Assign a number to each unique value in a list

Question

I have a list of strings. I want to assign a unique number to each string (the exact number is not important), and create a list of the same length using these numbers, in o

Answer 1

To make it more generic you can wrap it in a function, so these hard-coded values don't do any harm, because they are local.

If you use efficient lookup-containers (I'll use a plain dictionary) you can keep the first index of each string without loosing to much performance:

def your_function(list_of_strings):

    encountered_strings = {}
    result = []

    idx = 0
    for astring in list_of_strings:
        if astring in encountered_strings:  # check if you already seen this string
            result.append(encountered_strings[astring])
        else:
            encountered_strings[astring] = idx
            result.append(idx)
            idx += 1
    return result

And this will assign the indices in order (even if that's not important):

>>> your_function(['ll', 'll', 'll', 'hl', 'hl', 'hl', 'LL', 'LL', 'LL', 'HL', 'HL', 'HL'])
[0, 0, 0, 1, 1, 1, 2, 2, 2, 3, 3, 3]

This needs only one iteration over your list of strings, which makes it possible to even process generators and similar.