Question

我在大约一周前开始使用Objective-C编程，并且一直在从传递给函数的字符串数组创建一个新字符串。我对此进行了彻底的研究，似乎无法弄清楚为什么我可以使用stringByAppendingString一次（使用NSLog检查）但不是第二次没有抛出此错误：

感谢您的光临。

2012-06-29 20：46：35.761 Calculator [32883：f803] - [__ NSCFNumber stringByAppendingString：]：无法识别的选择器发送到实例0x6e43d50

2012-06-29 20：46：35.763计算器[32883：f803] * 由于未捕获的异常'NSInvalidArgumentException'而终止应用程序，原因：' - [__ NSCFNumber stringByAppendingString：]：发送无法识别的选择器例如0x6e43d50'

+ (NSString *)descriptionOfTopOfStack:(NSMutableArray *)stack
{
    NSString *result;
    NSString *displayString = [[NSString alloc]init];
    id topOfStack = [stack lastObject];
    if (topOfStack) [stack removeLastObject];

    if ([self isNoOperandOperation:topOfStack]) {
        displayString = [displayString stringByAppendingString:topOfStack];
        result = displayString;
        }
    else if ([self isOperation:topOfStack]){
        id nextInStack = [stack lastObject];
        if (topOfStack) [stack removeLastObject];
        displayString = [displayString stringByAppendingString:nextInStack];
        displayString = [displayString stringByAppendingString:topOfStack];
        result = displayString;

Answer 1

stringByAppendingString是NSString的一个方法，你必须将displayString作为NSString返回，

+ (NSString *)descriptionOfTopOfStack:(NSMutableArray *)stack
{
NSLog(@"stack array :: %@",stack);
NSString *result;
NSString *displayString = [[NSString alloc]init];
id topOfStack = [stack lastObject];
if (topOfStack) [stack removeLastObject];

NSLog(@"topOfStack :: %@", (NSString*)topOfStack);

if ([self isNoOperandOperation:topOfStack]) {
    displayString = [[(NSNumber*)displayString stringValue] 
                                stringByAppendingString:(NSString*)topStack];
    result = displayString;
    }
else if ([self isOperation:topOfStack]){
    id nextInStack = [stack lastObject];
    if (topOfStack) [stack removeLastObject];
    displayString = [[(NSNumber*)displayString stringValue] 
                                stringByAppendingString:(NSString*)nextInStack];
    displayString = [[(NSNumber*) displayString stringValue] 
                                stringByAppendingString:(NSString*)topStack];
    result = displayString;

编辑：stringByAppendingString方法需要NSString的参数类型，你的topOfStack是类型ID，你需要输入强制转换为NSString，假设你的topOfStack是字符串，对于nextInStack是相同的

Answer 2

您需要先将id转换为字符串。对于NSNumbers，您可以使用-stringValue。

使用stringByAppendingString连接类型ID的字符串

2 个答案: