Split a list into sublists based on a set of indexes in Python

Question

I have a list similar to below

['a','b','c','d','e','f','g','h','i','j']

and I would like to separate by a list of index

[1,4]

In this case, it will be

[['a'],['b','c'],['d','e','f','g','h','i','j']]

As

[:1] =['a']

[1:4] = ['b','c']

[4:] = ['d','e','f','g','h','i','j']

Case 2: if the list of index is

[0,6]

It will be

[[],['a','b','c','d','e'],['f','g','h','i','j']]

As

[:0] = []

[0:6] = ['a','b','c','d','e']

[6:] = ['f','g','h','i','j']

Case 3 if the index is

[2,5,7]

it will be [['a','b'],['c','d','e'],['h','i','j']] As

[:2] =['a','b']
[2:5] = ['c','d','e']
[5:7] = ['f','g']
[7:] = ['h','i','j']

If your question is how to obtain these sublists: why not use a simple loop? — offeltoffel
– offeltoffel, Commented Jan 15, 2020 at 9:54
This sounds only 99% like a homework. What do you want to achieve with this? Or in other words, what does the homework want you to do? You allready split lists based on a set of indexes. — hajef
– hajef, Commented Jan 15, 2020 at 9:55
Does this answer your question? Split a list into parts based on a set of indexes in Python — Georgy
– Georgy, Commented Jan 15, 2020 at 10:04
The slices that you are showing aren't consistent. [0:6] = ['a','b','c','d','e'] should be ['a','b','c','d','e','f'] and [1:4] = ['b','c'] should be ['b','c','d'] — Alex
– Alex, Commented Jan 15, 2020 at 10:05

Fourier · Accepted Answer · 2020-01-15 10:02:05Z

8

Something along these lines:

mylist = ['a','b','c','d','e','f','g','h','i','j']
myindex = [1,4]

[mylist[s:e] for s, e in zip([0]+myindex, myindex+[None])]

Output

[['a'], ['b', 'c', 'd'], ['e', 'f', 'g', 'h', 'i', 'j']]

answered Jan 15, 2020 at 10:02

Fourier

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Comments

cristian hantig · Accepted Answer · 2020-01-15 10:41:09Z

2

This solution is using numpy:

import numpy as np

def do_split(lst, slices):
    return [sl.tolist()for sl in np.split(lst, slices)]

splits = do_split(a, [2,5,7])

Out[49]:
[['a', 'b'], ['c', 'd', 'e'], ['f', 'g'], ['h', 'i', 'j']]

answered Jan 15, 2020 at 10:41

cristian hantig

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1 Comment

Fourier Over a year ago

This will be faster than my approach by avoiding copies, +1!

Strange · Accepted Answer · 2020-01-15 09:59:29Z

1

a = [1,2,3,4,5,6,7,8,9,10]

newlist = []

divide = [2,5,7]
divide = [0]+divide+[len(a)]
for i in range(1,len(divide)):
  newlist.append(a[divide[i-1]:divide[i]])

print(newlist)

Output:

[[1, 2], [3, 4, 5], [6, 7], [8,9,10]]

answered Jan 15, 2020 at 9:59

Strange

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Comments

MattFrench019 · Accepted Answer · 2020-01-15 10:20:13Z

0

I wrote this function to do what you're asking

def splitter(_list, *args):
    args_list = [0]
    args_list += args
    args_list.append(len(_list))
    new_list = []
    for i, arg in enumerate(args_list):
        try:
            new_list.append(_list[arg:args_list[i+1]])

        except IndexError:
            continue
    return new_list

The function can be used like this:

mylist = ['1', '2', '3', '4', '5', '6']
splitter(mylist, 2, 4)

Which returns:

[['1', '2'], ['3', '4'], ['5', '6']]

answered Jan 15, 2020 at 10:20

MattFrench019

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