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
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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]
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Posted on: March 12, 2021 by Deven
In this article, you will learn how to multiply array by scalar in python.
Let’s say you have 2 arrays that need to be multiplied by scalar n
.
array1 = np.array[[1, 2, 3]]
array2 = np.array[[[1, 2], [3, 4]]]
n = 5
Numpy multiply array by scalar
In order to multiply array by scalar in
python, you can use np.multiply[]
method.
import numpy as np
array1 = np.array[[1, 2, 3]]
array2 = np.array[[[1, 2], [3, 4]]]
n = 5
np.multiply[array1,n]
np.multiply[array2,n]
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PreviousNextYou can multiply numpy arrays by scalars and it just works.
>>> import numpy as np
>>> np.array[[1, 2, 3]] * 2
array[[2, 4, 6]]
>>> np.array[[[1, 2, 3], [4, 5, 6]]] * 2
array[[[ 2, 4, 6],
[ 8, 10, 12]]]
This is also a very fast and efficient operation. With your example:
>>> a_1 = np.array[[1.0, 2.0, 3.0]]
>>> a_2 = np.array[[[1., 2.], [3., 4.]]]
>>> b = 2.0
>>> a_1 * b
array[[2., 4., 6.]]
>>> a_2 * b
array[[[2., 4.],
[6., 8.]]]
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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]