@raschemmel, thanks for posting the code correctly.
@insanehad, if you want people to study a long piece of code make life easy for them by following the Forum guidelines.
As I understand it you want to be able (say) to add 2 + 3 = 6 followed by (say) 6 * 3 = 18 with the calculator remembering the 6 between calculations.
From a quick glance the problem seems to be that all of the "code" is within the IF statement that checks the keypad inputs and I think, to do what you want, the code needs to separate the keypad inputs from the calculations. If I'm write that is not a trivial change, though it shouldn't be difficult.
Rather than thiink about how to modify that piece of code it will probably be easier to think about the problem from scratch and just use that code for ideas about how to read the keypad etc.
...R