Python – K Summation from Two lists

Last Updated : 06 Mar, 2020

Sometimes, while working with Python lists we can have a problem in which we need to find summation of lists taking elements from two different lists. This kind of application can come in different domains such as web development. Lets discuss certain ways in which this task can be performed.

Method #1 : Using loop
This task can be done using loop in a brute force manner. We run a nested loop one for each list and when we find a match we update the result list with the pair.

# Python3 code to demonstrate 
# K Summation from Two lists
# using loop
  
# Initializing lists
test_list1 = [3, 2, 5]
test_list2 = [4, 3, 6, 8, 7]
  
# printing original lists
print("The original list 1 is : " + str(test_list1))
print("The original list 2 is : " + str(test_list2))
  
# Initializing K 
K = 9
  
# K Summation from Two lists
# using loop
res = []
for idx in test_list1:
    val_req = K - idx
    for j in test_list2:
        if j == val_req:
            x, y = j, idx
            res.append((x, y))
  
# printing result 
print ("Summation pairs among lists : " + str(res))

Output :

The original list 1 is : [3, 2, 5]
The original list 2 is : [4, 3, 6, 8, 7]
Summation pairs among lists : [(6, 3), (7, 2), (4, 5)]

Method #2 : Using list comprehension
This is yet another way in which this task can be performed. In this, we perform similar task as above. The difference is that this is one liner and we perform task in one line.

# Python3 code to demonstrate 
# K Summation from Two lists
# using list comprehension
  
# Initializing lists
test_list1 = [3, 2, 5]
test_list2 = [4, 3, 6, 8, 7]
  
# printing original lists
print("The original list 1 is : " + str(test_list1))
print("The original list 2 is : " + str(test_list2))
  
# Initializing K 
K = 9
  
# K Summation from Two lists
# using list comprehension
res = [(a, b) for a in test_list1 for b in test_list2 if a + b == K]
  
# printing result 
print ("Summation pairs among lists : " + str(res))

Output :

The original list 1 is : [3, 2, 5]
The original list 2 is : [4, 3, 6, 8, 7]
Summation pairs among lists : [(6, 3), (7, 2), (4, 5)]

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Python – K Summation from Two lists