Python | Multiplying Alternate elements in List

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

Method #1 : Using list comprehension + list slicing + loop 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, product by the external prod function. 

Python3

# Python3 code to demonstrate
# Multiplying Alternate elements in List
# using list comprehension + list slicing

# getting Product
def prod(val) :
    res = 1
    for ele in val:
        res *= ele
    return res

# 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
# Multiplying Alternate elements in List
res = [prod(test_list[i : : 2]) for i in range(len(test_list) // (len(test_list)//2))]

# print result
print("The alternate elements product list : " + str(res))

Output

The original list : [2, 1, 5, 6, 8, 10]
The alternate elements product list : [80, 60]

Time Complexity: O(n*n) where n is the number of elements in the list “test_list”. 
Auxiliary Space: O(n) where n is the number of elements in the list “test_list”. 

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

Python3

# Python3 code to demonstrate
# Multiplying Alternate elements in List
# using loop

# initializing list
test_list = [2, 1, 5, 6, 8, 10]

# printing original list
print("The original list : " + str(test_list))

# using loop
# Multiplying Alternate elements in List
res = [1, 1]
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 product list : " + str(res))

Output : 

The original list : [2, 1, 5, 6, 8, 10]
The alternate elements product list : [80, 60]

Time Complexity: O(n*n), where n is the number of elements in the list “test_list”.
Auxiliary Space: O(n), where n is the number of elements in the list “test_list”.

Method #3 : Using reduce() function
We can use the built-in function reduce() to get the product of alternate elements in a list.

Python3

#initializing list
from functools import reduce
test_list = [2, 1, 5, 6, 8, 10]
# printing original list
print("The original list : " + str(test_list))
#using built-in function reduce() to get the product of alternate elements
res = reduce(lambda x, y: x*y, test_list[::2])
res1 = reduce(lambda x, y: x*y, test_list[1::2])
# print result
print("The alternate elements product : " + str([res,res1]))
#This code is contributed by Edula Vinay Kumar Reddy

Output

The original list : [2, 1, 5, 6, 8, 10]
The alternate elements product : [80, 60]

Time complexity: O(n)
Auxiliary Space: O(1)