Pairwise multiplication of elements between two lists

Question

I am trying to calculate the product of every element from one list multiplied by every element from another list. from each list multiplied by each other.

For example, in list1 I have 4, which needs to be multiplied by 3 in list2. Next, the 4 in list1 needs to be multiplied by 1 in list2. The pattern continues until I receive the output: [12,4,36,8,6,2,18,4,21,7,63,14]. I haven't been able to achieve this -- here is the code I have so far:

def multiply_lists(list1,list2):
    for i in range(0,len(list1)):
        products.append(list1[i]*list2[i])
    return products

list1 = [4,2,7]
list2 = [3,1,9,2]
products = []
print ('list1 = ',list1,';','list2 = ', list2) 
prod_list = multiply_lists(list1,list2)
print ('prod_list = ',prod_list)

score 2 · Answer 1 · answered Apr 06 '22 at 05:27

2

Use list comprehension like this

print([i * j for i in list1 for j in list2])

Output:

[12, 4, 36, 8, 6, 2, 18, 4, 21, 7, 63, 14]

answered Apr 06 '22 at 05:27

Manjunath K Mayya

1,078
1
11
20

BrokenBenchmark · Answer 2 · 2022-04-06T05:56:32.697

Here are two concise approaches.

The first uses itertools.product() and a list comprehension:

from itertools import product

[x * y for x, y in product(list1, list2)]

But, this problem is very well-suited for itertools.starmap(), which motivates the second approach. If you're unfamiliar with the function, it takes in two parameters:

A function that takes in two parameters. In this case, we use operator.mul, a version of the multiplication operator that we can pass into a function. Note that functions can be passed into other functions in Python because functions are first class.
An iterable of iterables of size two. In this case, it's our output from itertools.product().

For each element in our iterable, it unpacks the element and passes each element as a parameter into the function specified by the first parameter. This gives us:

from itertools import product, starmap
import operator

list(starmap(operator.mul, product(list1, list2)))

Both of these output:

[12, 4, 36, 8, 6, 2, 18, 4, 21, 7, 63, 14]

If you want to extend this approach to more than two iterables, you can do (as suggested by flakes):

from math import prod
list(map(prod, product(list1, list2, <specify more iterables>)))

Other answers have suggested using multiple for loops inside the comprehension. Note that some consider this approach to be poor style; the use of itertools.product() avoids this issue entirely.

_{If you have any questions, feel free to comment -- I'm more than happy to clarify any confusion. I realize that these solutions may not be the most beginner-friendly. At the very least, I hope that these approaches may be useful for future readers.}

score 1 · Accepted Answer · answered Apr 06 '22 at 05:25

You're close. What you really want is two for loops so you can compare each value in one list against all of the values in the second list. e.g.

def multiply_lists(list1, list2):
    for i in range(len(list1)):
        for j in range(len(list2)):
            products.append(list1[i] * list2[j])
    return products

You also don't need a range col, you can directly iterate over the items of each list:

def multiply_lists(list1, list2):
    for i in list1:
        for j in list2:
            products.append(i * j)
    return products

And you could also do this as a comprehension:

def multiply_lists(list1, list2):
    return [i * j for i in list1 for j in list2]

score 0 · Answer 4 · answered Apr 06 '22 at 05:31

You should use nested loops to multiply elements of 2 list or more list to multiply with each other.

Corrected Answer:

def multiply_lists(list1,list2):
    for i in list1:
        for j in list2:
            products.append(i*j)
    return products

list1 = [4,2,7]
list2 = [3,1,9,2]
products = []
print ('list1 = ',list1,';','list2 = ', list2) 
prod_list = multiply_lists(list1,list2)
print ('prod_list = ',prod_list)

Pairwise multiplication of elements between two lists

4 Answers4