Why I like RPN

RPN Notation in calculators is unique to HP these days, and even then only limited to very few calculators. Personally, I love RPN entry and think it’s way better than DAL (Direct Algebraic Logic) that is used by all other calculators.

Wrong reason

First let me explain what is not the reason I prefer RPN. Somehow the “lesser keystrokes” argument always comes up, but I don’t think it is that convincing. Consider this calculation that HP uses as an example to tout how much more efficient RPN is:

simple_calc

In algebraic notation you would enter it like either like this:

( 3 + 5 ) ÷ ( 7 + 6 ) =

Which is a whopping 12 keystrokes. Or, if you have a primitive desk calculator that doesn’t offer parenthesis, you would enter it like this:

7 + 6 [M+] 3 + 5 ÷ [MRC] =

Which is an impressive 10 keystrokes! No, then RPN:

3 [ENTER] 5 + 7 [ENTER] 6 + ÷

Which is 9 keystrokes. Wow! The difference! “But with complex calculations the difference will add up.” Well yes, with complex calculations your numbers tend to get longer too, so the relative difference will only get less. Will it really matter if a calculation takes 35 keystrokes or 40?

So what is the advantage then?

The example is actually good, but for the wrong reason. What I truly like about RPN is that it makes entering the calculation so much easier. Let’s review the same example and see what happens when you enter it:

simple_calc

First you calculate the numerator:

3 [Enter] (“start a new calculation with this entry”)
5 [+] (“take 5, and add it to the running total” — Calculator shows 8)

Then you calculate the denominator:

6 [Enter] (“start a new calculation”)
7 [+] (“take 7, and add it to the running total” — Calculator shows 13)

Now, take the original total and divide that by what I have right now:

[÷] (Calculator shows 0.6154)

Because previous calculations are automatically stored in memory (“the stack”) complex calculations become far more logical to enter. Consider this calculation:

complex_calc

In algebraic notation you’d enter this as:

(( 1 × 3 × 5 ) + ( 2 × 4 × 6 )) ÷ ( 1 + 3 + 4 + 7 ) =

Or maybe as:

1 × 3 × 5 = + ( 2 × 4 × 6) = ÷ ( 1 + 3 + 4 + 7 ) =

But you’ll have to pay attention to the parenthesis. How much easier in RPN!

“Let’s do 1×3×5 first” 1 [Enter] 3 [×] 5 [×]
“Then do the other half of the numerator” 2 [Enter] 4 [×] 6 [×]
“Add those two together” [+]
“Now let’s do the denominator…” 1 [Enter] 3 [×] 5 [×] 7 [×]
“…and divide by it” [÷]

Entering your calculation, despite the “reverse entry” goes in a much more natural fashion than the acrobatics with memory and parenthesis that algebraic input demands.

(Equations provided by the online latex equation editor)

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