How to multiply an array by a constant in python

Multiplying a constant to a NumPy array is as easy as multiplying two numbers. To multiply a constant to each and every element of an array, use multiplication arithmetic operator *. To multiplication operator, pass array and constant as operands as shown below.

b = a * c

Run

where a is input array and c is a constant. b is the resultant array.

Example

In the following python example, we will multiply a constant 3 to an array a. The resulting array is stored in b.

import numpy as np

#2D array
a = (np.arange(8)*2).reshape(2,4)

#print array
print("The array\n",a)

#multiplying a constant to all the elements of array
b = a * 3

print("\nAfter multiplying a constant to all the elements of array\n",b)

Run

View Discussion

Improve Article

Save Article

View Discussion

Improve Article

Save Article

While working with the python lists, we can come over a situation in which we require to multiply constant to each element in the list. We possibly need to iterate and multiply constant to each element but that would increase the line of code. Let’s discuss certain shorthands to perform this task.

Method #1 : Using List Comprehension 
List comprehension is just the short way to perform the task we perform using the naive method. This is mainly useful to save time and also is best among others when it comes to the readability of the code.

Python3

test_list = [4, 5, 6, 3, 9]

print ("The original list is : " + str(test_list))

K = 4

res = [x * K for x in test_list]

print ("The list after constant multiplication : " + str(res))

Output

The original list is : [4, 5, 6, 3, 9]
The list after constant multiplication : [16, 20, 24, 12, 36]

 
Method #2 : Using map() + operator.mul 
This is similar to the above function but uses the operator.mul to multiply each element to other element from the other list of K formed before applying the map function. It multiplies the similar index elements of list.

Python3

import operator

test_list = [4, 5, 6, 3, 9]

print ("The original list is : " + str(test_list))

K_list = [4] * len(test_list)

res = list(map(operator.mul, test_list, K_list))

print ("The list after constant multiplication : " + str(res))

Output

The original list is : [4, 5, 6, 3, 9]
The list after constant multiplication : [16, 20, 24, 12, 36]