Python | Alternate element summation in list

Last Updated : 29 Mar, 2019

The problem of getting summation of a list is quite generic and we might some day face the issue of getting the summation of alternate elements and get the list of 2 elements containing summation of alternate elements. Let’s discuss certain ways in which this can be performed.

Method #1 : Using list comprehension + list slicing + sum()
List slicing clubbed with list comprehension can be used to perform this particular task. We can have list comprehension to get run the logic and list slicing can slice out the alternate character, summation by the sum function

# Python3 code to demonstrate
# alternate elements summation
# using list comprehension + list slicing
  
# initializing list 
test_list = [2, 1, 5, 6, 8, 10]
  
# printing original list 
print("The original list : " + str(test_list))
  
# using list comprehension + list slicing
# alternate elements summation
res = [sum(test_list[i : : 2])
      for i in range(len(test_list)//(len(test_list)//2))]
  
# print result
print("The alternate elements summation list : " + str(res))

Output :

The original list : [2, 1, 5, 6, 8, 10]
The alternate elements summation list : [15, 17]

Method #2 : Using loop
This is the brute method to perform this particular task in which we have the summation of alternate elements in different element indices and then return the output list.

# Python3 code to demonstrate
# alternate elements summation
# using loop
  
# initializing list 
test_list = [2, 1, 5, 6, 8, 10]
  
# printing original list 
print("The original list : " + str(test_list))
  
# using loop
# alternate elements summation
res = [0, 0]
for i in range(0, len(test_list)):
    if(i % 2):
        res[1] += test_list[i]
    else :
        res[0] += test_list[i]
  
# print result
print("The alternate elements summation list : " + str(res))

Output :

The original list : [2, 1, 5, 6, 8, 10]
The alternate elements summation list : [15, 17]

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