This is from a Nov 2012 posting on http://hardware.slashdot.org
Comparison between RPN and algebraic calculators
Why do you, Slashdot users, like RPN?
I don't know that I can really articulate it, either, but I can report
the results of an interesting experiment I participated in about 20
I was an RPN-lover even then, having recently graduated from my 15C to
a 28S, but most of the other geeks in the university computer lab
where I spent a ridiculous amount of time couldn't see the sense in
it. Late one night the discussion got somewhat heated and someone said
that an advantage of RPN was that it was faster because it required
fewer keystrokes. A measurable claim like that immediately sparked a
demand for proof, so we decided to do some comparisons.
One guy got on the whiteboard and wrote down four very complex
arithmetic expressions. Then for each expression, two candidates were
selected, one from the RPN camp and one from the infix camp. Each was
to write down on the board, under the expression, the series of
keystrokes that would be needed to evaluate it. In all cases the RPN
keystroke list was shorter, often considerably, but after the first
was done everyone noticed a second interesting and unexpected outcome:
The RPN-wielder finished writing down his keystroke list long before
the infix-wielder -- and not just because of the number of keystrokes.
Everyone watching noticed that the infix-proponent often paused for a
second or two to think about how to handle the next bit, or stopped
for a moment to go back to count up parentheses. In contrast, the
RPN-er never paused, never hesitated, just wrote down keystrokes as
fast as he could.
After that, we all decided that we should also time the remaining
trials, which were all conducted with different candidates. The RPN
user consistently finished 25% faster than the infix user, even though
the keystroke list was only about 5% shorter.
Then someone (I think it was actually someone from the RPN camp)
decided to write a truly horrendously complex expression. It had
fractions nested at least ten layers deep and was, frankly,
ridiculous. Two more stepped up to try and, once again, the RPN user
wrote down keystrokes in a long list, without any more hesitation than
it took to find his place in the expression. The infix guy, on the
other hand got badly bogged down, backed up several times and
ultimately gave up after his RPN competitor had been watching him
struggle for five minutes.
To top it all off, actually punching all those keystrokes into real
calculators showed that RPN was more accurate. On only one of the five
problems was the RPN calculation not correct, while the infix
calculation was incorrect on three out of five (determining which
answers were correct took significant time and much arguing).
Bottom line, per our impromptu tests and my personal experience, RPN
is faster and easier.
I could easily explain why it requires fewer keystrokes, but why
exactly it requires less cognitive effort is harder to describe. I
believe, though, that it's because when you use RPN you pick a "path"
through the expression, and then just follow it. At each point along
the path you only have to remember where you've been and where you're
going. The calculator keeps track of the stack. With infix you have to
manage the "stack" in your head, figuring out when to add and remove
nesting levels with parentheses. That's not exactly right, but it's as
close as I've been able to come.
--- swillden @ Why do you, Slashdot users, like RPN?
Back to Guido's RPN calculator page
© Guido Socher, guidosocher at