How to find the starting and ending index of elements in list automatically in python

Question

I would like to find the starting and ending index of all the userId in the list, I want to do this without specifying every single userId, because the dataset is large.

[1, 1, 1, 1, 1, 1, 1, 1, 1, 1.......213,213,213,213]

I want the output to be

[{1: 0, 20},{2: 21, 40}.....{213: 29,703, 30,000}]

Is there a package or function that can do this automatically in python?

Answer 1

You can do this:

from collections import Counter

a = ...

a_counter = Counter(a)
a_indices = []

running_count = 0

for x, x_count in sorted(a_counter.items()):
   a_indices.append({x: (running_count, running_count + x_count - 1)}) 
   running_count += x_count

For example, if a = [1, 1, 2, 2, 3, 3] , a_indices = [{1: (0, 1)}, {2: (2, 3)}, {3: (4, 5)}] (closest to your output format, while being valid).

If you're willing to slightly change your output format, use:

a_indices = {}

running_count = 0

for x, x_count in sorted(a_counter.items()):
   a_indices[x] = (running_count, running_count + x_count - 1) 
   running_count += x_count

Now a_indices , for the a above, will be {1: (0, 1), 2: (2, 3), 3: (4, 5)} , a much nicer structure to work with.

Both of these solutions will make each end index for x inclusive. If you want to make it exclusive, replace running_count + x_count - 1 with running_count + x_count .