How to create a list that represent the number of times a given item was shown?

This problem seems really stupid bu I can’t get my head around it.

I have the following list:

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

I have to produce a second list which have the same size as the previous one but the values that appear should be the amount of times that a value showed up in the array until that point. For example:

b = [1, 1, 1, 2, 3, 2, 2, 3, 3]

So b[0] = 1 because it’s the first time the item ‘2’ appear on the ‘a’ list. b[5] = 2 and b[7] = 3 because it’s the second and third time that the item ‘2’ appear on the list ‘a’.

Answer

Here a solution:

from collections import defaultdict
a = [2, 1, 3, 1, 1, 2, 3, 2, 3]
b = []
d = defaultdict(int)

for x in a:
  d[x] +=1
  b.append(d[x])

print(b)

Output:

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

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Answer