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How Computers Think


A good reminder of what happens when we become careless with technology. I needed to convert temperature from degrees Celsius to degrees Fahrenheit. Out comes the trusty TI-85 calculator. Bring up the conversion menu, and into the temperature subsection. I entered my conversion:

-30 C > F

and get back -86. Almost subconsciously, this answer struck me as incorrect. A sanity check with a co-worker confirmed my suspicion that it was indeed incorrect. What was going on? After pondering, I tried the conversion again in a slightly different manner:

(-30) C > F

this time getting back -22. A quick check using the conversion formula itself:

TF=95*TC+32

verified the answer:

TF=95*(-30)+32=-22

So what happened? It turns out that in the first attempt, my trusty TI was really doing this:

-1*(30 C > F)

TI should warn users about this. Maybe they do. It has been a long time since I opened that user manual. I wonder how this works on RPN-style calculators? Or if this behavior is corrected or accounted for on the newer models like the TI-86?

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Comments (6)

Lamar:

I like to say, "Computers always do exactly what you tell them to do. You just don't always understand what you're telling them."

I've got an HP 11C RPN calculator at the house. If I think about it, I'll give that conversion a try.

Jonathan:

To me the bigger question is "does it really matter?" i.e., does anyone really use calculators anymore or are they not doomed to go the way of the abacus?

I can't even remember the last time I used one, and I would bet our kids never will unless they have an interest in "vintage electronics."

Jonathan:

BTW, I didn't mean this wasn't an interesting find. I guess it just got me thinking about the future of the calculator.

Lamar:

Perhaps I'm weird, or just behind the times, but I use my HP 11C calculator at least once a month to balance my checkbook.

Lamar:

Finally brought in the 'ole HP 11C RPN calculator today. Turns out that it's not fancy enough to have a built-in C to F conversion key. I have to flip it over and read the conversion formula off the cheat sheet printed on the back of the unit and key it in manually:

30

1.8
32
Result: -22.0

I really love RPN. IMO It's the way all scientific calculators should operate. To this day, I have a hard time using non-RPN calculators.

Lamar:

Hmmm...my RPN notation conflicted with HTML tags. Let's try that again

30[enter]
[chs]
1.8[*]
32[+]
Result: -22.0

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This page contains a single entry from the blog posted on December 4, 2009 6:33 AM.

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