从1d数组中的值开始切片2d数组

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 .

Answer 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))