Question

I am parsing XML that gets returned from a web service. If it is a valid user than I should be getting a long integer back. (for example: 2110504192344912815 or 2061128143657912001). If it is invalid then I get a 0. However if you see my code, I am getting negative values sometimes for userID and that is throwing my validation code hay wire. Any tips?

NSMutableString *loginString;
    NSInteger userID;

-(void)parser:(NSXMLParser *)parser
didEndElement:(NSString *) elementName
 namespaceURI:(NSString *)namespaceURI
qualifiedName:(NSString *)qName{

    if ([elementName isEqual:@"GetUIDResult"]) {        
        userID = [loginString longLongValue]; / 0 if invalid password or LONG 
        [loginString release];
        loginString = nil;
    }   

}


//MY VALIDATION CODE..

    NSLog(@"The value of userID is: %ld",userID);

    if (userID > 0) {

    NSLog(@"YOU ARE NOW LOGGED IN");

    }else {

        UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Login Validation Error" message:@"Unable to validate Login/Password combination" delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil]; 
        [alert show];
        [alert release];

    }

Answer 1

check if loginString is not equal to @"0"

if (![loginString isEqualToString:@"0"]) {
       NSLog(@"YOU ARE NOW LOGGED IN");
}

EDIT: I jut realised that your are actually asking why. Well, the documentation for NSString longLongValue methods says

http://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Classes/NSString_Class/Reference/NSString.html#//apple_ref/doc/uid/20000154-intValue

Return Value The long long value of
the receiver’s text, assuming a
decimal representation and skipping
whitespace at the beginning of the
string. Returns LLONG_MAX or LLONG_MIN
on overflow. Returns 0 if the receiver
doesn’t begin with a valid decimal
text representation of a number.

So, what is happening is that loginString’s value is an overflow, either because you are receiving it like this, or parsing it wrongly.

EDIT 2:

I saw that you edited your code. Now you are trying to store a long long value into a NSInteger type variabel:userID. This definitely will cause an overflow.