label assignment from lookup dictionary keys and value in Python

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Given a list

[0,1,2,3,4,5,6,7,8,9,10,11,12,13]

I would like to label at each of the list position as below

['fir', 'fir', 'sec', 'fir', 'fir', 'fir', 'thr', 'sec', 'fou', 'fou', 'fou', 'thr', 'sec', 'fou']

with respect to a lookup dictionary m

m={ "fir": [0,1,3,4,5],
        "sec": [2,7,12],
        "thr": [6,11],
        "fou": [8,9,10,13]}

Let take for example the first three position.

From the dict m, the position 0 and 1 fell under the key fir. Whereas, the third position (e.g., 2) is under the key sec.

Hence, the list for the three are ['fir', 'fir', 'sec'].

To achieve the above objective, the following is drafted

m={ "fir": [0,1,3,4,5],
        "sec": [2,7,12],
        "thr": [6,11],
        "fou": [8,9,10,13]}

all_key=m.keys()
all_opt=[]
for idx in range (sum(len(v) for v in m.values())):
  for nkey in all_key:
    if idx in m[nkey]:
      all_opt.append(nkey)
      break

which produced

opt=['fir', 'fir', 'sec', 'fir', 'fir', 'fir', 'thr', 'sec', 'fou', 'fou', 'fou', 'thr', 'sec', 'fou']

I wonder whether there is better alternative than the nested for-loop?

dictionary python performance
2 Answers

You can invert the mapping dictionary so that mapping would be:

  • valuei ⟶ labelj instead of label ⟶ [value0...valuen]

You can invert the dictionary using dict comprehension

inv_m = {v: k for k, lst in m.items() for v in lst}
print(inv_m)
# {0: 'fir', 1: 'fir', 3: 'fir', 4: 'fir', 5: 'fir', 2: 'sec',
#  7: 'sec', 12: 'sec', 6: 'thr', 11: 'thr', 8: 'fou', 9: 'fou',
#  10: 'fou', 13: 'fou'}

Now, iterate over the given list and map each value to its corresponding label.

lst = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]
out = [inv_m[v] for v in lst]
print(out)
# ['fir', 'fir', 'sec', 'fir', 'fir', 'fir', 'thr', 'sec', 'fou',
#  'fou', 'fou', 'thr', 'sec', 'fou']
user_124164530

Create a fixed size list and fill it by putting labels to corresponding indexes:

lst = [0,1,2,3,4,5,6,7,8,9,10,11,12,13]
m = { 
    "fir": [0,1,3,4,5],
    "sec": [2,7,12],
    "thr": [6,11],
    "fou": [8,9,10,13]
}

label_lst = [''] * len(lst)
for label, idxs in m.items():
    for idx in idxs:    
        label_lst[idx] = label

print(label_lst)

Output

['fir', 'fir', 'sec', 'fir', 'fir', 'fir', 'thr', 'sec', 'fou', 'fou', 'fou', 'thr', 'sec', 'fou']
user_29134770
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