Python – Group similar value list to dictionary

Sometimes, while working with Python list, we can have a problem in which we need to group similar value lists indices to values into a dictionary. This can have a good application in domains in which we need grouped dictionary as output for pair of lists. Lets discuss certain ways in which this task can be performed. 

Method #1 : Using dictionary comprehension This is way in which this task can be performed. In this, we iterate one list along with other and keep constructing dictionary with similar key from one list and corresponding values from other. This is one liner solution. 

Python3

# Python3 code to demonstrate
# Group similar value list to dictionary
# using dictionary comprehension

# Initializing lists
test_list1 = [4, 4, 4, 5, 5, 6, 6, 6, 6]
test_list2 = ['G', 'f', 'g', 'i', 's', 'b', 'e', 's', 't']

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

# Group similar value list to dictionary
# using dictionary comprehension
res = {key : [test_list2[idx]
      for idx in range(len(test_list2)) if test_list1[idx]== key]
      for key in set(test_list1)}

# printing result
print ("Mapped resultant dictionary : " + str(res))

Output

The original list 1 is : [4, 4, 4, 5, 5, 6, 6, 6, 6]
The original list 2 is : ['G', 'f', 'g', 'i', 's', 'b', 'e', 's', 't']
Mapped resultant dictionary : {4: ['G', 'f', 'g'], 5: ['i', 's'], 6: ['b', 'e', 's', 't']}

Method #2 : Using defaultdict() + loop This is yet another way in which this task can be performed. In this we avoid testing for keys presence in dictionary by creating a defaultdict(). 

Python3

# Python3 code to demonstrate
# Group similar value list to dictionary
# using defaultdict() + loop
from collections import defaultdict

# Initializing lists
test_list1 = [4, 4, 4, 5, 5, 6, 6, 6, 6]
test_list2 = ['G', 'f', 'g', 'i', 's', 'b', 'e', 's', 't']

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

# Group similar value list to dictionary
# using defaultdict() + loop
res = defaultdict(set)
for key, val in zip(test_list1, test_list2):
    res[key].add(val)
res = {key: list(val) for key, val in res.items()}

# printing result
print ("Mapped resultant dictionary : " + str(res))

Output

The original list 1 is : [4, 4, 4, 5, 5, 6, 6, 6, 6]
The original list 2 is : ['G', 'f', 'g', 'i', 's', 'b', 'e', 's', 't']
Mapped resultant dictionary : {4: ['g', 'G', 'f'], 5: ['s', 'i'], 6: ['t', 's', 'b', 'e']}

Method #3 : Using for loop,list() and set() methods

Python3

# Python3 code to demonstrate
# Group similar value list to dictionary

# Initializing lists
test_list1 = [4, 4, 4, 5, 5, 6, 6, 6, 6]
test_list2 = ['G', 'f', 'g', 'i', 's', 'b', 'e', 's', 't']

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

# Group similar value list to dictionary
res = dict()
x = list(set(test_list1))
for i in x:
    a = []
    for j in range(0, len(test_list1)):
        if(i == test_list1[j]):
            a.append(test_list2[j])
    res[i] = a

# Printing result
print("Mapped resultant dictionary : " + str(res))

Output

The original list 1 is : [4, 4, 4, 5, 5, 6, 6, 6, 6]
The original list 2 is : ['G', 'f', 'g', 'i', 's', 'b', 'e', 's', 't']
Mapped resultant dictionary : {4: ['G', 'f', 'g'], 5: ['i', 's'], 6: ['b', 'e', 's', 't']}