Thursday, July 20, 2017

HP35s Scientific Calculator with RPN

Review: General review of HP's RPN calculators and why you should switch. I just bought a new HP35s.

HP35s Scientific Calculator (RPN)

Author's note: Although this article is dated, first written in 2009, I smiled today as I reviewed it and decided to re-publish it.  I am still using these same calculators!

This is a digression from the usual computer topics. If you need to calculate more than simply adding and multiplying numbers, consider using an HP "RPN" calculator. In this short article, I will explain why this type of calculator is better than a standard "Algebraic" calculator.

(Illustration: My new calculator and my daughter's statistics text book)

Consider this formula. Using your current calculator, compute the result:

  • Did you get the right answer the first time?
  • Did you write down intermediate answers?
  • Did you type parenthesis?
  • Were you confident, as you entered the numbers, that you were on the right track or did you have to wait for the final equal-sign before you were done?

Now try this formula:

Again, the same questions:
  • Were you confident?
  • Did you see the intermediate results?
  • Did you have to save values into Memory (M1), (M2)?
  • Did you type a boat-load of parenthesis?

My daughter, with her expensive TI calculator, tried these formulas several times and was not confident in her results.

Here is the surprise:

With an HP (RPN) calculator, you would get the correct answer the first time. You would have a high degree of confidence, knowing each intermediate step was reasonable -- with results displayed as you type formulas.

For example, on the HP, you would see the following:

- As you typed "2+3" you would see "5"
- As you typed "4+5" you would see "9"
- and when multiplied together (2+3) x (4+5) you would see "45"

RPN calculators show intermediate results and there are no parenthesis, nor do you need to store values in "M" (memory) -- the calculator handles all of this for you.   

What is RPN?

RPN stands for Reverse Polish Notation. The gist is this: You enter the formulas in the same way you would if solving on paper and pencil; solving from left-to-right, inner-parenthesis first, using "My Dear Aunt Sally" (multiplication and division before addition and subtraction). These are the same rules learned in 3rd-grade.

RPN keystrokes are different than an algebraic calculator. With RPN, you type the first number,then press ENTER and type the second number.  After the two numbers are typed, press the function (add, subtract, etc.)  Of interest, there is no "Equal" key.

This works to your advantage. With this, you can type the most horrendous function and never once have to type a parenthesis or save an intermediate value.

Using the first example, the keystrokes would be this:

2 (Enter, separates this first number from the second)
3 +
The calculator shows the intermediate result: 5

4 (Enter, separates this from the above "5" *)
5 +
The calculator shows the intermediate result: 9
x (times)
The calculator shows: 45
The calculator shows the intermediate result: 6.708

Without pressing any other keys, begin the second part of the equation by typing:
6 (Enter *Technically, this ENTER is not required because the SQRT resolved)
7 + .... etc.
completing this part the same as the first.
Once completed, press "+", adding the two big sections together.

At each stage, intermediate results are always displayed. Yes, the keystrokes are counter-intuitive, but after practicing for ten minutes, you will never forget and you will never go back.

I have used HP's RPN for so many years I can no longer use a "standard" calculator and I am embarrassed when forced to -- I can only solve the simplest equations on those types of calculators -- Algebraic calculators now seem completely foreign to me.

Choosing an HP Calculator:
  • HP makes both Algebraic (press "Equals) and RPN calculators (press "Enter") -- be sure to pick the right model.
HP divides the world into two types: Financial and Scientific. If you are a banker or real estate, choose a Financial calculator (12c). Otherwise, choose the Scientific calculators (even if you are not a scientist). Here are my recommendations:

Scientific/Non-business Calculators:
HP33s ($40.00) low-end calculator. 
If you can, buy the 35s; it has a better keyboard.

HP35s ($60.00) Recommended

HP50g ($150.00) Graphing (now dated)

HPG8X92AA (graphic)  I will be buying this calculator next:

Business: (Interest/PMT/Financial)
HP12C ($70)

This is an Industry Standard calculator for all financial users. HP reviewed this calculator a few years ago and decided, even after 30 years of production, nothing needed changed. This same calculator, with the same features, has been produced since then. Remarkable.

Look and Feel:
These calculator are pleasurable to use. The buttons "feel" substantial; with a solid, satisfying 'click' as you type. People who use HP calculators rave about the keyboard -- it is unlike any other calculator. When HP has made some cheaper models, with cheaper keyboards (the 33s, for example), users complained.

I believe all of the scientific calculators are programmable (you can write your own functions, automating common tasks).

The other thing about these calculators is they never die. My first HP11c is now approaching 30 years old and I still use it several times a week. I think I've changed the batteries three times in all those years.  It uses standard watch batteries.

My other HP calculator, which sits at home, is a 20-year-old HP32s and I only recently replaced it with a newer 35s for no other reason than I wanted to see what the new calculators looked like.

In short: If you are in school, or if you need to do a bit more than common math, spend a few dollars, get a quality calculator. Take twenty minutes studying the first couple of chapters in the manual, and you will never go back to those old, stinky, run-of-the-mill calculators again.


  1. I've been helping my daughter with linear-regression analysis. The HP35s has been absolutely fun to use. It calculates y-hat, x-bar, Sx, Sy, etc., with ease; the only thing it can't do is a scatter graph. (This is where I wished I had the HP50G graphing calculator. HP: you are welcome to send me one if you'd like.)

  2. Additional comments on my daughter's statistics homework. I am saddened to report that TI-85 calculator does some statistics work that the HP35s can't. I have found several areas lacking in the HP's statistical functions:

    1. I've not found a way to review the XY Data Points; the TI shows them as List-1 and List-2 and they are easy to view and remarkably easy to remove whole groups of data-points.

    2. Data-entry on the HP is cumbersome when compared to the TI. With HP, you enter Y, then X points; On the TI, you enter all the X's in one list and all the Y's in another. My daughter can easily enter the same data-points in a third of the time.

    3. Probably the most damning flaw: On the TI you can transform an entire list, all data-points, by applying a formula. For example, we have been transforming all List-2 points with a logarithm (handy for converting exponential functions to linear). On the HP, I have to convert them one-by-one as I enter the points.

    In HP's defense, with two or three simple keystrokes, I can view x-bar, y-bar, y-hat, sx, sy and other registers. On the TI it appears to take no less than 30 keystrokes to get these same values.

    I may have to look more carefully at HP's graphing calculator (as you can see, I am still unwilling to go back to an algebraic calculator).

    Your comments are welcome.

  3. RPN Logic is awesome. I'd like that option on a calculator on my PC's desktop (free of course). I loaned a friend my HP 12-C and haven't seen him for over a year to get it back. I guess I shouldn't have done that.

  4. There are plenty of free RPN calculators for PCs, I use one called "rpncalc.exe" from in the Shareware section. I just got the HP-35s and absolutely love it (thanks HP!), possibly more than my 41C and 48sx. The keyboard layout and choice of functions is perfect.

    For me the argument over whether or not one can do "less" or "more" on a TI is completely irrelevant because unless it's RPN it's unusable - so that leaves HP.

  5. ...without RPN a calculator is irrelevant... I smiled at your comment. How true.

  6. Just bought the HP Prime Graphing calculator. Amazing. Solves the statistics problems I had with my previous comments. Tons of Youtube videos for help. Review sometime in the future.


Comments are moderated and published upon review.