Name: RPN 2+
Rating: 4.4 (5 reviews)
Author: ogri

RPN 2+ is a calculator with Reverse Polish Notation that you can use (mostly) without your +2 Dioptre reading glasses.

* TL;DR
RPN - as on HP calculators with an ENTER button.
Calculate 5 - 3: 
5
Swipe left (ENTER)
3
Swipe down (to see commands)
-

* What is RPN? 
RPN means "Reverse Polish Notation" and is really convenient once you get the hang of it.
In RPN you enter operands first and then the operator. Instead of keying in 5 + 3 = you press 5 ENTER 3 +. In this way you can calculate complicated expressions in the same order as you would do manually: from the inside out, without any need for parenthesis or memory.
For example, if you were to calculate 8 divided by the sum of 1 and 3, with a "normal" calculator you would press:
8 / ( 1 + 3 =
With RPN you do like this:
1 ENTER 3 + 8 X(swap)Y /
You start at the innermost expression, as you would have done for a manual calculation. First sum 1 and 3. Then input the 8. Switch order between the 8 and the calculated sum, then divide.
(In this short example you also could have pressed 8 ENTER 1 ENTER 3 + / . For longer expressions with many pending operations (like the division here), you have to start with the innermost expression.) 

Here is a tutorial for using RPN on HP calculators: https://hansklav.home.xs4all.nl/rpn/index.html
RPN 2+ is "Classical RPN" (not "Entry RPN") in the tutorial's terminology.

* RPN 2+ Features

- The display area grows to show large numbers. 
- The input "buttons" can be touched as normal even when the display area covers them.
- Swipe left (or press the Start button) for ENTER.
- Swipe right (or press the Back button) to go back or exit the app. During number input the last digit is deleted.
- Swipe up or down to enter the command loop with 5 pages of commands.
- The Menu button shows the menu with an Exit command, settings, and more.
- 4-level stack (X, Y, Z, T registers, X is shown in the display)
- ENTER lifts the stack and duplicates X; X is copied into Y (and so on), the old T is lost
- Use ENTER to separate one number from the next, not to end number input.
- T is copied to Z when the stack drops.
-  % calculates (X × Y ) / 100 and replaces X with the result. Y is kept so you can do the same calculation with a new X by dropping the result and inputing a new X.
- The frame around the display area is green but changes to blue if an operation resulted in the same value as was already displayed (a kind of visual "key-click"). The frame is white during number input.

* Accuracy
Calculations are done with the binary DOUBLE datataype, which can't represent all decimal fractions exactly. 

A well known example is 0.1 + 0.2 - 0.3 which is not 0.0. RPN 2+ rounds this to 0 in the main display, but if you look at the X register in the Stack view, the result is a very small, but non-zero, number.
