Numpy:finding Difference Between Two Files Containing Float Values

I am trying to use Python to compute the difference between two text files and print the first value and location where they start to diverge. I am not sure how to use loadtxt: i

Solution 1:

You could use

idx = np.where(np.abs(a-b) > 1e-6)[0]
firstidx = idx[0]

to find the first index where the values in a and b differ by more than some nominal amount like 1e-6:

import numpy as np

a = np.loadtxt("path/to/file", float)
b = np.loadtxt("path/to/file2", float)

idx = np.where(np.abs(a-b) > 1e-6)[0]
firstidx = idx[0]
print(firstidx, a[firstidx], b[firstidx])

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Note that when dealing with floats, you rarely if ever want to compare with equality, such as with

np.abs(a-b) == 0

or the converse,

np.abs(a-b) != 0

because the inaccuracy of floating point representations can cause a and b to be slightly different even when their values should be exactly the same if their values were represented with infinite precision.

So use something like

np.abs(a-b) > 1e-6

instead. (Note that you have to choose a level of tolerance, e.g. 1e-6).

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Here is a simple example demonstrating the pitfall of comparing floats using equality:

In [10]: 1.2-1.0==0.2Out[10]: False