Can't get negative sign in calculator to disappear
===

I'm working on an iOS 5 calculator that contains the following code:

    - (IBAction)digitPressed:(UIButton *)sender 
    {

    //Enter digit, avoiding two-decimal-point case.
    NSString *digit = sender.currentTitle;
    NSRange range = [self.display.text rangeOfString:@"."];
    if (range.location==NSNotFound || (![digit isEqualToString:@"."]))
        if (self.userIsEnteringNumber)
        {
            self.display.text = [self.display.text stringByAppendingString:digit];
        }
        else 
        {
            if (![sender.currentTitle isEqualToString:@"."])
            {
                self.display.text = digit;
            }
            else 
            {
                self.display.text = @"0.";
            }
            self.userIsEnteringNumber = YES;
        }
    }

and

    - (IBAction)changeSignPressed 
    {

    //Reverse sign from positive to negative or vice versa.
    if (self.numberIsNegative) self.numberIsNegative = NO;
    if (!(self.numberIsNegative)) self.numberIsNegative = YES;

    //Correct display to reflect change in sign.
    if (self.numberIsNegative)
    {
        if (![[self.display.text substringToIndex:1] isEqualToString:@"-"])
        {
            self.display.text = [@"-" stringByAppendingString:self.display.text];
        }
    }
    if (!self.numberIsNegative)
    { 
         /*This isn't working--why not?
        if ([[self.display.text substringToIndex:1] isEqualToString:@"-"])
        {
            NSLog(@"Okay."); //This doesn't log, so obv. the problem is in the previous line.
            self.display.text = [self.display.text substringFromIndex:1];
        }
         */
    }
    }

I can't get the commented-out code in changeSignPressed to work no matter how I fiddle with it.  Any ideas about how I can fix this?
