Subtracting temperatures causes confusion

Question

Subtracting temperatures causes confusion

thejpster opened this issue 8 years ago · 4 comments

I confused myself for a good 20 minutes today, trying to get the difference between two temperatures. They were both around room temp, but the result was coming up as -272 degrees C.

it took me a while to realise that the difference was 1 degree, and if you interpret that as Kelvin, you get -272 degrees C. I don't know what the correct answer is, but I'm not sure allowing native subtraction and addition for temperatures makes sense.

Answer 1 · 2017-02-23T12:36:13.000Z

@thejpster Maybe some example code would help clarify this case? I'm not sure I fully understand what happened, but I see how there could be an issue.

Internally, all the units are treated as some kind of floating point base unit. For temperature I believe it's degrees Celsius. Everything else is a conversion of some kind.

For really highly precise measurements or those that vary greatly in magnitude, I'm sure this is bound to cause a whole load of problems.

Answer 2 · 2017-02-23T12:49:27.000Z

Ah. I figured it out at lunch today. We need to distinguish between absolute temperature and relative temperature. 21°C - 20°C != 1 °K, it's 1°. We can do that in the type system. (It's Kelvin internally as it happens).

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On Thu, 23 Feb 2017, 12:36 jocull, ***@***.***> wrote: @thejpster <https://github.com/thejpster> Maybe some example code would help clarify this case? I'm not sure I fully understand what happened, but I see how there could be an issue. Internally, all the units are treated as some kind of floating point base unit. For temperature I believe it's degrees Celsius. Everything else is a conversion of some kind. For really highly precise measurements or those that vary greatly in magnitude, I'm sure this is bound to cause a whole load of problems. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#2 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AA6ljyOY-EZeGSP-uw30Ltm6ZyWihAzLks5rfX09gaJpZM4MGrPH> .

Answer 3 · 2017-02-23T12:52:43.000Z

Ah! That makes sense... I think, haha. As I wrote in the README I'm pretty far from any kind of math or science expert so I'm sure cases like this where I didn't understand the units are not isolated.

Answer 4 · 2017-04-11T21:48:12.000Z

Added a TemperatureDelta type - you can add/subtract to/from a Temperature to get another Temperature and subtract two Temperatures to get a TemperatureDelta.