Python program to concatenate every elements across lists

Given 2 lists, perform concatenations of all strings with each other across list.

Input : test_list1 = ["gfg", "is", "best"], test_list2 = ["love", "CS"] 
Output : ['gfg love', 'gfg CS', 'is love', 'is CS', 'best love', 'best CS'] 
Explanation : All strings are coupled with one another.

Input : test_list1 = ["gfg", "best"], test_list2 = ["love", "CS"] 
Output : ['gfg love', 'gfg CS', 'best love', 'best CS'] 
Explanation : All strings are coupled with one another. 
 

Method #1: Using list comprehension

In this, we form pairs with each using list comprehension and then perform task of concatenation using another list comprehension.

# Python3 code to demonstrate working of  # All elements concatenation across lists # Using list comprehension # initializing lists test_list1 = ["gfg", "is", "best"] test_list2 = ["love", "CS"] # printing original lists print("The original list 1 is : " + str(test_list1)) print("The original list 2 is : " + str(test_list2)) # forming pairs temp = [(a, b) for a in test_list1 for b in test_list2] # performing concatenation res = [x + ' ' + y for (x, y) in temp] # printing result  print("The paired combinations : " + str(res)) 

Output:

The original list 1 is : ['gfg', 'is', 'best'] The original list 2 is : ['love', 'CS'] The paired combinations : ['gfg love', 'gfg CS', 'is love', 'is CS', 'best love', 'best CS']

Time Complexity: O(n²) -> two for loops

Space Complexity: O(n)

Method #2 : Using product() + list comprehension

In this, we perform task of forming combination using product() and list comprehension performs the task of concatenation.

# Python3 code to demonstrate working of  # All elements concatenation across lists # Using product() + list comprehension from itertools import product # initializing lists test_list1 = ["gfg", "is", "best"] test_list2 = ["love", "CS"] # printing original lists print("The original list 1 is : " + str(test_list1)) print("The original list 2 is : " + str(test_list2)) # concatenation using formatting and pairing using product res = ['%s %s' % (ele[0], ele[1]) for ele in product(test_list1, test_list2)] # printing result  print("The paired combinations : " + str(res)) 

Output:

The original list 1 is : ['gfg', 'is', 'best'] The original list 2 is : ['love', 'CS'] The paired combinations : ['gfg love', 'gfg CS', 'is love', 'is CS', 'best love', 'best CS']

Time Complexity: O(n²) -> time complexity of product is O(n) and a for loop, O(n²)

Space Complexity: O(n)

Method #3: Using map and join

In this approach, we use map function to perform the task of concatenation and then join the resultant strings.

# Python3 code to demonstrate working of # All elements concatenation across lists # Using map() and join() from itertools import product # initializing lists test_list1 = ["gfg", "is", "best"] test_list2 = ["love", "CS"] # printing original lists print("The original list 1 is : " + str(test_list1)) print("The original list 2 is : " + str(test_list2)) # concatenation using map() and join() res = list(map(' '.join, product(test_list1, test_list2))) # printing result print("The paired combinations : " + str(res)) 

The original list 1 is : ['gfg', 'is', 'best'] The original list 2 is : ['love', 'CS'] The paired combinations : ['gfg love', 'gfg CS', 'is love', 'is CS', 'best love', 'best CS']

Time Complexity: O(n2) -> time complexity of product is O(n) and a map function call O(n)

Auxiliary Space: O(n)

Method #4: Using itertools.product() and str.join()

• Import the itertools module, which provides various functions to work with iterators.
• Initialize two lists test_list1 and test_list2 with the desired elements.
• Use itertools.product() function to form all possible pairs of elements from test_list1 and test_list2. This function returns an iterator that generates the pairs one at a time.
• Store the iterator in a temporary variable called temp.
• Iterate over the temp iterator and for each pair, join the elements using a space separator (' ') and append the result to a list called res.
• Print the final result by converting the res list to a string using str() and passing it to the print() function

import itertools # initializing lists test_list1 = ["gfg", "is", "best"] test_list2 = ["love", "CS"] # forming pairs temp = itertools.product(test_list1, test_list2) # performing concatenation res = [' '.join(pair) for pair in temp] # printing result  print("The paired combinations : " + str(res)) 

The paired combinations : ['gfg love', 'gfg CS', 'is love', 'is CS', 'best love', 'best CS'] 

Time complexity: O(n^2), where n is the length of the longer list (since we form all possible pairs).
Auxiliary space: O(n^2) for the temp list, since we need to store all the pairs.