Well, this is a nice calculator, but... this isn't RPN (aka postfix), it's infix. Maybe you saw someone on Miiverse mention the shunting-yard algorithm, but that is for infix, not RPN.
RPN is a different way of writing math, it's not the usual syntax we normally use.
Infix: 1 * 2 + 3 / (4 - 5) RPN: 1 2 * 3 4 5 - / + PN: + * 1 2 / 3 - 4 5Note that RPN and PN don't require parentheses at all, nor an order of operations. In addition, RPN in particular is very simple to parse and evaluate. You can simply read it left to right, using a stack:
Push 1 onto stack (1) Push 2 onto stack (1, 2) Multiply top two items on stack (2) Push 3 onto stack (2, 3) Push 4 onto stack (2, 3, 4) Push 5 onto stack (2, 3, 4, 5) Subtract top two items on stack (2, 3, -1) Divide top two items on stack (2, -3) Add top two items on stack (-1)And -1 is indeed the correct result. P.S. If this seems interesting to you, there are actually programming languages which use RPN for their entire syntax, with functions called like 1 2 3 FUNC and so on. Forth is a very notable example.