[Richard Suchenwirth] 2004-02-05 - RPN is short for Reverse Polish Notation. Arithmetics is most frequently done (like Tcl's [expr]) with infix operators
 1 + 2
However, this gives rise to problems of operator precedence. Jan [Lukasiewicz] in Poland, early 1900's, devised a way of putting the operator always in front of its operands, which is known as ''Polish Notation'' - see [Parsing Polish notation]:
 + 1 2
Programming languages that employ Polish notation for easier coding are [Lisp], [Logo], and not least (except for [expr] arithmetics), [Tcl] :) Also, in most languages except APL, functions are invoked by giving first the function name and then the arguments, which is Polish:
 f(x,y)

As you have to visually decode Polish expressions from right to left, which is unusual except in Arabic and Hebrew, the alternative "Reverse Polish Notation" was created [[by whom, when?...]]]
 1 2 +
This was used in [Forth], HP pocket calculators, [Postscript] and Joy - see [Playing Joy]. RPN builds heavily on the concept of a stack, and operators pop their arguments, and push their results, to it. So RPN allows to code with no variables at all, which is a desideratum in functional programming - but of course the stack is the Big Variable, changing its value all the time.

Both PN and RPN allow for unparenthesized expressions - mostly because the user has to deliver the very parse tree in advance...

Pages on this Wiki experimenting with [RPN in Tcl]:
   * [HP Calculator Simulations]
   * [Forth]
   * [GPS RPN]
   * [Minimal RPN]
   * [RPN again]
   * [Programmable RPN calculator]
   * [Playing bytecode] - most minimal, since no space is needed between bytes
   * [Brute force meets Goedel] - discover algorithms in RPN by testing all possibilities
   * [Pocket Joy 2005]
----
[Category Concept] | [Arts and crafts of Tcl-Tk programming]