CS193P - Adding floating point button to iOS calculator

I recently started following the online course on iPhone development from Stanford University on iTunes U.

I'm trying to do the homework assignments now for the first couple of lectures. I followed through the walkthrough where I built a basic calculator, but now I'm trying the first assignment and I can't seem to work it out. It's a follows:

Your calculator already works with floating point numbers (e.g. if you press 3 / 4 = it will properly show the resulting value of 0.75), however, there is no way for the user to enter a floating point number. Remedy this. Allow only legal floating point numbers to be entered (e.g. “192.168.0.1” is not a legal floating point number).

First of all, I'm not sure whether a floating point counts as digitPressed or operationPressed. I even tried with a new method called floatingPointPressed but that didn't really work out. Could someone provide pointers on this?

When I saw it as digitPressed, I tried something like this:

    if (hasFloatingPoint) {
        NSRange range = [[display text] rangeOfString:@"."];
        if (range.location == NSNotFound) 
        {
            [display setText:[[display text] stringByAppendingFormat:digit]];
            hasFloatingPoint = YES;
        }
    }
    else {
        [display setText:[[display text] stringByAppendingFormat:digit]];
    }

But I'm missing the key concept here, I think.

I have also tried another solution which I have, sadly, undone already so I can't provide the code but what happend was this: I could press a number, say 5 and then a floating point and 3 so I would end up with 5.3. I then managed to 'disable' the floating point for the remainder of the input.. But I guess I was a little too strict on it: I couldn't, for example, put in 5.3.2 but after pressing an operation button (+, etc), it still wouldn't let me press the floating point button. I guess I should reset the bool I used for this?

I'm not looking for a completely written out solution here, but could someone be so kind to provide some basic advice on how to tackle this problem? Some kind of step-by-step overview of what I should do and think about, without actually providing a code solution.

Thanks in advance.

Solution

Since you want a hint of where to look, have a look at attaching a NSNumberFormatter to the input fields.

This is the proper way of validating input, rather than manually checking each character the user enters yourself.

edit I've just read the assignment that you are trying to work through. I think my answer here, although still the right way to do it, not the best way to do what the assignment is trying to get you to do.

Since you are reading the digits one by one as they are added to the view and appending them to the number you are operating on you may be on the right track with your original answer:

Keep accepting numbers as they are added
Add a decimalEntered flag - initialise it to 'NO'
If a decimal is entered, check the flag, if it is no, accept it and set the flag to YES
If a decimal is entered and the flag is YES, don't accept it.
Whenever you press an operand, set this flag back to NO.

When you learn a bit more, you'll realise that another way to do this is to let the user type in the field. Hook up a formatter to the field to validate that the input is a valid number, but it will also be able to turn the string into a number for you. And when an operand is pressed, this number will be kept aside as one of the numbers upon which the operand will operate.