Objective-C麻烦：EXC_BAD_ACCESS code = 2

Question

我试图编写一个计算器程序，但是当按下与函数expressionEvaluation关联的特定键时，我遇到了这个错误。我的计算器工作正常，但我试图添加使用/存储变量的功能。为此，我使用expressionEvaluation按钮。但是，只要我按下链接到该方法的按钮，整个应用程序崩溃，xcode给我错误EXC_BAD_ACCESS code = 2。我是新来的Objective-C，所以我不知道如何调试这个错误。据我所知，问题是与行

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

...在我的视图控制器。我在这条线之前设置了一个断点，但它没有崩溃，但将它设置在这条线上导致了我遇到的崩溃。 我试图在下面切下代码，并且只包含直接链接到expressionEvaluation按钮的东西。感谢您的帮助！

下面是我的大脑方法.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 

这里的.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 

下面是我的看法控制器.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 

这里的.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 

编辑：就像一个侧面说明，我没有得到任何错误，只有一个警告：不完整的实现。这是因为我还没有完成一对夫妻班的方法，但我不认为这会导致车祸。

Answer 1

通过在xCode中添加异常断点，您将获得有关引发异常的代码行的更多信息。

Answer 2

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

原来是错误的一行。它应该是：

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

感谢所有帮助过的人！