Question

I am doing some iOS learning and I have an NSNumber which appears to be in order but when I ask for the doubleValue of it, I get null.

[stack addObject: [NSNumber numberWithDouble:operand]]
id topOfStack = [stack lastObject];
if([topOfStack isKindOfClass:[NSNumber class]]){
    NSLog(@"%@",topOfStack);
    NSLog(@"%@",[topOfStack class]);
    result  = [topOfStack doubleValue];
    NSLog(@"%@",result);
}

This prints:

2012-04-11 17:52:04.909 RPNCalc[4127:f803] 3
2012-04-11 17:52:04.910 RPNCalc[4127:f803] __NSCFNumber
2012-04-11 17:52:04.910 RPNCalc[4127:f803] (null)

Why would the doubleValue come out as null?

Answer 1

You are getting NULL because %@ is the string literal for an object. Doubles are not objects. Use %f to NSLog a float or double.