Numpy: copying numpy array at specific indexes

Question

I have arrays like

arr1['a'] = np.array([1, 1, 1])
arr1['b'] = np.array([1, 1, 1])
arr1['c'] = np.array([1, 1, 1])

b_index = [0, 2, 5]

arr2['a'] = np.array([2, 2, 2, 2, 2, 2])
arr2['b'] = np.array([2, 2, 2, 2, 2, 2])
arr2['c'] = np.array([2, 2, 2, 2, 2, 2])
arr2['f'] = np.array([2, 2, 2, 2, 2, 2])

b_index is the list of indexes. I want to copy from arr1 to arr2 at indexes in b_index. so the result should be something like

arr2['a'] = np.array([1, 2, 1, 2, 2, 1])
arr2['b'] = np.array([1, 2, 1, 2, 2, 1])
arr2['c'] = np.array([1, 2, 1, 2, 2, 1])
arr2['f'] = np.array([2, 2, 2, 2, 2, 2])

I can obviously do using loops, but not sure if that is a right way to do that. We are talking about 100 columns('a','b','c') and around a 1 million rows.

Big Bro · Accepted Answer · 2021-05-04 10:33:26Z

1

One solution, which might not be optimal, is to use advanced array indexing:

In [1]: arr = np.ones((5, 3))

In [2]: arr2 = np.full((5, 5), 2)

In [3]: arr2[:, [1, 2, 4]] = arr

In [4]: arr2
Out[4]:
array([[2, 1, 1, 2, 1],
       [2, 1, 1, 2, 1],
       [2, 1, 1, 2, 1],
       [2, 1, 1, 2, 1],
       [2, 1, 1, 2, 1]])

Does it help ?

answered May 4, 2021 at 10:33

Big Bro

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