objective-c – 为什么isEqualToString对NSString不起作用？

我正在尝试在XCode中为iPhone应用程序的登录过程编写代码.问题在于下面的NSString serverOutput.当我使用printf(serverOutput.UTF8String)打印它时;它向控制台打印“是”.但是,当我将serverOutput与“Yes”进行比较时,它不起作用.任何帮助,将不胜感激.这是我的代码：

- (IBAction) loginButton: (id) sender
{
    // TODO: spawn a login thread

    indicator.hidden = FALSE;
    [indicator startAnimating];
    NSString *post =[NSString stringWithFormat:@"username=%@&password=%@",userName.text,password.text];

    NSString *hostStr = @"http://10.243.1.184/connectDB.php?";
    hostStr = [hostStr stringByAppendingString:post];
    NSData *dataURL =  [NSData dataWithContentsOfURL: [ NSURL URLWithString: hostStr ]];
    NSString *serverOutput = [[NSString alloc] initWithData:dataURL encoding: NSASCIIStringEncoding];
    printf(serverOutput.UTF8String);


    if([serverOutput isEqualToString:@"Yes"]){


        UIAlertView *alertsuccess = [[UIAlertView alloc] initWithTitle:@"Congrats" message:@"You are authorized"
        delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil,nil];

        [alertsuccess show];
        [alertsuccess release];


    }
    else {
        UIAlertView *alertsuccess = [[UIAlertView alloc] initWithTitle:@"Error" message:@"Username or Password Incorrect"
        delegate:self cancelButtonTitle:@"OK"otherButtonTitles:nil,nil];
        [alertsuccess show];
        [alertsuccess release];
        //[self.navigationController pushViewController:DashboardViewController animated:YES];
        loginbutton.enabled = TRUE;

    }


    loginbutton.enabled = FALSE;
}

NSString *serverOutput = [[NSString alloc] initWithData:dataURL encoding: NSASCIIStringEncoding];
NSLog(@"Result = '%@'",serverOutput); // look for space between quotes
serverOutput = [serverOutput stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];