Add a new column to a numpy array with values based on list of indices

Question

Having an original numpy array like:

a = np.array([
    [12, 43],
    [42, 23],
    [33, 22],
    [53, 15],
    [34, 31]
])

I want to add a new column to a based on an array of indices:

indices = np.array([2, 3])

such that a becomes:

# if index is in "indices", new value is 1, otherwise 0

a = np.array([
    [12, 43, 0],
    [42, 23, 0],
    [33, 22, 1],
    [53, 15, 1],
    [34, 31, 0]
])

Note: I cannot use Pandas in this context, it has to be a pure numpy solution

Andy L. · Accepted Answer · 2020-06-07 18:39:20Z

1

Create a new nx1 zeros array and assign indices location to 1 and column_stack

b = np.zeros((a.shape[0],1))
b[indices] = 1
c = np.column_stack([a, b])

Out[28]:
array([[12., 43.,  0.],
       [42., 23.,  0.],
       [33., 22.,  1.],
       [53., 15.,  1.],
       [34., 31.,  0.]])

answered Jun 7, 2020 at 18:39

Andy L.

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