Question

我制作了一些方法，它从堆栈中获取数字和操作数，并以更加用户友好的方式显示它。问题在于descriptionString变量，当topOfStack为“+”时，它会返回null。我在下面显示一个日志。

+(NSString *)descriptionOfTopOfStack:(NSMutableArray *)stack
{

    NSString *descriptionString;
    id topOfStack = [stack lastObject];
    NSString *secondInStack;
    NSString *thirdInStack;
    if (topOfStack)
    [stack removeLastObject];    
    if ([topOfStack isKindOfClass:[NSNumber class]]) {
        descriptionString = [topOfStack stringValue];
    }
    else if([topOfStack isKindOfClass:[NSString class]]){
        if(([topOfStack isEqualToString:@"+"]) || ([topOfStack isEqualToString:@"—"])){
            secondInStack = [self descriptionOfTopOfStack:stack];
            thirdInStack = [self descriptionOfTopOfStack:stack];
           descriptionString = [descriptionString stringByAppendingFormat:@"%@ %@ %@",thirdInStack,topOfStack,secondInStack]; 
          NSLog(@"description is %@",descriptionString);

        }

    }
    return descriptionString;
}

我以2 + 6为例，这是日志：

2012-02-21 22：09：39.983计算器[12536：f803] stack =（ 2， 6， “+” ）
2012-02-21 22：09：39.983计算器[12536：f803]描述为（null）

为什么descriptionString为空？我犯了什么错误？感谢

Answer 1

在行中：

descriptionString = [descriptionString stringByAppendingFormat:@"%@ %@ %@",thirdInStack,topOfStack,secondInStack];

变量descriptionString为nil。用以下内容替换该行。

descriptionString = [NSString stringWithFormat:@"%@ %@ %@",thirdInStack,topOfStack,secondInStack];

Answer 2

我怀疑您没有为descriptionString的其他分支中的if变量设置值，因此您要将字符串附加到null。您可以设置变量，也可以使用[NSString stringWithFormat:format]：

if ([topOfStack isKindOfClass:[NSNumber class]]) {
    descriptionString = [topOfStack stringValue];
}
else if([topOfStack isKindOfClass:[NSString class]]){
    if(([topOfStack isEqualToString:@"+"]) || ([topOfStack isEqualToString:@"—"])){
        secondInStack = [self descriptionOfTopOfStack:stack];
        thirdInStack = [self descriptionOfTopOfStack:stack];
       descriptionString = [NSString stringWithFormat:@"%@ %@ %@",thirdInStack,topOfStack,secondInStack]; 
      NSLog(@"description is %@",descriptionString);

    }

}

如何从几个字符串？ Xcode中

2 个答案: