Question

I’m looking for a way to make a list containing list (a below) into a single list (b below) with 2 conditions:

1. The order of the new list (b) is based on the number of times the value has occurred in some of the lists in a.
2. A value can only appear once

Basically turn a into b:

a = [[1,2,3,4], [2,3,4], [4,5,6]]
# value 4 occurs 3 times in list a and gets first position
# value 2 occurs 2 times in list a and get second position and so on...
b = [4,2,3,1,5,6]

I figure one could do this with set and some list magic. But can’t get my head around it when a can contain any number of list. The a list is created based on user input (I guess that it can contain between 1 – 20 list with up 200-300 items in each list).

My trying something along the line with [set(l) for l in a] but don’t know how to perform set(l) & set(l).... to get all matched items.

Is possible without have a for loop iterating sublist count * items in sublist times?

Answer 1

I think this is probably the closest you’re going to get:

from collections import defaultdict

d = defaultdict(int)

for sub in outer:
  for val in sub:
    d[val] += 1

print sorted(d.keys(), key=lambda k: d[k], reverse = True)
# Output: [4, 2, 3, 1, 5, 6]

There is an off chance that the order of elements that appear an identical number of times may be indeterminate – the output of d.keys() is not ordered.