Indexing a list with an unique index

Question

I have a list say l = [10,10,20,15,10,20]. I want to assign each unique value a certain \"index\" to get [1,1,2,3,1,2].

This is my code:

Answer 1

Your solution is slow because its complexity is O(nm) with m being the number of unique elements in l: a.index() is O(m) and you call it for every element in l.

To make it O(n), get rid of index() and store indexes in a dictionary:

>>> idx, indexes = 1, {}
>>> for x in l:
...     if x not in indexes:
...         indexes[x] = idx
...         idx += 1
... 
>>> [indexes[x] for x in l]
[1, 1, 2, 3, 1, 2]

If l contains only integers in a known range, you could also store indexes in a list instead of a dictionary for faster lookups.