Python: scale conversion

Let’s suppose we have two different scales, one ranging from A to B and the other from C to D.

Now, we have a value X in the range A <= X <= B and we want to convert it into the other scale.

Here it is a function to do that.

def rescale(X,A,B,C,D,force_float=False):
    retval = ((float(X - A) / (B - A)) * (D - C)) + C
    if not force_float and all(map(lambda x: type(x) == int, [X,A,B,C,D])):
        return int(round(retval))
    return retval

Source code: [[attachment:rescale.py]]

Example: Celsius <-> Fahrenheit conversion

An example application of this function, is conversion between Celsius and Fahrenheit degrees, assuming we don’t know the conversion ratio / delta, but just two fixed points in the scale.

Knowing, for example, that 0 °C = 32 °F and 100 °C = 212 °F, we can do some conversions:

°C to °F

>>> rescale(20,0,100,32,212,True)
68.0
>>> rescale(200,0,100,32,212,True)
392.0
>>> rescale(-5,0,100,32,212,True)
23.0

°F to °C

>>> rescale(0,32,212,0,100,True)
-17.777777777777779
>>> rescale(23,32,212,0,100,True)
-5.0
>>> rescale(392,32,212,0,100,True)
200.0
>>> rescale(68,32,212,0,100,True)
20.0

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