Numpy: expanding subarray of a larger array

Question

I have a Numpy array like this,

[[1 2 3
  2 0 0
  3 0 0]]

and I want to turn it into this form,

[[1 2 2 3 3
  2 0 0 0 0
  2 0 0 0 0
  3 0 0 0 0
  3 0 0 0 0]]

My idea is to extract the sub-array contains zero from the original array, and use Kronecker product to get the sub-array, which is in the array I want. But I have no idea to get the first row and column of the output array.

How to achieve this goal? Please give me any suggestions.

I adjusted the title as you suggested in your question - do feel free to change it back or improve it further if you feel it doesn't quite fit with what you're asking. — Alex Riley
– Alex Riley, Commented Feb 5, 2015 at 15:29

Alex Riley · Accepted Answer · 2015-02-05 16:42:57Z

3

Another way would be to use np.repeat. If arr is your 3x3 array:

>>> arr.repeat([1, 2, 2], axis=0).repeat([1, 2, 2], axis=1)
array([[1, 2, 2, 3, 3],
       [2, 0, 0, 0, 0],
       [2, 0, 0, 0, 0],
       [3, 0, 0, 0, 0],
       [3, 0, 0, 0, 0]])

For example, arr.repeat([1, 2, 2], axis=0) means that the first row of arr is repeated once, the second row is repeated twice and the third row is repeated three times.

The same thing is then done for the columns.

edited Feb 5, 2015 at 16:42

answered Feb 5, 2015 at 14:11

Alex Riley

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2 Comments

Heinz Over a year ago

Thanks for replying me with this helpful answer, but if there are many rows, say, 20 in the input array, that will be inconvenient to get the output array via using these codes. And I'm still thinking about how to solve this problem.

Alex Riley Over a year ago

@Heinz: if arr has 20 rows, you could always do, for example, arr.repeat([1]+[2]*19, axis=0) to repeat each row after the first row two times.

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