[Numpy-discussion] searchsorted
Perry Greenfield
perry at stsci.edu
Tue Jun 1 09:17:00 CDT 2004
Nadav Horesh writes
> I am currently working on a simulation that makes a heavy use of
> searchsorted. But it does not precisely fit to what I need --- if a
> value v is between p and q searchsorted returns the index of q, while
> what I need is the index of p.
>
> Currently my solution is to turn to floating points numbers:
>
> ======================================
>
> Python 2.3.4 (#1, May 31 2004, 09:13:03)
> [GCC 3.4.0] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
> from numarray import *
>
> bins = array((0,10,20,30))
> val = array((10, 15))
> searchsorted(bins, val)
> array([1, 2]) # I really would like to get array([1, 1])
>
> # Here is the trick:
>
> fb = bins - 0.1
> fb
> array([ -0.1, 9.9, 19.9, 29.9])
>
> searchsorted(fb, val) - 1
> array([1, 1]) # That's it!
>
This is only approximate, right? If val = array([9.95, 15])
you will get the wrong answer won't you?
> ============================================
>
> My questions are:
>
> 1. Is there a more elegant solution?
> 2. I am thinking of letting "searchsorted" return a second boolean
> array which has the value True for every exact match:
> >>> searchsorted(bins, val)
> >>> [array([1, 2]), array([1, 0], type=Bool)]
> Any comments?
>
> Nadav.
>
To get the latter, you could so something like
ind = searchsorted(bins, val)
neq_mask = bins[ind]-val
ind[neq_mask] -= 1. # well, you need to handle where ind = 0 and
# is not equal as well
Would that suffice?
Perry
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