How to parse XML data when it returns two different categories depending on data entered?

I am trying to get weather information for an iPhone application. If I enter a 5 digit zipcode, the code is working. However, I want to make sure that if the user does not enter a valid city or zip code, it doesnt crash. The problem is that with Google Weather API, the XML data reads "city 开发者_高级运维data" when a valid place is entered, and "problem_cause data" when it is not a valid place. Please help!

-(IBAction) getlocation {
    NSString *locationentered = location.text;
    if ([locationentered isEqualToString: @""])  {
    locationlabel.text = @"Please Enter a Location";
}
    else {

    NSString * locationen =  locationentered;
    NSString * address = @"http://www.google.com/ig/api?weather=";
    NSString * request = [NSString stringWithFormat:@"%@%@",address,locationen];
    NSURL * URL = [NSURL URLWithString:request];
    NSError * error;    
    NSString* XML = [NSString stringWithContentsOfURL:URL encoding:NSASCIIStringEncoding error:&error];
    NSString * temptoday = [[[[XML componentsSeparatedByString:@"city data=\""] objectAtIndex:1] componentsSeparatedByString:@"\""] objectAtIndex:0];


    NSString * errorlocation = [[[[XML componentsSeparatedByString:@"problem_cause data=\""] objectAtIndex:1] componentsSeparatedByString:@"\""] objectAtIndex:0];
    if ([errorlocation isEqualToString: @""]) {
    locationlabel.text = @"Invalid Location";
    }


if ([temptoday isEqualToString:@"(null)"]) {

        locationlabel.text = @"Location present";
    }
else {

    locationlabel.text = temptoday;

    }
    }
}

The result is XML so use an XMLParser to look at it. In this case the standard Cocoa one is NSXMLParser and then setup a delegate which will get called for each element.

For the start of an element you will get called parser:didStartElement:namespaceURI:qualifiedName:attributes: and this will pass in the element name in this case problem_cause or forecast_information depending if the input was good.

See Apple's Event-Driven XML Programming Guide for more or look at DOM parsers from this Stack Exchange question