[Numpy-discussion] contiguous true

John Hunter jdh2358@gmail....
Fri Feb 29 11:53:09 CST 2008

[apologies if this is a resend, my mail just flaked out]

I have a boolean array and would like to find the lowest index "ind"
where N contiguous elements are all True.  Eg, if x is

In [101]: x = np.random.rand(20)>.4

In [102]: x
array([False,  True,  True, False, False,  True,  True, False, False,
        True, False,  True, False,  True,  True,  True, False,  True,
       False,  True], dtype=bool)

I would like to find ind=1 for N=2 and ind=13 for N=2.  I assume with
the right cumsum, diff and maybe repeat magic, this can be vectorized,
but the proper incantation is escaping me.

for N==3, I thought of

In [110]: x = x.astype(int)
In [112]: y = x[:-2] + x[1:-1] + x[2:]

In [125]: ind = (y==3).nonzero()[0]

In [126]: if len(ind): ind = ind[0]

In [128]: ind
Out[128]: 13


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