operate on expressions on the stack. The [ENTER] key pushes expressions on the stack. For
example, the keystrokes to add 2 to 4 are [2] [ENTER] [4] [+], and this illustrates the origin of the
acronym RPN. Polish notation is named in honor of the Polish mathematician Jan Lukasiewicz (1878 -
1956), who developed a syntax for representing any expression without parenthesis. In Lukasiewicz'
'Polish notation', the expression to add 2 and 4 is + 2 4. The 'reverse' in RPN comes from the fact that
the operator comes after the operands, instead of before.
In the simple 2 + 4 example, there are no keystroke savings: both methods take four keystrokes.
However, consider this example:
(a+b)/(c+d) [ENTER] algebraic; 12 keystrokes
a [ENTER] b + c [ENTER] d + / RPN; 9 keystrokes
While the keystroke savings are real, even more significant is the fact that intermediate results are
displayed immediately, which leads to earlier troubleshooting of incorrect entries. In addition, there is
no mental effort expended on matching parentheses, and there are no calculation errors from
mismatched parentheses. RPN is as useful with symbolic expressions as it is with numeric ones.
RPN calculators also have various stack manipulation functions, such as swap, drop, dup and rotate,
which make calculations even more efficient.
There can be no argument that learning RPN takes some effort. But that effort will be repaid in orders
of magnitude by faster, more accurate results.
You can get Lars' RPN at Roberto Perez-Franco's site:
http://www.perez-franco.com/symbulator/download/rpn.html
RPN is fully documented, with plenty of examples to get you started.
[9.20] Use a graphical equation editor (equation writer)
Entering complicated expressions in the command line can be prone to error. Expressions can be
entered more quickly without mistakes if you can see the equation, in pretty-print, as you enter it. This
feature is called an equation writer, and has been available on the HP-48 and HP-49 calculators. E.W.
has written an equation writer called EQW for the TI-89/92+. EQW is the winner of the 2001 Texas
Instruments App Development Contest, 68000-platform division. You can get a free version at
http://tiger.towson.edu/~bbhatt1/ti/
or you can buy EQW as a flash application from TI. The flash app version has more features, but the
free version is more than adequate for most uses.
This screen shot shows a typical EQW screen:
9 - 24
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