Just before I called it a night, I decided to delete the “.” (decimal) and “E/-” buttons off my keypad. My thinking is that in most cases, it’s not a big surprise that the decimal point is the second character in the string of a number. I’m having the app automatically detect the position of the decimal and put it in right after the previous digit has correctly been entered.
I also decided that the “E/-” (exponent) button is similarly unimportant. Right now, none of my selected number have an exponent. I can see adding numbers, like the Avogadro constant, which would require an exponent, and some with a negative exponent, thus requiring the “-“. Curiously (and at this late hour I don’t want to be bothered thinking about it) I don’t know of any special numbers that themselves are negative.
In the case of numbers with exponents, I think I will automatically append the characters “10²³” or “E23” to the right side of the number and do the user a favor. Yes, they won’t have to remember this last bit. For now, I’m ok with it.
I got the sounds working – and on the iPhone, if you type in the incorrect digit, you’ll get a vibration!
Next comes scoring or some form of performance feedback.