Making n-length sublists for every item within reach of an index

Question

Let's say I have a list l = ['a', 'b', 'c', 'd', 'e', 'f', 'g'] and a given element e = 'b' and I want to make 3-length sublists off of elements no further than 3 positions away from our element, including the element. We would want to produce the following result for our example:

[['a', 'b', 'c'], ['b', 'c', 'd']]

As you can see it can't go further left because the element's position is smaller than the reach, and it goes as far as the element 'd' (index 4) on the right.

This is my native solution:

l = ['a', 'b', 'c', 'd', 'e', 'f', 'g']
n = len(l)
reach = 3
e = 'b'
idx = l.index(e)
result = []
for i in range(reach):
  lb, rb = idx - i, idx - i + reach
  if lb >= 0:
    result.append(l[lb:rb])

Is there a more elegant way to do it, e.g. with itertools or zip?

If you change `reach` to 2 or 4, the result doesn't look right... — superb rain, Jul 28 '20 at 12:08
Or if you use `e = 'g'` (with reach=3), it also doesn't look right. — superb rain, Jul 28 '20 at 12:10

score 0 · Answer 1 · answered Jul 28 '20 at 12:51

I'd compute the min/max starting index for the sublists instead of going too far (and filtering) or not going far enough (and missing something!):

idx = l.index(e)
min_start = max(idx - reach + 1, 0)
max_start = min(idx, n - reach)
result = [l[i:i+reach] for i in range(min_start, max_start + 1)]

Making n-length sublists for every item within reach of an index

1 Answers1