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

Well I guess it depends on if you want it to return the indexes in that specific order or not. If you want the example to return:

    [1,1,2,3,1,2]

then you can look at the other answers submitted. However if you only care about getting a unique index for each unique number then I have a fast solution for you

    import numpy as np
    l = [10,10,20,15,10,20]
    a = np.array(l)
    x,y = np.unique(a,return_inverse = True)

and for this example the output of y is:

    y = [0,0,2,1,0,2]

I tested this for 1,000,000 entries and it was done essentially immediately.