Python | Merge two list of lists according to first element

Last Updated : 20 May, 2019

Given two list of lists of equal length, write a Python program to merge the given two list of lists, according to the first common element of each sublist.

Examples:

Input : lst1 = [[1, 'Alice'], [2, 'Bob'], [3, 'Cara']]
        lst2 = [[1, 'Delhi'], [2, 'Mumbai'], [3, 'Chennai']]
Output : [[1, 'Alice', 'Delhi'], [2, 'Bob', 'Mumbai'], [3, 'Cara', 'Chennai']]

Input : lst1 = [ ['c', 'class'], ['g', 'greek'], ]
        lst2 = [['c', 'coder'], ['g', 'god'], ]
Output : [['c', 'class', 'coder'], ['g', 'greek', 'god']]

Method #1 : Python zip() with list comprehension

# Python3 program to Merge two list of 
# lists according to first element
  
def merge(lst1, lst2):
    return [a + [b[1]] for (a, b) in zip(lst1, lst2)]
      
# Driver code
lst1 = [[1, 'Alice'], [2, 'Bob'], [3, 'Cara']]
lst2 = [[1, 'Delhi'], [2, 'Mumbai'], [3, 'Chennai']]
print(merge(lst1, lst2))

Output:

[[1, 'Alice', 'Delhi'], [2, 'Bob', 'Mumbai'], [3, 'Cara', 'Chennai']]

Method #2 : Python enumerate() with list comprehension

# Python3 program to Merge two list of 
# lists according to first element
import collections
  
def merge(lst1, lst2):
    return [(sub + [lst2[i][-1]]) for i, sub in enumerate(lst1)]
      
# Driver code
lst1 = [[1, 'Alice'], [2, 'Bob'], [3, 'Cara']]
lst2 = [[1, 'Delhi'], [2, 'Mumbai'], [3, 'Chennai']]
print(merge(lst1, lst2))

Output:

[[1, 'Alice', 'Delhi'], [2, 'Bob', 'Mumbai'], [3, 'Cara', 'Chennai']]

Method #3 : Python dictionary
In this method, we initialise ‘dict1’ with collections.defaultdict and traverse through ‘lst1’+’lst2’ and append the first element of ‘lst1’ as key and tupled second element of both respective sublists as value. Finally we traverse through ‘dict1’ and initialize ‘dictlist’ with the desired output.

# Python3 program to Merge two list of 
# lists according to first element
import collections
  
def merge(lst1, lst2):
    dict1 = collections.defaultdict(list)
      
    for e in lst1 + lst2:
        dict1[e[0]].append(e[1])
    dictlist = list()
      
    for key, value in dict1.items():
        dictlist.append([key]+value)
      
    return dictlist
      
# Driver code
lst1 = [[1, 'Alice'], [2, 'Bob'], [3, 'Cara']]
lst2 = [[1, 'Delhi'], [2, 'Mumbai'], [3, 'Chennai']]
print(merge(lst1, lst2))

Output:

[[1, 'Alice', 'Delhi'], [2, 'Bob', 'Mumbai'], [3, 'Cara', 'Chennai']]

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