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Issue 5.6


An RPN Interpreter

"Hello World" For Compilers

Issue: 5.6 (September/October 2007)
Author: Charles Yeomans
Author Bio: Charles is the author of "I Declare: Calling External Functions in REALbasic", available online at http://www.declareSub.com/
Article Description: No description available.
Article Length (in bytes): 16,774
Starting Page Number: 42
Article Number: 5616
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Excerpt of article text...

Writing an RPN interpreter is a standard five-finger exercise for computer science students. We will see why in this and several subsequent columns. But, first, what is RPN?

RPN stands for "reverse Polish notation", and is a syntax for writing mathematical expressions involving operators. The usual form of a mathematical expression is in the form operand - operator - operand; 2 + 3, for example. This is called "infix" notation. In postfix notation, an expression is written in the form operator - operator - operand. In postfix, 2 + 3 is written as 2 3 +. An example of posfix with which you may be familiar is the factorial operator -- N!.

There is also prefix notation, in which the operator comes first: + 2 3. This is also called "Polish notation", in honor of the mathematician Jan Łukasiewicz. Prefix notation came before postfix, so postfix is known as reverse Polish notation, or RPN.

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