Question

If I have a list of numpy arrays, then using remove method returns a value error.

For example:

import numpy as np

l = [np.array([1,1,1]),np.array([2,2,2]),np.array([3,3,3])]

l.remove(np.array([2,2,2]))

Would give me

ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

I can’t seem to get the all() to work, is it just not possible?

Answer 1

The problem here is that when two numpy arrays are compared with ==, as in the remove() and index() methods, a numpy array of boolean values (the element by element comparisons) is returned which is interpretted as being ambiguous. A good way to compare two numpy arrays for equality is to use numpy’s array_equal() function.

Since the remove() method of lists doesn’t have a key argument (like sort() does), I think that you need to make your own function to do this. Here’s one that I made:

def removearray(L,arr):
    ind = 0
    size = len(L)
    while ind != size and not np.array_equal(L[ind],arr):
        ind += 1
    if ind != size:
        L.pop(ind)
    else:
        raise ValueError('array not found in list.')

If you need it to be faster then you could Cython-ize it.