Label a list following the unique elements appearing in it

Question

Given a list of strings such as:

foo = \['A', 'A', 'B', 'A', 'B', 'C', 'C', 'A', 'B', 'C', 'A'\]

How can we label them such that the output would be:

output = \['A1', 'A2', 'B1', 'A3', 'B2', 'C1', 'C2', 'A4', 'B2', 'C3', 'A5'\] 

(keeping the order of the original list)

In the following case there are only 3 unique variables to look at, so the first think I tried was looking at the unique elements:

import numpy as np

np.unique(foo)

Output = \['A', 'B', 'C'\]

But then I get stacked when trying to find the proper loop to reach the desired output.

Answer 1

Using pure python, take advantage of a dictionary to count the values:

foo = ['A', 'A', 'B', 'A', 'B', 'C', 'C', 'A', 'B', 'C', 'A']

d = {}
out = []
for val in foo:
    d[val] = d.get(val, 0)+1
    out.append(f'{val}{d[val]}')

If you can use pandas:

import pandas as pd

s = pd.Series(foo)
out = s.add(s.groupby(s).cumcount().add(1).astype(str)).tolist()

Output: ['A1', 'A2', 'B1', 'A3', 'B2', 'C1', 'C2', 'A4', 'B3', 'C3', 'A5']

Answer 2

foo = ['A', 'A', 'B', 'A', 'B', 'C', 'C', 'A', 'B', 'C', 'A']
foo_dict = {}
new_foo = []
for item in foo:
    foo_dict[item] = foo_dict.get(item,0)+1
    new_foo.append(f"{item}{foo_dict[item]}")
print(new_foo)