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

If the condition is that the numbers are unique and the exact number is not important, then you can build a mapping relating each item in the list to a unique number on the fly, assigning values from a count object:

from itertools import count

names = ['ll', 'll', 'hl', 'hl', 'LL', 'LL', 'LL', 'HL', 'll']

d = {}
c = count()
numbers = [d.setdefault(i, next(c)) for i in names]
print(numbers)
# [0, 0, 2, 2, 4, 4, 4, 7, 0]

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You could do away with the extra names by using map on the list and a count object, and setting the map function as {}.setdefault (see @StefanPochmann's comment):

from itertools import count

names = ['ll', 'll', 'hl', 'hl', 'LL', 'LL', 'LL', 'HL', 'll']
numbers  = map({}.setdefault, names, count()) # call list() on map for Py3
print(numbers)
# [0, 0, 2, 2, 4, 4, 4, 7, 0]

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As an extra, you could also use np.unique, in case you already have numpy installed:

import numpy as np

_, numbers = np.unique(names, return_inverse=True)
print(numbers)
# [3 3 2 2 1 1 1 0 3]