Can't get negative sign in calculator to disappear - objective-c

Question

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?

Answer 1

It's probably this code:
//Reverse sign from positive to negative or vice versa.
if (self.numberIsNegative) self.numberIsNegative = NO;
if (!(self.numberIsNegative)) self.numberIsNegative = YES;
There is no else so if self.numberIsNegative is initially YES it will become NO; it will then be immediately seen as NO on the following line where it is changed back to YES.
In any case, that code is more complex than it needs to be; you can just say this to invert a flag:
self.numberIsNegative = ! self.numberIsNegative;