Insert a 2d array into another 2d array at multiple positions

Question

I have an array like this, although much larger:

X = np.array([
    [1, 2, 3, 4, 5],
    [1, 2, 3, 4, 5],
    [1, 2, 3, 4, 5],
]);

and I want to insert a 2d array every k positions. For example:

dim = 2
to_insert = np.zeros((dim, X.shape[1]))

I'm using this code, but it's too slow for the size of the arrays I'm using:

for k in range(X.shape[0]):                                                                                              
    X = np.insert(X, k*(dim+1), to_insert, axis=0)  
                                                                     
#[[0 0 0 0 0]
# [0 0 0 0 0]
# [1 2 3 4 5]
# [0 0 0 0 0]
# [0 0 0 0 0]
# [1 2 3 4 5]
# [0 0 0 0 0]
# [0 0 0 0 0]
# [1 2 3 4 5]]

I am trying to do it using np.insert:

positions = [k for k in range(X.shape[0])]                                                                               
X = np.insert(X, positions, to_insert, axis=0)

But it only works with dim=1. For dim>1 I get the error:

  File "./test.py", line 25, in <module>
    X = np.insert(X, positions, to_insert, axis=0)
  File "/usr/local/lib/python3.6/dist-packages/numpy/lib/function_base.py", line 4629, in insert
    new[tuple(slobj)] = values
ValueError: shape mismatch: value array of shape (2,5) could not be broadcast to indexing result of shape (3,5)

What is the most efficient way to do it?

mozway · Accepted Answer · 2022-09-23 16:24:45Z

1

Using a different example for clarity:

# X
array([[ 0,  1,  2,  3,  4],
       [ 5,  6,  7,  8,  9],
       [10, 11, 12, 13, 14]])

# to_insert
array([[ -1,  -2,  -3,  -4,  -5],
       [ -6,  -7,  -8,  -9, -10]])

You can tile a destination array and use indexing to "insert" the rows of X:

Y = np.tile(np.r_[to_insert, np.zeros((1, X.shape[1]))], (X.shape[0], 1))
Y[to_insert.shape[0]::to_insert.shape[0]+1] = X

output Y:

array([[ -1.,  -2.,  -3.,  -4.,  -5.], # to_insert
       [ -6.,  -7.,  -8.,  -9., -10.], #
       [  0.,   1.,   2.,   3.,   4.],     # X row 0
       [ -1.,  -2.,  -3.,  -4.,  -5.], # to_insert
       [ -6.,  -7.,  -8.,  -9., -10.], #
       [  5.,   6.,   7.,   8.,   9.],     # X row 1
       [ -1.,  -2.,  -3.,  -4.,  -5.], # to_insert
       [ -6.,  -7.,  -8.,  -9., -10.], #
       [ 10.,  11.,  12.,  13.,  14.]])    # X row 2

edited Sep 23, 2022 at 16:24

answered Sep 23, 2022 at 13:07

mozway

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