Find the previous number in a list of float and integer

Question

i have a list of float and integers like below

[2,4.635,9,18,27.548,36]

For a given input i want to find the previous number of that input in the list by comparing that number with the numbers in list For example: if i give 8.9 as input i should get 4.635 as the output, is there any builtin function for this

Answer 1

You can use bisect to find the insertion point of a value in a sorted list:

import bisect 
li=[2,4.635,9,18,27.548,36]

>>> li[bisect.bisect_left(li,8.9)-1]    
4.635     

You could also just use a simple for loop and zip on a sorted list as well:

for lo,hi in zip(li,li[1:]):
    if lo< 8.9 <=hi:
        print lo 
        break   

Answer 2

I cannot remember any builtin method for this, but it is quite easy to implement:

def get_previous(l, query):
    return max([x for x in l if x<query])

query = 8.9
l = [2,4.635,9,18,27.548,36]
result = get_previous(l, query)

If the list is sorted, there could be better ways to achieve this (binary search), but you don't state that fact.