Question

This is the basic problem: I have an array of integers with possibly duplicate elements. I need to know the indices of each element, but when I sort the array, whenever I select an element from the new array, I want to be able to reference the same element from the original array.

I am looking for a solution to the problem, or maybe a solution to the approach I am taking.

Here is an array

a = [1, 2, 3, 4, 3, 5, 2]

There are two 2’s and two 3’s, but if I’m working with the first 2 (from the left), I want to work with index 1, and if I’m working with the second 2, I want to be working with index 6. So I use a helper array to allow me to do this:

helper = [0, 1, 2, 3, 4, 5, 6]

Which I will iterate over and use to access each element from a.
I could have accomplished this with each_with_index, but the problem begins when I sort the array.

Now I have a sort order

sort_order = [2, 4, 1, 5, 3]

I use sort_by to sort a according to sort_order, to produce

sorted_a = [2, 2, 4, 1, 5, 3, 3]

You may assume all elements in the input exist in sort_order to avoid sort_by exceptions.

Now the problem is that my helper array should be updated to match the new positions. Each element should be sorted the same way as a was sorted, because it is unclear whether the first 2 in the new array was at index 1 or at index 6 of the original array.

So my new helper array might look like

new_helper = [1, 6, 3, 0, 5, 2, 4]

So if I were to go with this approach, how would I produce the new_helper array, given the original array and the sort order?

Maybe there is a better way to do this?

Answer 1

Make a list of pairs of the original data and that data’s index. Like this:

a = [(1, 0), (2, 1), (3, 2), (4, 3), (3, 4), (5, 5), (2,6)]

Sort that list (lexicographically, or just ignore the second part of the pair except to carry it along). The second item in every pair tells you where the element was in the original array.