When I was working as an engineer, I fell in love with the RPN calculators produced by HP. Once one gets used to it, RPN is a fast and elegant way of entering complex calculations while displaying intermediate results on the fly. However, as much as I love RPN, it doesn't make it easy to verify that you entered the whole calculation correctly, which is something that today's textbook format calculators do very well.

So, the thought occurred to me, why can't a calculator do both? Why not enter calculations using RPN just like the old HP calculators, but then display the calculation in textbook format on the stack along with the answer? Then on top of that, make it possible to go back and change the calculation numbers, or undo the calculation entirely and move the parts around the stack and redo it again? That's what I created Visual RPN Calculator to do - combine the fast and elegant input method of an RPN calculator with the ability to verify and edit your calculations like an algebraic calculator.

I chose to implement my idea on Windows 10 using C# and Visual Studio because Microsoft has a reputation for making a good tools and I wanted to learn them. Once I chose Universal Windows Platform (UWP) as my target, I wanted to make the calculator interface fit the platform, not be an emulation of an HP calculator on windows.

My first windows store submission back in July 2016 was pretty basic, but over time I've added more features on a regular basis - sometimes in response to requests from users (if you want something, click on the feedback button on the calculator under the hamburger menu).

Over time Visual RPN Calculator's feature set has been slowly building up to approach the calculators which cost lots of money at the store:

- All the common scientific, trig, and statistics functions - this was the easy part because they're all built into c#
- Extensive editing capabilities - change numbers in calculations, undo calculations and copy and move entries around on the stack
- Sets - work with numbers combined into sets, making possible single calculations using multiple data points. This is a super powerful feature once you start using it.
- Graphing - graph multiple equations at once, with highlight marks and pop-up labels for roots, graph intersects, and local minimums/maximums
- Searchable library of physical constants
- User defined buttons for saving off new functions and constants
- Lots of keyboard shortcuts so if you're working on a PC you can keep your hands on the keyboard if you want

There are a few items I want in the calculator that I'm getting to as time permits:

- Matrices
- Complex numbers
- Exact results option - i.e., sqrt(8) = 2sqrt(2), 1/2 + 1/3 = 5/6, etc.
- Teacher arithmetic - i.e. the calculation of 23 * 45 shown the way a student should do it so a teacher can check work. (I'm a math tutor so this is something I could use personally.)
- Numerous UI improvements - a dark mode, a keypad for inputing graph coordinates (because using the phone keyboard is not really optimal), etc.

Visual RPN Calculator has an option to store and load calculator state to a file and folder of the users choice. Other than that, the application doesn't mess with any of your files, and no other data from your your computer is accessed by this application.