indexing lists/arrays question

[Neil Cerutti]

On 2010-05-13, a <oxfordenergyservices at googlemail.com> wrote:
> this must be easy but its taken me a couple of hours already
>
> i have
>
> a=[2,3,3,4,5,6]
>
> i want to know the indices where a==3 (ie 1 and 2)
>
> then i want to reference these in a
>
> ie what i would do in IDL is
>
> b=where(a eq 3)
> a1=a(b)
>
> any ideas?

For a sorted sequence the bisect module is a good start.

>>> start = bisect.bisect_left(a, 3)
>>> end = bisect.bisect_right(a, 3, bs)
>>> b = list(range(start, end))
>>> b
[1, 2]

If the list isn't necessarily sorted, try filter and enumerate.

>>> b = [a for a,b in filter(lambda x: x[1]==3, enumerate(a))]
>>> b
[1, 2]

-- 
Neil Cerutti
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