Small python quirk: Finding the index of first and last number within boundaries in a list - list

Question

I have encountered a small annoying problem. My problem is this:
I have a series of number between 0 and 1:
[0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9]
and two boundaries, say 0.25 and 0.75.
I need a quick and pretty way to find the index of the first number and last number in the series, that are within the boundaries, in this case (2, 6)
I have so far only come up with a clumsy way using for loops and the break command.
thanks in advance for any help!

Answer 1

If your series of numbers is always sorted, you can use the bisect module to perform binary search for the endpoints:
>>> a = [.1, .2, .3, .4, .5, .6, .7, .8, .9]
>>> import bisect
>>> bisect.bisect_left(a, 0.25)
2
>>> bisect.bisect_right(a, 0.75) - 1
6
bisect_left(a, x) returns the position p such that every element of a[:p] is less than x, and every element of a[p:] is greater than or equal to x; this is exactly what you want for the lower bound.
bisect_right returns the position p such that every element of a[:p] is less than or equal to x, and a[p:] are all greater than x. So for the right bound, you need to subtract one to get the largest position <= x.

Answer 2

if you can use numpy:
import numpy as np
data = np.array([0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9])
max_b = .75
min_b = .25
wh = np.where((data < max_b)*(data > min_b))[0]
left, right = wh[0], wh[-1] + 1
or simply (thanks to dougal):
left, right = np.searchsorted(data, [min_b, max_b])
if you can't:
import bisect
data = [0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9]
max_b = .75
min_b = .25
left = bisect.bisect_left(data, min_b)
right = bisect.bisect_right(data, max_b)
plus or minus 1 on the right depending on if you want data[right] to be in the set, or data[left:right] to give you the set.