String to Double Trouble Shooting

How do I correctly convert 开发者_如何学编程a NSString to a double in xcode 3.2? This is my code(abbreviated), but when I run it, the message says (null). What am I doing wrong? Thanks so much!

@synthesize class1sem1;

-(IBAction)buttonPressed
{   
NSString *myString = class1sem1.text;
double myDouble = [myString doubleValue];


NSString *message = [[NSString alloc] initWithFormat:
                     @"%@", myDouble];

UIAlertView *alert =[[UIAlertView alloc] initWithTitle:@"Result" message:message delegate:nil cancelButtonTitle:@"Great!" otherButtonTitles:nil];
[alert show];
[alert release];
[message release];

}

Use %g for your double .. not %@

NSString *message = [[NSString alloc] initWithFormat:
                 @"%g", myDouble];

You try to display a double as if it were an object. Use this instead:

NSString *message = [[NSString alloc] initWithFormat:
                      @"%f", myDouble];
                     //  ^ note 'f' instead of '@'

f means "double precision floating point number", while @ means "Objective-C object". A double is a double precision floating point number, not an Objective-C object.