Question

I am reading “Programming in Objective-C” 4th Ed. by Stephen G. Kochan. In the book, there is a sample code for creating a Calculator application for the iPhone. I understand the code, at least 90% of it. – There is a Fraction class that has methods to store fraction objects and that describe how to perform different basic fraction arithmetic operations
– In addition to that, there is a calculator class that runs the appropriate methods from the Fraction class depending on whether the user is trying to sum, divide etc.

The view controller has the following method for when the user presses a number in the interface:

-(IBAction)clickDigit:(UIButton *)sender {
int digit = sender.tag;     //sender or the argument inthis case is  the button

[self processDigit:digit];
}

As you see this method is now called:

-(void) processDigit:(int)digit {
currentNumber = currentNumber * 10 + digit;

[displayString appendString:
 [NSString stringWithFormat:@"%i", digit]];
display.text = displayString;

}

My ONLY question (and probably the one with the most simple answer) is: Why is currentNumber always multiplied by 10? The value of currentNumber is always 0 by the time the compiler enters the method above (I verified this using the debugger in XCode) so i dont get why we even have to multiply it by 10. I did delete that multiplication and the results are incorrect, i just cannot figure out why yet.

Thank you

Answer 1

Maybe the best way to think about this is with an example.

Imagine the user has clicked the following digits in order: 1, 2. Then, assuming the code is working, the numeric value of currentNumber should be 12.

Now imagine the user next clicks on ‘3’. Now you want the value to be 123. You can get this by multiplying the previous value (12) by 10 and then adding the 3. If they then click on a ‘4’, the value should become 1,234, which is achieved by 10 * 123 + 4. And so on.