Objective-C問題：EXC_BAD_ACCESS代碼= 2

Question

我正在嘗試編寫一個計算器程序，但是在按下鏈接到函數expressionEvaluation的特定鍵時遇到了此錯誤。 我的計算器工作正常，但是我試圖增加使用/存儲變量的能力。 為此，我使用expressionEvaluation按鈕。 但是，一旦我按下鏈接到該方法的按鈕，整個應用程序就會崩潰，並且xcode給我錯誤EXC_BAD_ACCESS code = 2。 我是Objective-c的新手，所以不確定如何調試此錯誤。 據我所知，問題在於生產線

double result = [CalculatorBrain evaluateExpression:self.brain.expression usingVariableValues:testValues];

...在我的視圖控制器中 我在此行之前設置了一個斷點，但它沒有崩潰，但是在此行上設置它導致了我遇到的崩潰。 我試圖將代碼縮減為以下內容，僅包含直接鏈接到expressionEvaluation按鈕的內容。 謝謝您的幫助！

這是我的大腦方法的.h：

//  CalculatorBrain.h

#import <Foundation/Foundation.h>

@interface CalculatorBrain : NSObject
- (void) pushOperand:(double)operand;
- (void) setVariableAsOperand:(NSString *)variableName;
- (void) performWaitingOperation;
- (double) performOperation:(NSString *)operation;

@property (readonly) id expression;
+ (double)evaluateExpression: (id)anExpression
         usingVariableValues: (NSDictionary *)variables;
+ (NSSet *)variablesInExpression:(id)anExpression;
+ (NSString *)descriptionOfExpression:(id)anExpression;

+ (id)propertyListForExpression:(id)anExpression;
+ (id)expressionForPropertyList:(id)propertyList;

@end

這是.m：

//  CalculatorBrain.m

#import "CalculatorBrain.h"
#define VARIABLE_PREFIX @"^"

@interface CalculatorBrain()
@property (nonatomic) double operand;
@property (nonatomic, strong) NSString *waitingOperation;
@property (nonatomic) double waitingOperand;
@property (nonatomic) double storedOperand;
@property (nonatomic) NSMutableArray *internalExpression;
@end

@implementation CalculatorBrain
@synthesize operand = _operand;                     //operand in use
@synthesize waitingOperation = _waitingOperation;   //waiting to be computed
@synthesize waitingOperand = _waitingOperand;       //waiting to be used
@synthesize storedOperand = _storedOperand;         //stored in memory
@synthesize internalExpression = _internalExpression; //private expression stored
@synthesize expression = _expression;

- (id) expression {;
    return [NSMutableArray arrayWithArray:self.internalExpression];
}

//here I have instance methods that add objects to the array internalExpression
//these work fine as far as I can tell

+ (double) evaluateExpression:(id)anExpression usingVariableValues:(NSDictionary *)variables {
    double result = 0;
    int count = [anExpression count];
    CalculatorBrain *brain;
    for (int i = 0; i < count; i++) {
        if([[anExpression objectAtIndex:i] isKindOfClass:[NSNumber class]]) {
            double operand = [[anExpression objectAtIndex:i] doubleValue];
            [brain pushOperand:operand];
        }
        if([[anExpression objectAtIndex:i] isKindOfClass:[NSString class]]) {
            NSString *string = [anExpression objectAtIndex:i];
            if([string characterAtIndex:0] == '^') {
                NSString* variable = [NSString stringWithFormat:@"%c",[string characterAtIndex:1]];
                double valueOfVariable = [[variables objectForKey:variable] doubleValue];
                [brain pushOperand:valueOfVariable];

            }
            else {
                NSString* operator = [anExpression objectAtIndex:i];
                result = [brain performOperation:operator];
            }
        }
    }
    return result;
}

@end

這是我的視圖控制器的.h：

//  CalcViewController.h

#import <UIKit/UIKit.h>
//view controller for calculator
@interface CalcViewController : UIViewController
@property (weak, nonatomic) IBOutlet UILabel *display;          //real-time display
@property (strong, nonatomic) IBOutlet UILabel *miniDisplay;    //past display
@property (strong, nonatomic) IBOutlet UILabel *memDisplay;     //display stored digits
- (IBAction)digitPressed:(UIButton *)sender;    //pressed number
- (IBAction)operandPressed:(UIButton *)sender;  //pressed operation
- (IBAction)variablePressed:(UIButton *)sender; //pressed variable
- (IBAction)expressionEvaluation:(UIButton *)sender; //evaluate expression with variables
@end

這是.m：

//  CalcViewController.m

#import "CalcViewController.h"
#import "CalculatorBrain.h"

@interface CalcViewController ()
@property (nonatomic, strong) CalculatorBrain *brain;
@property (nonatomic) BOOL userIsInTheMiddleOfTypingANumber;
@end

@implementation CalcViewController
- (CalculatorBrain *) brain {
    if (!_brain) _brain = [[CalculatorBrain alloc] init];
    return _brain;
}

- (IBAction)expressionEvaluation:(UIButton *)sender {
    NSDictionary* testValues = [NSDictionary dictionaryWithObjectsAndKeys:@"x", 2, @"y", 3, @"z", 4, nil];
    double result = [CalculatorBrain evaluateExpression:self.brain.expression usingVariableValues:testValues];
    NSString *resultString = [NSString stringWithFormat:@"%g", result];
    self.display.text = resultString;
}


@end

編輯：作為一個旁注，我沒有收到任何錯誤，只有一個警告：不完整的實現。 這是因為我還沒有完成幾個類方法，但是我不認為這會導致崩潰。

Answer 1

NSDictionary* testValues = [NSDictionary dictionaryWithObjectsAndKeys:@"x", 2, @"y", 3, @"z", 4, nil];

原來是錯誤的路線。 應該是：

NSDictionary* testValues = [NSDictionary dictionaryWithObjectsAndKeys:@"x", [NSNumber numberWithInt:2], @"y", [NSNumber numberWithInt:3], @"z", [NSNumber numberWithInt:4], nil];

感謝所有提供幫助的人！

Answer 2

通過在xCode中添加異常斷點，您將獲得更多有關引發異常的代碼行的信息。