keeping only elements in a list at a certain distance at least - changing iterator while looping - Python

Question

as the title suggests, I have developed a function that, given an ORDERED ascending list, you keep only the elements which have a distance of at least k periods but it does so while dynamically changing the iterator while looping. I have been told this is to be avoided like the plague and, though I am not fully convinced as to why this is such a bad idea, I trust in those whom I have been leaning on for training and thus asking for advice on how to avoid such practice. The code is the following:

import pandas as pd
from datetime import days
a = pd.Series(range(0,25,1), index=pd.date_range('2011-1-1',periods=25))
store_before_cleanse = a.index

def funz(x,k):
    i = 0
    while i < len(x)-1:
        if (x[i+1]-x[i]).days < k:
            x = x[:i+1] + x[i+2:]
            i = i-1
        i = i + 1
    return x

print(funz(store_before_cleanse,10))

what do you think can be done in order to avoid it? p.s.: do not worry about solutions in which the list is not ordered. the list that will be given will always be ordered in an ascending fashion.

Answer 1

The biggest default of your function his to have a quadratic complexity, since x = x[:i+1] + x[i+2:] copy the whole x each time.

The simplest an more efficient way to do that want is probably

a.resample('10D').first().index.

If you prefer a loop you can just do :

def funz1(dates,k):
    result=[dates[0]]
    for date in dates:
        if (date-result[-1]).days >= k:
            result.append(date)
    return result