objective-c - "if"条件不起作用

Question

由于某种原因，if条件中声明的变量得到的是一个未使用的变量，而稍后要使用的变量得到了未声明的变量。为什么会发生这种情况以及解决方法是什么？

NSString *name = [NSString stringWithFormat:@"Josh"];

if ([name isEqualToString:@"Josh"])
{
    NSString *greeting = [NSString stringWithFormat:@"Hello Josh"]; //Warning: Unused variable 'greeting'
}
else 
{
    NSString *greeting = [NSString stringWithFormat:@"Hello %@", name]; //Warning: Unused variable 'greeting'
}
NSLog (@"%@", greeting); //Error: 'greeting' undeclared

Answer 1

您绝对应该阅读 variables scope .

这是一个正确的代码:

NSString *name = [NSString stringWithFormat:@"Josh"];
NSString *greeting = nil;

if ([name isEqualToString:@"Josh"])
{
    greeting = [NSString stringWithFormat:@"Hello Josh"];
}
else 
{
    greeting = [NSString stringWithFormat:@"Hello %@", name];
}

NSLog (@"%@", greeting);