How to return indices of values between two numbers in numpy array

Question

I would like to return the indices of all the values in a python numpy array that are between two values. Here is my code:

inEllipseIndFar = np.argwhere(excessPathLen * 2 < ePL < excessPathLen * 3)

But it returns an error:

inEllipseIndFar  = np.argwhere((excessPathLen * 2 < ePL < excessPathLen * 3).all())
ValueError: The truth value of an array with more than one element is ambiguous. Use 
a.any() or a.all()

I'd like to know if there is a way of doing this without iterating through the array. Thanks!

Answer 1

You can combine multiple boolean expressions by using parentheses and the correct operation:

In [1]: import numpy as np

In [2]: A = 2*np.arange(10)

In [3]: np.where((A > 2) & (A < 8))
Out[3]: (array([2, 3]),)

You can also set the result of np.where to a variable to extract the values:

In [4]: idx = np.where((A > 2) & (A < 8))

In [5]: A[idx]
Out[5]: array([4, 6])

Answer 2

Since > < = return masked arrays, you can multiply them together to get the effect you are looking for (essentially the logical AND):

>>> import numpy as np
>>> A = 2*np.arange(10)
array([ 0,  2,  4,  6,  8, 10, 12, 14, 16, 18])

>>> idx = (A>2)*(A<8)
>>> np.where(idx)
array([2, 3])