检查 if(country == @"(null)" 不起作用

发布于 2024-08-27 10:06:42 字数 345 浏览 3 评论 0 原文

我遇到的问题是 if 语句不起作用。在第一个代码行之后，变量包含值“(null)”，因为从 iPhone 地址簿中选择联系人的用户没有为此联系人设置国家/地区键，到目前为止一切顺利。

但如果我检查变量，它不会是真的，但值肯定是“（null）”......有人有想法吗？

 NSString *country = [NSString [the dict objectForKey:(NSString *)kABPersonAddressCountryKey]];

 if(country == @"(null)")
 {
      country = @"";
 }

提前致谢
肖恩

原文

I got the problem that the if-statement doesn't work. After the first code line the variable contains the value "(null)", because the user who picked the contact from his iphone address book doesn't set the country key for this contact, so far so good.

but if I check the variable, it won't be true, but the value is certainly "(null)"... does someone have an idea?

 NSString *country = [NSString [the dict objectForKey:(NSString *)kABPersonAddressCountryKey]];

 if(country == @"(null)")
 {
      country = @"";
 }

thanks in advance
sean

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画尸师 2024-09-03 10:06:42

正确的表达式是：

if (country == nil)

可以进一步缩短为：

if (!country)

如果您确实想使用 @"(null)" 字符串测试相等性，则应该使用 isEqual: 方法或 isEqualToString:

if ([country isEqualToString:@"(null)"])

当您使用 == 运算符进行比较时，您正在比较对象地址，而不是其内容：

NSString *foo1 = [NSString stringWithString:@"foo"];
NSString *foo2 = [NSString stringWithString:@"foo"];
NSAssert(foo1 != foo2, @"The addresses are different.");
NSAssert([foo1 isEqual:foo2], @"But the contents are same.");
NSAssert([foo1 isEqualToString:foo2], @"True again, faster than isEqual:.");

The correct expression would be:

if (country == nil)

which can be further shortened to:

if (!country)

And if you really want to test equality with the @"(null)" string, you should use the isEqual: method or isEqualToString:

if ([country isEqualToString:@"(null)"])

When you compare using the == operator, you are comparing object addresses, not their contents:

NSString *foo1 = [NSString stringWithString:@"foo"];
NSString *foo2 = [NSString stringWithString:@"foo"];
NSAssert(foo1 != foo2, @"The addresses are different.");
NSAssert([foo1 isEqual:foo2], @"But the contents are same.");
NSAssert([foo1 isEqualToString:foo2], @"True again, faster than isEqual:.");

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