Filter a list of strings by frequency

Question

I have a list of strings:

a = ['book','book','cards','book','foo','foo','computer'] 

I want to return anything in this list that's x > 2

Final output:

a = ['book','book','book']

I'm not quite sure how to approach this. But here's two methods I had in mind:

Approach One:

I've created a dictionary to count the number of times an item appears:

a = ['book','book','cards','book','foo','foo','computer'] 

import collections

def update_item_counts(item_counts, itemset):
    for a in itemset:
        item_counts[a] +=1

test = defaultdict(int)
update_item_counts(test, a)
print(test)

Out: defaultdict(<class 'int'>, {'book': 3, 'cards': 1, 'foo': 2, 'computer': 1})

I want to filter out the list with this dictionary but I'm not sure how to do that.

Approach two: I tried to write a list comprehension but it doesn't seem to work:

res = [k for k in a if a.count > 2 in k]

Answer 1

A very barebone answer is that you should replace a.count by a.count(k) in your second solution.

Although, do not attempt to use list.count for this, as this will traverse the list for each item. Instead count occurences first with collections.Counter. This has the advantage of traversing the list only once.

from collections import Counter
from itertools import repeat

a = ['book','book','cards','book','foo','foo','computer']
count = Counter(a)

output = [word for item, n in count.items() if n > 2 for word in repeat(item, n)]

print(output) # ['book', 'book', 'book']

Note that the list comprehension is equivalent to the loop below.

output = []

for item, n in count.items():
    if n > 2:
        output.extend(repeat(item, n))

Answer 2

Try this:

a_list = ['book','book','cards','book','foo','foo','computer'] 
b_list = []

for a in a_list:
    if a_list.count(a) > 2:
        b_list.append(a)

print(b_list)
#  ['book', 'book', 'book']

Edit: You mentioned list comprehension. You are on the right track! You can do it with list comprehension like this:

a_list = ['book','book','cards','book','foo','foo','computer'] 
c_list = [a for a in a_list if a_list.count(a) > 2]

Good luck!