>>> import numpy as np
>>> a=np.arange(0,2,0.2)
>>> a
array([ 0. , 0.2, 0.4, 0.6, 0.8, 1. , 1.2, 1.4, 1.6, 1.8])
>>> a=a.tolist()
>>> a
[0.0, 0.2, 0.4, 0.6000000000000001, 0.8, 1.0, 1.2000000000000002, 1.4000000000000001, 1.6, 1.8]
>>> a.index(0.6)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: 0.6 is not in list
it appears that some values in the list have changed and I can't find them with index(). How can I fix that?
0.6 hasn't changed; it was never there:
>>> import numpy as np
>>> a = np.arange(0, 2, 0.2)
>>> a
array([ 0. , 0.2, 0.4, 0.6, 0.8, 1. , 1.2, 1.4, 1.6, 1.8])
>>> 0.0 in a
True # yep!
>>> 0.6 in a
False # what?
>>> 0.6000000000000001 in a
True # oh...
The numbers in the array are rounded for display purposes, but the array really contains the value you subsequently see in the list; 0.6000000000000001. 0.6 cannot be precisely represented as a float, therefore it is unwise to rely on floating-point numbers comparing precisely equal!
One way to find the index is to use a tolerance approach:
def float_index(seq, f):
for i, x in enumerate(seq):
if abs(x - f) < 0.0001:
return i
which will work on the array too:
>>> float_index(a, 0.6)
3
