Return elements between specific indexes

Question

Imagine a situation where you have a list of elements for example: arr = ["h4", "p", "p", "h4", "p", "p", "p"] and list of indexes where h4 value is: idx = [0, 3]. My goal is to print out values between these indexes, so the output will be: [["p", "p"], ["p", "p", "p"]]. In another words I want to print arr[0:3] and arr[3:] but I want to make it dynamic so for example with another array like arr2 = ["h4", "p", "p", "h4", "p", "p", "p", "h4", "p"], idx2 = [0, 3, 7] the output will be: [["p", "p"], ["p", "p", "p"], ["p"]] (i.e. arr2[0:3], arr[3:7], arr[7:]).

In every case h4 value will always be at the index 0.

I don't know if similar question was asked because I couldn't find and also I don't know if it is possible to do it. Maybe similar task is on leetcode, if someone knows something let me know.

Answer 1

By using the groupby function from the itertools package you can group the term of the list by a custom criteria, lambda term: term == 'p'. See docs for details.

import itertools as it

a = ["h4", "p", "p", "h4", "p", "p", "p", "h4", "p"]
 
grouped = [list(grp) for match, grp in it.groupby(a, lambda term: term == 'p') if match]
    
print(grouped)

Output

[['p', 'p'], ['p', 'p', 'p'], ['p']]