How to extract sub-strings based on indices

Question

My goal is to extract from the string below, sub-strings that covers the range of each given indices.

_string='the old school teacher is having a nice time at school'
index_list=[[0,1,2],[4,7,20]]

My attempt:

1)>>> [[[_string[y] for y in x]for x in index_list]
[['t', 'h', 'e'], ['o', ' ', 'e']]

2)>>>[_string[x[0:]] for x in index_list]
TypeError: string indices must be integers, not list

The first attempt extracted only the characters that corresponded with the indices, while the second resulted in TypeError.

Desired output:

['the', 'old school teach']

Any suggestions on how to arrive at the desired output? thanks.

Answer 1

If you are delimiting each selection using just the first and last indexes of each selector:

[ _string[x[0]:x[-1]] for x in index_list]

If your last index is inclusive, you should at 1 to the right limit:

[ _string[x[0]:(x[-1]+1)] for x in index_list]

Answer 2

_string='the old school teacher is having a nice time at school'
index_list=[[0,1,2],[4,7,20]]
print [_string[x[0]:x[-1]+1] for x in index_list]

Is it what you are looking for? You only need the first (x[0]) and the last (x[-1]) indexes. Maybe you'll have to change 20 to 21 if you want the whole sentence.

Answer 3

If it is just the range that matters, then you can do this:

>>> _string='the old school teacher is having a nice time at school'
>>> index_list=[[0,1,2],[4,7,20]]
>>> [_string[i[0]:i[-1]+1] for i in index_list]
['the', 'old school teache']

So, you should change the index list to [[0,1,2],[4,7,21]]. And if it is just the first and last item you care about, perhaps you can get rid of the middle element altogether.

Answer 4

You can try in a different approach. First, if I understand correctly your problem, you only need the first and the last index of your list(if it is sorted). So you can try and remove the other values:

second_index = [[x[0],x[-1]] for x in index_list]

and then you can produce your output like that:

[_string[x[0]:x[1]+1] for x in second_index]