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Find index of numpy array in list

Can someone explain why the following is happening? My use case is that I have a python list whose elements are all numpy ndarray objects and I need to search through the list to find the index of a particular ndarray obj.

Simplest example:

>>> import numpy as np
>>> a,b = np.arange(0,5), np.arange(1,6)
>>> a
array([0, 1, 2, 3, 4])
>>> b
array([1, 2, 3, 4, 5])
>>> l = list()
>>> l.append(a)
>>> l.append(b)
>>> l.index(a)
0
>>> l.index(b)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

Why l

can find the index a

, but not b

?

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2 answers


Applying the idea at fooobar.com/questions/1491903 / ... (see the "Linked side" section)

[np.array_equal(b,x) for x in l].index(True)


should be more reliable. It provides the correct array for array comparison.

Or [id(b)==id(x) for x in l].index(True)

if you want it to compare IDs.

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The idea is to convert the numpy arrays to lists and convert the problem to finding a list in another list:



def find_array(list_of_numpy_array,taregt_numpy_array):
   out = [x.tolist() for x in list_of_numpy_array].index(taregt_numpy_array.tolist())
   return out
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