Calculator App - strange error: 'unrecognized selector sent to instance' when trying to update display - objective-c

Question

I'm relatively new to Objective-C, so I'm not 100% about everything I'm coding. However, I'm tackling my errors as they happen, and I am getting a run-time error that I'm not sure how to fix. The error claims that an 'unrecognized selector [was] sent to instance.'
- (IBAction) equalsPressed{
self.userIsInTheMiddleOfTypingANumber = NO;
if (self.brain.operationIsPicked) {
[self.brain pushOperand: [self.display.text doubleValue]];
double result = [self.brain performOperation: self.brain.operation];
// The line below this
self.display.text = [NSString stringWithFormat: #"%g", result];
}
}
I'm doing the Calculator app taught by the Stanford professor whose course is posted in iTunes U. However, I edited it to not include an enterPressed command and instead use an altered and more user-friendly 'equalsPressed.' After equalsPressed is finished (meaning when I click the equals sign on the calculator), the error is displayed. Might anyone know what is causing this problem? I already found someone else who has an error very similar to this one, but occurring in a slightly different place. Here are the other code snippets that could help elucidate the problem.
#import "CalculatorBrain.h"
#interface CalculatorBrain()
#property (nonatomic, strong) NSMutableArray *operandStack;
#end
#implementation CalculatorBrain
#synthesize operandStack = _operandStack;
#synthesize operandStackIsEmpty = _operandStackIsEmpty;
#synthesize operationIsPicked = _operationIsPicked;
#synthesize operation = _operation;
- (NSMutableArray *) operandStack {
if (!_operandStack) {
_operandStack =[[NSMutableArray alloc] init];
_operandStackIsEmpty = YES;
_operationIsPicked = NO;
}
return _operandStack;
}
- (void) resetStack {
[self.operandStack removeAllObjects];
self.operandStackIsEmpty = YES;
self.operationIsPicked = NO;
}
- (void) pushOperand : (double) operand{
[self.operandStack addObject: [NSNumber numberWithDouble:operand]];
self.operandStackIsEmpty = NO;
}
- (double) popOperand {
NSNumber *num = [self.operandStack lastObject];
if (num) {[self.operandStack removeLastObject];}
return [num doubleValue];
}
- (double) performOperation : (NSString *) operation{
double result = 0;
double num2 = [self popOperand];
double num1 = [self popOperand];
if ([operation isEqualToString:#"+"])
result = num2 + num1;
else if ([operation isEqualToString:#"-"])
result = num2 - num1;
else if ([operation isEqualToString:#"*"] || [operation isEqualToString:#"x"])
result = num2 * num1;
else if ([operation isEqualToString:#"/"]){
if (num2 == 0)
[self resetStack];
else
result = num2 / num1;
}
[self pushOperand:result];
self.operationIsPicked = NO;
return result;
}
#end
Also, the debugging NSLog that I had print the display in the console works correctly, BUT the display in the Calculator view doesn't actually update (when I used breakpoints to slow the function down).
#import "CalculatorViewController.h"
#import "CalculatorBrain.h"
#interface CalculatorViewController ()
#property (nonatomic) BOOL userIsInTheMiddleOfTypingANumber;
#property (nonatomic, strong) CalculatorBrain *brain;
#end
#implementation CalculatorViewController
#synthesize userIsInTheMiddleOfTypingANumber = _userIsInTheMiddleOfTypingANumber;
#synthesize brain = _brain;
#synthesize display = _display;
- (CalculatorBrain *) brain {
if (!_brain) {
_brain =[[CalculatorBrain alloc] init];
}
return _brain;
}
- (IBAction) digitPressed: (UIButton *)sender {
NSString *digit = [sender currentTitle];
if (self.userIsInTheMiddleOfTypingANumber)
self.display.text = [self.display.text stringByAppendingString: digit];
else {
self.display.text = digit;
self.userIsInTheMiddleOfTypingANumber = YES;
}
}
- (IBAction) clearPressed {
self.userIsInTheMiddleOfTypingANumber = NO;
self.display.text = #"0";
[self.brain resetStack];
}
- (IBAction) equalsPressed {
self.userIsInTheMiddleOfTypingANumber = NO;
if (self.brain.operationIsPicked) {
[self.brain pushOperand: [self.display.text doubleValue]];
double result = [self.brain performOperation: self.brain.operation];
self.display.text = [NSString stringWithFormat: #"%g", result];
NSLog(#"%#",self.display.text);
}
NSLog(#"%#",self.display.text);
}
- (IBAction) operationPressed:(UIButton *) sender {
if (self.userIsInTheMiddleOfTypingANumber) {
[self.brain pushOperand: [self.display.text doubleValue]];
self.brain.operation = [sender currentTitle];
self.brain.operationIsPicked = YES;
self.userIsInTheMiddleOfTypingANumber = NO;
NSLog(#"%#", sender.currentTitle);
}
}
Thank you!

Answer 1

To quote the answer in the comments:
"The full error said that I sent an invalid argument to "[self.brain equalsPressed:]". In reality, equalsPressed takes no argument, but when I was linking the '=' button in my view to its respective method, I forgot to indicate that the function took no arguments. Hence even though the function as I wrote it has no arguments, I told the compiler (to begin with) that it was supposed to take an argument."