FriskyGrub FriskyGrub - 1 year ago 166
Python Question

How can I create a slice object for Numpy array?

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?

Answer Source

Yes you can use numpy.s_:


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

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.

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