[Numpy-discussion] contiguous true

[Robert Kern]

On Fri, Feb 29, 2008 at 11:53 AM, John Hunter <jdh2358 at gmail.com> wrote:
> [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
>  Out[102]:
>  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 smallish N (< 100 perhaps), I'd do something like this:


In [57]: from numpy import *

In [58]: prng = random.RandomState(1234567890)

In [59]: x = prng.random_sample(50) < 0.5

In [60]: x
Out[60]:
array([False, False, False, False,  True, False,  True, False, False,
       False,  True, False,  True, False,  True,  True,  True,  True,
        True, False, False, False,  True, False,  True, False, False,
       False,  True,  True,  True,  True, False, False,  True, False,
       False, False, False, False, False, False, False,  True, False,
       False,  True, False,  True, False], dtype=bool)

In [61]: N = 2

In [62]: mask = ones(len(x) - N + 1, dtype=bool)

In [63]: for i in range(N):
   ....:     mask &= x[i:len(x)-N+1+i]
   ....:
   ....:

In [64]: mask
Out[64]:
array([False, False, False, False, False, False, False, False, False,
       False, False, False, False, False,  True,  True,  True,  True,
       False, False, False, False, False, False, False, False, False,
       False,  True,  True,  True, False, False, False, False, False,
       False, False, False, False, False, False, False, False, False,
       False, False, False, False], dtype=bool)

In [65]: nonzero(mask)[0][0]
Out[65]: 14

In [66]: x[13:20]
Out[66]: array([False,  True,  True,  True,  True,  True, False], dtype=bool)


-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless
enigma that is made terrible by our own mad attempt to interpret it as
though it had an underlying truth."
  -- Umberto Eco