What is all this RPN stuff?
Do not be frightened away by RPN calculators - they are not so different from "algebraic" calculators. Reverse Polish Notation (RPN) logic merely uses an "entry" key instead of an "=" key, which operates immediately upon the displayed result. This allows the solving of complicated calculations with a minimum of keystrokes, and without the need for parentheses.
A number is entered, followed by the operator, and the result is immediately displayed. Further operations can be performed on the displayed number, with the result of each operation displayed as you go along.
Intermediate results are held on a stack of 4 registers, which are all displayed. The X register holds the current result.
To calculate: (4 + 5 + 9) x 3
| Number | Key | Display (X-register) | Result |
| 4 | enter | 4 | 4 |
| 5 | + | 9 | 4 + 5 |
| 9 | + | 18 | 4 + 5 + 9 |
| 3 | x | 54 | (4 + 5 + 9) x 3 |
A more complicated calculation: ( (2 x 3) + (4 + 5)² )³
| Number | Key | Display (X-register) | Result |
| 2 | enter | 2 | 2 |
| 3 | x | 6 | 2 x 3 |
| 4 | enter | 4 | 4 |
| 5 | + | 9 | 4 + 5 |
| X² | 81 | (4 + 5)² | |
| + | 87 | (2 x 3) + (4 + 5)² | |
| X³ | 658503 | ( (2 x 3) + (4 + 5)² )³ |
Note that only 7 keystrokes are required. Using the Psion scientific calculator, 15 keystrokes are required - 6 of them for the parentheses. In RPN, the expressions in parentheses are calculated independently. Using this technique removes the need for parentheses, and a stack of 4 is sufficient for almost all calculations. Also, arithmetic functions such as sin, log, etc. operate immediately on the displayed result - the built-in Psion scientific calculator seems rather cumbersome in this area (requiring use of the "ans" key). If you are not familiar with RPN, this method may same strange, but believe me, after getting used to it, normal calculators will seem slow by comparison.