If you're a
frequent calculator user, you owe it to yourself to
investigate the advantages of RPN.

RPN stands for Reverse Polish Notation which was
developed in 1920 by Jan Łukasiewicz (de.wikipedia) as a way to write a
mathematical expression without using parentheses and
brackets.

Hewlett-Packard (tm), realizing that
Łukasiewicz's method was superior to standard
algebraic(1)
expressions when using calculators and computers,
adapted RPN for its first hand-held scientific
calculator, the HP35, in 1972.

- RPN saves time and keystrokes. You never have
to account for the parentheses while doing
calculations. The process is similar to the way you
learned math on paper.
- You can see the intermediary results as you
perform your computations rather than just the
answer at the end. This is an extremely helpful
byproduct. Math teachers are using this feature to
improve student understanding of mathematics.
- An intermediate result allows the user to check
the results and correct errors more easily. It's
easier to follow the stream of calculation. The
user defines the priority of operators.
- RPN is logical because the user first gives the
number and then tells what to do with it.

RPN is also consistent in its usage. Most non-RPN
scientific calculators are half RPN and half
algebraic. For example, to perform addition, you need
to enter 2+4 (algebraic), but to perform a sine
calculation, you need first to enter the number and
then to press the SIN button, which is a RPN method
of entering the equation. Our RPN calculators do not
suffer from this idiosyncrasy.

Believe it or not, the process
of using RPN is similar to the way you learned math.
If you think about it, you have to modify the way you
learned math in order to use an algebraic mode
calculator.
Here's an example:

__3+5__

7+6

Or (3+5) / (7+6) = x

Algebraic method: Add 3+5=8. Write down the answer
or store it in memory. Add 7+6=13. Now enter the 8
from the first answer and then divide it by entering
the second answer to get x=0.62.

RPN method: Press 3 then the ENTER key. Press 5
then the + key. Press 7, and then ENTER. Press 6 then
the + key. Note that the answer to the second sum is
displayed. Now here's the magic part. Press the
divide key and the calculator gives the answer,
0.62.

Algebraic: 13 strokes, not counting the effort to
write down or memorize the first answer while you
calculated the second answer.

RPN: 9 strokes, and no need to write anything
down.

RPN keeps a record of
calculations by placing them in a stack(2). In the above example, when
you pressed ENTER the second time, the answer to the
first sum was pushed(3) up in the stack awaiting the next
action. After entering the second sum, pressing the
divide key looked at the first sum and divided it by
the second and popped(4) the answer out of the stack. In
other words, RPN performed the calculation in a
logical order.

Learning how to use an RPN calculator usually takes
just a few minutes and it can save lots of time and
effort over the long run.
- Algebraic mode: This is the name of the
mathematic notation used on all non-RPN calculators
where you enter a mathematic equation like this:
1+3*(3+(2-5)/3). In algebraic mode, parentheses and
order of operations are extremely important.
- Stack: A stack, also called LIFO (last-in,
first-out), is the basis of the RPN system as it is
the 'memory' that allows the user to enter
numbers.
- Push/pushing: This is the action of adding a
number at the bottom of a stack, pushing all the
other numbers up.
- Pop/popping: This is the action of removing the
last number that was pushed on a stack.