Implementations that use RPN are stackbased; that is, operands are popped from a stack, and calculation results are pushed back onto it. Although this concept may seem obscure at first, RPN has the advantage of being extremely easy for a computer to analyze due to it being a regular grammar. RPN has been used in HewlettPackard calculators as well as in some programming languages such as Forth and PostScript. Most Unix systems also comes with a calculator program that uses RPN: dc.
Practical implications From the practical point of view, in RPN:
Example For instance, the calculation: ((1 + 2) * 4) + 3 can be written down like this in RPN:
1 2 + 4 * 3 +The expression is evaluated in the following way (the Stack is displayed after Operation has taken place):
Input  Stack  Operation 

1  1  Push operand 
2  1, 2  Push operand 
+  3  Addition 
4  3, 4  Push operand 
*  12  Multiplication 
3  12, 3  Push operand 
+  15  Addition 
Compare to Polish notation.
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