[Numpy-discussion] Bin selection
Todd Miller
jmiller at stsci.edu
Tue Dec 31 05:25:04 EST 2002
Magnus Lie Hetland wrote:
>I have a set of limits, e.g. array([0, 35, 45, 55, 75]) and I want to
>use these to "classify" a set of numbers (another one-dimensional
>array). The "class" is the number of the first limit that is lower
>than or equal to the number I want to classify. E.g. I'd classify 17
>as 0 and 42 as 1.
>
>My current approach is:
>
> sum(nums[:,NewAxis] >= lims, dim=-1)
>
>It seems a bit unnecessary to compare each number with all the limits
>when O(log(n)) would suffice (the limits are ordered); or even with
>O(n) running time, a smarter implementation could get an average of
>n/2 comparisons...
>
>Suggestions?
>
>
>
Try searchsorted(). Searchsorted returns the index of the first bin >=
the number being classified and has O(log(n)) running time.
>>> a=numarray.array([0, 35, 45, 55, 75])
>>> numarray.searchsorted(a, [1,42,35])
array([1, 2, 1])
Todd
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