Comment by cranium
6 years ago
One thing I got bitten by was the handling of Fahrenheit/Celsius, because it's a non-linear conversion between the two. When you ask to convert `10 degC` to `degF` you get 18, which is the delta of ºF corresponding to increment of 10ºC. To get the absolute temperature, you have to ask to convert `tempC(10)` to `tempF` which is 50, as expected.
"Non-linear" threw me off for a second - I almost never see the mathematically correct definition of linear in computer science spaces. For anyone wondering, Celsius to Fahrenheit is an affine transform, technically not linear, because you have to add an offset, not just multiply.
On the other hand, an equation of the form y = a x + b is a linear equation. If you have Celsius and want Fahrenheit you accomplish that by applying a linear equation (F = 1.8 C + 32), so I certainly can't fault people for saying that the transformation they are doing is linear.
I wonder what people would say for something using an equation of the form y = a x^2 + b x + c to transform something? I can't say that I've heard anyone talk of quadratic transformations. On the other hand, I can't think of ever transforming anything with a quadratic equation, so never had the need t speak of it.
(Also, he called it a linear conversion, not a linear transformation).
FWIW, units on macOS (not GNU) handles conversion of `10 degC` to `degF` correctly, although it dates back to 1993.
It seems that GNU units at some point added support for several non-linear units, which may have prompted them to rethink their syntax.