Key in values, hitting the Enter key after each one. The operation keys (-/+, +, -, *, /) cause a calculation to be performed automatically, so you don't need to click on the Enter key for those. With an RPN calculator, you put in the values before putting in the operation. So, for example, to calculate 7 + 2, the sequence woud be '7', Enter, '2', Enter, '+'. The calculator would then calculate the result and display it via the result variable. The Backspace (back arrow) key can be used to correct the current value and the Clear key will clear out the stack and result.
This project implements an RPN calculator (http://en.wikipedia.org/wiki/RPN_calculator). RPN calculators are interesting because when implemented appropriately, you don't need parentheses to group operations. Something like 2 * (3 + 4), which requires parentheses to get the addition before the multiplication, is just 2, 3, 4, +, * using an RPN calculator.
