Python – Frequency of elements from other list

Given two lists, compute frequency of elements of list 2 that occur in list 1.

Input : test_list1 = [4, 6, 8, 9], test_list2 = [4, 6, 6, 5, 8, 10, 4, 9, 8, 10, 1]
Output : {4: 2, 6: 2, 8: 2, 9: 1}
Explanation : 4 occurs 2 times, 6 occurs 2 times in second list and so on.

Input : test_list1 = [4, 6, 8, 9], test_list2 = [4, 6, 5, 8, 10, 4, 9, 8, 10, 1]
Output : {4: 2, 6: 1, 8: 2, 9: 1}
Explanation : 4 occurs 2 times, 6 occurs 1 time in second list and so on.

Method #1 : Using dictionary comprehension + count()

This is one of the ways in which this task can be performed. In this, count() is used to extract count of elements in list 2 and dictionary comprehension is used to compile result and iterate through list 1 values.

Python3

# Python3 code to demonstrate working of
# Frequency of elements from other list
# Using dictionary comprehension + count()

# initializing lists
test_list1 = [4, 6, 8, 9, 10]
test_list2 = [4, 6, 6, 5, 8, 10, 4, 9, 8, 10, 1]

# printing original lists
print("The original list 1 : " + str(test_list1))
print("The original list 2 : " + str(test_list2))

# count() used to perform count.
# returns 0 is no occurrence.
# ignores different new elements in list 2
res = {key : test_list2.count(key) for key in test_list1}

# printing result
print("Lists elements Frequency : " + str(res))

Output

The original list 1 : [4, 6, 8, 9, 10]
The original list 2 : [4, 6, 6, 5, 8, 10, 4, 9, 8, 10, 1]
Lists elements Frequency : {4: 2, 6: 2, 8: 2, 9: 1, 10: 2}

Method #2 : Using Counter() + setdefault() + loop

This is another way in which this task can be performed. In this, we compute the frequency using Counter() and assign values using loop. The setdefault() is used to initialize the counter to 0 for not present values.

Python3

# Python3 code to demonstrate working of
# Frequency of elements from other list
# Using Counter() + setdefault() + loop
from collections import Counter

# initializing lists
test_list1 = [4, 6, 8, 9, 10]
test_list2 = [4, 6, 6, 5, 8, 10, 4, 9, 8, 10, 1]

# printing original lists
print("The original list 1 : " + str(test_list1))
print("The original list 2 : " + str(test_list2))

# Counter() used to perform count on list 2
res = dict(Counter(test_list2))

# filtering only list 1 keys
res = {key : val for key, val in res.items() if key in test_list1}

for key in test_list1:

    # initializing elements not present in list 2 of list 1 to 0
    res.setdefault(key, 0)


# printing result
print("Lists elements Frequency : " + str(res))

Output

The original list 1 : [4, 6, 8, 9, 10]
The original list 2 : [4, 6, 6, 5, 8, 10, 4, 9, 8, 10, 1]
Lists elements Frequency : {4: 2, 6: 2, 8: 2, 10: 2, 9: 1}