我正在尝试编写一个计算器程序，但是在按下与函数 expressionEvaluation 链接的特定键时遇到此错误。我的计算器工作正常，但我正在尝试添加使用/存储变量的能力。为此，我使用了 expressionEvaluation 按钮。但是，一旦我按下链接到该方法的按钮，整个应用程序就会崩溃，并且 xcode 会给我错误 EXC_BAD_ACCESS code=2。我是objective-c的新手，所以我不确定如何调试这个错误。不过，据我所知，问题出在这条线上
double result = [CalculatorBrain evaluateExpression:self.brain.expression usingVariableValues:testValues];
...在我的视图控制器中。我在这条线之前设置了一个断点并且它没有崩溃，但是在这条线设置它导致了我正在经历的崩溃。我试图将代码删减到下面，并且只包含直接链接到 expressionEvaluation 按钮的内容。谢谢您的帮助！
这是我的大脑方法的.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
这是.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
这是我的视图控制器的 .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
这是.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
编辑：顺便说一句，我没有收到任何错误，只有一个警告：不完整的实现。这是因为我还没有完成几个类方法，但我认为它不会导致崩溃。