How can I add small 2D array to larger array?

Question

I have a larger 2D array, and I would like to add a smaller 2D array.

from numpy import *
x = range(25)
x = reshape(x,(5,5))
print x
[[ 0  1  2  3  4]
 [ 5  6  7  8  9]
 [10 11 12 13 14]
 [15 16 17 18 19]
 [20 21 22 23 24]]
y = [66,66,66,66]
y = reshape(y,(2,2))
print y
[[66 66]
 [66 66]]

I would like to add the values from array y to x starting at 1,1 so that x looks like this:

[[ 0  1  2  3  4]
 [ 5 72 73  8  9]
 [10 77 78 13 14]
 [15 16 17 18 19]
 [20 21 22 23 24]]

Is this possible with slicing? Can someone please suggest the correct formatting of the slice statement to achieve this?

Thanks

ArtemB · Accepted Answer · 2013-11-23 00:46:07Z

12

Yes, you can use slicing on numpy arrays:

In [20]: x[1:3,1:3] += y

In [21]: print x
[[ 0  1  2  3  4]
 [ 5 72 73  8  9]
 [10 77 78 13 14]
 [15 16 17 18 19]
 [20 21 22 23 24]]

answered Nov 23, 2013 at 0:46

ArtemB

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user2357112 · Accepted Answer · 2013-11-23 00:45:38Z

8

x[1:3, 1:3] += y

Add y in-place to the slice of x you want to modify. Note that numpy indexing counts from 0, not 1. Also, note that for this particular choice of y,

x[1:3, 1:3] += 66

will achieve the same effect in a simpler manner.

answered Nov 23, 2013 at 0:45

user2357112

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2 Comments

the dood Over a year ago

Same solution s suggested by @ArtemB. Thanks fro the quick reply.

ArtemB Over a year ago

+1 for pointing that a scalar could've been used. It would potentially be faster for large operations.

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