How can I create a slice object for Numpy array?

Question

I’ve tried to find a neat solution to this, but I’m slicing several 2D arrays of the same shape in the same manner. I’ve tidied it up as much as I can by defining a list containing the ‘x,y’ center e.g. cpix = [161, 134] What I’d like to do is instead of having to write out the slice three times like so:

a1 = array1[cpix[1]-50:cpix[1]+50, cpix[0]-50:cpix[0]+50] 
a2 = array2[cpix[1]-50:cpix[1]+50, cpix[0]-50:cpix[0]+50] 
a3 = array3[cpix[1]-50:cpix[1]+50, cpix[0]-50:cpix[0]+50]

is just have something predefined (like maybe a mask?) so I can just do a

a1 = array1[predefined_2dslice] 
a2 = array2[predefined_2dslice] 
a3 = array3[predefined_2dslice]

Is this something that numpy supports?

Asked By: FriskyGrub

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Source

Answer 1

Yes you can use numpy.s_:

Example:

>>> a = np.arange(10).reshape(2, 5)
>>> 
>>> m = np.s_[0:2, 3:4]
>>> 
>>> a[m]
array([[3],
       [8]])

And in this case:

my_slice = np.s_[cpix[1]-50:cpix[1]+50, cpix[0]-50:cpix[0]+50]

a1 = array1[my_slice] 
a2 = array2[my_slice] 
a3 = array3[my_slice]

You can also use numpy.r_ in order to translates slice objects to concatenation along the first axis.

Answered By: Mazdak

Answer 2

You can index a multidimensional array by using a tuple of slice objects.

window = slice(col_start, col_stop), slice(row_start, row_stop)
a1 = array1[window]
a2 = array2[window]

This is not specific to numpy and is simply how subscription/slicing syntax works in python.

class mock_array:
    def __getitem__(self, key):
        print(key)
m = mock_array()
m[1:3, 7:9] # prints tuple(slice(1, 3, None), slice(7, 9, None))

Answered By: benjimin

How can I create a slice object for Numpy array?

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