rlee827 rlee827 - 21 days ago 5
Python Question

Copy numpy array into part of another array

If I run the following:

import numpy as np
a = np.arange(9)
a = a.reshape((3,3))


I will get this:

a = [[0 1 2]
[3 4 5]
[6 7 8]]


If I create a larger array like this:

b = np.zeros((5,5))
b = [[ 0. 0. 0. 0. 0.]
[ 0. 0. 0. 0. 0.]
[ 0. 0. 0. 0. 0.]
[ 0. 0. 0. 0. 0.]
[ 0. 0. 0. 0. 0.]]


How do I efficiently copy
a
into
b
to get an array like this?

# border of 0 surrounding a to be filled in with other data later
b = [[ 0. 0. 0. 0. 0.]
[ 0. 1. 2. 3. 0.]
[ 0. 4. 5. 6. 0.]
[ 0. 7. 8. 9. 0.]
[ 0. 0. 0. 0. 0.]]


I am looking for a function built into
numpy
if it exists.

Answer

You can specify b[1:4, 1:4] to denote the part:

>>> import numpy as np
>>> a = np.arange(9)
>>> a = a.reshape((3, 3))
>>> b = np.zeros((5, 5))
>>> b[1:4, 1:4] = a
>>> b
array([[ 0.,  0.,  0.,  0.,  0.],
       [ 0.,  0.,  1.,  2.,  0.],
       [ 0.,  3.,  4.,  5.,  0.],
       [ 0.,  6.,  7.,  8.,  0.],
       [ 0.,  0.,  0.,  0.,  0.]])

>>> b[1:4,1:4] = a + 1  # If you really meant `[1, 2, ..., 9]`
>>> b
array([[ 0.,  0.,  0.,  0.,  0.],
       [ 0.,  1.,  2.,  3.,  0.],
       [ 0.,  4.,  5.,  6.,  0.],
       [ 0.,  7.,  8.,  9.,  0.],
       [ 0.,  0.,  0.,  0.,  0.]])