Counting number of values between interval

Question

Is there any efficient way in python to count the times an array of numbers is between certain intervals? the number of intervals i will be using may get quite large

like:

mylist = [4,4,1,18,2,15,6,14,2,16,2,17,12,3,12,4,15,5,17]

some function(mylist, startpoints):
   # startpoints = [0,10,20]
   count values in range [0,9]
   count values in range [10-19]

output = [9,10]

Answer 1

you will have to iterate the list at least once.

The solution below works with any sequence/interval that implements comparision (<, >, etc) and uses bisect algorithm to find the correct point in the interval, so it is very fast.

It will work with floats, text, or whatever. Just pass a sequence and a list of the intervals.

from collections import defaultdict
from bisect import bisect_left

def count_intervals(sequence, intervals):
    count = defaultdict(int)
    intervals.sort()
    for item in sequence:
        pos = bisect_left(intervals, item)
        if pos == len(intervals):
            count[None] += 1
        else:
            count[intervals[pos]] += 1
    return count

data = [4,4,1,18,2,15,6,14,2,16,2,17,12,3,12,4,15,5,17]
print count_intervals(data, [10, 20])

Will print

defaultdict(<type 'int'>, {10: 10, 20: 9})

Meaning that you have 10 values <10 and 9 values <20.