为什么我的比较 if 语句不起作用? [英] Why is my comparing if statement not working?

问题描述

为什么以下代码(在可可中)不起作用?

Why is the following code (in cocoa) not working?

NSString *extension = [fileName pathExtension];
NSString *wantedExtension = @"mp3";
if(extension == wantedExtension){
//work
}

在 Xcode 中，这只是在没有警告或错误的情况下运行，但没有做我认为应该做的事情.

in Xcode this just runs without warnings or errors but doesn't do what I think it SHOULD do.

推荐答案

不应该是

if ([extension isEqualToString:wantedExtension]) {
...
}

"==" 比较指针.isEqual: 和 isEqualToString: 比较字符串，不过如果你知道 extension 和 WantedExtension 都是 NSString(你在本例中这样做)，isEqualToString 会更好.

"==" compares the pointers. isEqual: and isEqualToString: compare the strings, although isEqualToString is better if you know both extension and wantedExtension are NSString (which you do in this case).

实际上，如果您是像我一样的老 C++ 和 Java 程序员，您可能会更乐意将已知不为 null 的wantedextension"放在第一位.在 Objective C 中这是不必要的，因为发送消息"(即调用方法)到 nil 会返回 0 或 false.

Actually, if you're an old C++ and Java programmer like me, you might be happier putting the one that is known not to be null, "wantedextension", first. In Objective C that is not necessary because "sending a message" (ie calling a method) to a nil returns 0 or false.

if ([wantedExtension isEqualToString:extension]) {
   ...
}

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