python - How to merge two lists into dictionary without using nested for loop

Question

I have two lists:

a = [0, 0, 0, 1, 1, 1, 1, 1, .... 99999]
b = [24, 53, 88, 32, 45, 24, 88, 53, ...... 1]

I want to merge those two lists into a dictionary like:

{
    0: [24, 53, 88], 
    1: [32, 45, 24, 88, 53], 
    ...... 
    99999: [1]
}

A solution might be using for loop, which does not look good and elegant, like:

d = {}
unique_a = list(set(list_a))
for i in range(len(list_a)):
    if list_a[i] in d.keys:
        d[list_a[i]].append(list_b[i])
    else:
        d[list_a] = [list_b[i]]

Though this does work, it’s an inefficient and would take too much time when the list is extremely large. I want to know more elegant ways to construct such a dictionary?

Thanks in advance!

Answer 1

You can use a defaultdict:

from collections import defaultdict
d = defaultdict(list)
list_a = [0, 0, 0, 1, 1, 1, 1, 1, 9999]
list_b = [24, 53, 88, 32, 45, 24, 88, 53, 1]
for a, b in zip(list_a, list_b):
   d[a].append(b)

print(dict(d))

Output:

{0: [24, 53, 88], 1: [32, 45, 24, 88, 53], 9999: [1]}