorViewController ()
#property (nonatomic)BOOL userIntheMiddleOfEnteringText;
#property(nonatomic,copy) NSMutableArray* operandStack;
#end
#implementation CalculatorViewController
BOOL userIntheMiddleOfEnteringText;
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
}
-(NSMutableArray*) operandStack {
if (_operandStack==nil) {
_operandStack=[[NSMutableArray alloc]init];
}
return _operandStack;
}
-(CalculatorBrain*)Brain
{
if (!_Brain) _Brain= [[CalculatorBrain alloc]init];
return _Brain;
}
- (IBAction)digitPressed:(UIButton*)sender {
if (self.userIntheMiddleOfEnteringText) {
NSString *digit= [sender currentTitle];
NSString *currentDisplayText=self.display.text;
NSString *newDisplayText= [currentDisplayText stringByAppendingString:digit];
self.display.text=newDisplayText;
NSLog(#"IAm in digitPressed method");
}
else
{
NSString *digit=[sender currentTitle];
self.display.text = digit;
self. userIntheMiddleOfEnteringText=YES;
}
}
-(void)pushElement:(double)operand {
NSNumber *operandObject=[NSNumber numberWithDouble:operand];
[_operandStack addObject:operandObject];
NSLog(#"operandObject is %#",operandObject);
NSLog(#"array contents is %#",_operandStack);
}
- (IBAction)enterPressed {
[self pushElement: [self.display.text doubleValue] ];
NSLog(#"the contents of array is %#",_operandStack);
userIntheMiddleOfEnteringText= NO;
}

Answer 1

It looks like the operand stack is never initialized.
When you directly access _operandStack you don't go through -(NSMutableArray*) operandStack, which is the only place where the operand stack is allocated and initialized. If the array isn't allocated you can't put anything in it, which is why it logs the contents as nil.
I'd recommend using either self.operandStack (which uses the method that checks if _operandStack is nil) everywhere except inside the -(NSMutableArray*) operandStack method, or allocating the operand stack in your viewDidLoad.