Slicing 2d array from values in 1d array onwards

Question

Having an arbitrary 2d array, say of zeros, and an array of indices:

z = np.zeros((5,5))
ix = np.array([1,4,2,3,0])

How could I add a 1 from the columns specified by a 1d array onwards, in order to obtain:

array([[0, 1, 1, 1, 1],
       [0, 0, 0, 0, 1],
       [0, 0, 1, 1, 1],
       [0, 0, 0, 1, 1],
       [1, 1, 1, 1, 1]])

I have not been able to find a simple way of doing so using numpy.

Divakar · Accepted Answer · 2019-04-19 13:56:58Z

1

One way would be -

In [50]: ncols = 5

In [51]: (ix[:,None] <= np.arange(ncols)).view('i1')
Out[51]: 
array([[0, 1, 1, 1, 1],
       [0, 0, 0, 0, 1],
       [0, 0, 1, 1, 1],
       [0, 0, 0, 1, 1],
       [1, 1, 1, 1, 1]], dtype=int8)

If you have to add to an existing array z -

z += (ix[:,None] <= np.arange(ncols))

answered Apr 19, 2019 at 13:56

Divakar

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3 Comments

yatu Over a year ago

Thanks a lot! Pretty nice solution. One doubt, what is the reason of using view here rather than say astype?

Divakar Over a year ago

@yatu Just for memory efficiency and hence performance, being a view.

yatu Over a year ago

Ah of course, astype is creating a copy of the array. Got it, thanks!

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