Python | Iterating two lists at once

Sometimes, while working with Python list, we can have a problem in which we have to iterate over two list elements. Iterating one after another is an option, but it’s more cumbersome and a one-two liner is always recommended over that. Let’s discuss certain ways in which this task can be performed.

Method #1 : Using loop + “+” operator
The combination of above functionalities can make our task easier. But the drawback here is that we might have to concatenate the list and hence would consume more memory than desired.

# Python3 code to demonstrate working of 
# Iterating two lists at once 
# using loop + "+" operator 
  
# initializing lists 
test_list1 = [4, 5, 3, 6, 2] 
test_list2 = [7, 9, 10, 0] 
  
# printing original lists 
print("The original list 1 is : " + str(test_list1)) 
print("The original list 2 is : " + str(test_list2)) 
  
# Iterating two lists at once 
# using loop + "+" operator 
# printing result  
print("The paired list contents are : ") 
for ele in test_list1 + test_list2: 
    print(ele, end =" ") 

Output :

The original list 1 is : [4, 5, 3, 6, 2]
The original list 2 is : [7, 9, 10, 0]
The paired list contents are : 
4 5 3 6 2 7 9 10 0

Method #2 : Using chain()
This is the method similar to above one, but it’s slightly more memory efficient as the chain() is used to perform the task and creates an iterator internally.

# Python3 code to demonstrate working of 
# Iterating two lists at once 
# using chain() 
from itertools import chain 
  
# initializing lists 
test_list1 = [4, 5, 3, 6, 2] 
test_list2 = [7, 9, 10, 0] 
  
# printing original lists 
print("The original list 1 is : " + str(test_list1)) 
print("The original list 2 is : " + str(test_list2)) 
  
# Iterating two lists at once 
# using chain() 
# printing result  
print("The paired list contents are : ") 
for ele in chain(test_list1, test_list2): 
    print(ele, end =" ") 

Output :

The original list 1 is : [4, 5, 3, 6, 2]
The original list 2 is : [7, 9, 10, 0]
The paired list contents are : 
4 5 3 6 2 7 9 10 0

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