How can I change values for a section of a numpy array?

Question

My question is a little more specific than that actually. Consider the following arrays:

from numpy import zeros, ones

array1 = ones((3, 3), bool)
array1[0][0] = 0
array1[0][2] = 0
array1[2][0] = 0
array1[2][2] = 0

array2 = zeros((12, 12), bool)

Now what I'm looking for is a way that I can refer to a 2 dimensional portion of array2 of the same proportions as array1 so that I can add the positive values from array1 to it. I know there are ways I can do this using loops, but I'd prefer to have a single statement like array2[(some way of getting a 3x3 portion of array2)] |= array1

Croad Langshan · Accepted Answer · 2011-04-22 23:13:53Z

The output from an example using floats seems easier to understand:

>>> a1 = numpy.ones((3, 3))
>>> a2 = numpy.ones((12, 12))
>>> a2[:3,:3] += a1
>>> a2
array([[ 2.,  2.,  2.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.],
       [ 2.,  2.,  2.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.],
       [ 2.,  2.,  2.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.],
       [ 1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.],
       [ 1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.],
       [ 1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.],
       [ 1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.],
       [ 1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.],
       [ 1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.],
       [ 1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.],
       [ 1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.],
       [ 1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.]])

Also, note you can do things like this:

>>> a2[slice(None, a1.shape[0]), slice(None, a1.shape[1])]
array([[ 2.,  2.,  2.],
       [ 2.,  2.,  2.],
       [ 2.,  2.,  2.]])

YXD · Accepted Answer · 2011-04-22 23:23:24Z

2

array2[start:end,start:end] |= array1

answered Apr 22, 2011 at 23:23

YXD

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