How to count number of consecutive Trues in a numpy array along a given axis?

Question

I have for example the following np array

array([[ True, False, False],
       [False,  True,  True],
       [ True,  True,  True],
       [False,  True,  True],
       [False,  True,  True],
       [ True, False, False],
       [ True,  True, False],
       [False, False,  True]])

I want to count the number of consecutive Trues along the columns, so in the above example, the first column contains 3 blocks of consecutive Trues, the second column contains 2 blocks, and the third contains 2 blocks. The output should then be

array([3, 2, 2])

I know I can do loops for each columns, like in this answer for a one-dimensional array, but what is a numpy-way for doing this on a 2-d array?

Asked By: TTY

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Source

Answer 1

Use boolean arithmetic to identify the True that are not followed by a True (using slicing and pad), then sum the True per column:

out = ((a != np.pad(a[1:], ((0,1), (0,0)), constant_values=False))
       & a).sum(axis=0)

Or:

out = (a & ~np.pad(a[1:], ((0,1), (0,0)), constant_values=False)).sum(axis=0)

Output:

array([3, 2, 2])

Answered By: mozway

Answer 2

You could do this by comparing elements with their successor along the first axis and adding up the transitions from False to True to the True values of the first row:

A[0,:] + (A[:-1,:]<A[1:,:]).sum(axis=0)

Answered By: Alain T.

How to count number of consecutive Trues in a numpy array along a given axis?

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