np.outer: How to Calculate Product of Vectors in Python

To calculate the outer product of two vectors in Python, use the numpy outer() function. Numpy is a mathematical library used to calculate vector algebra.

np.outer

The np.outer() is a numpy library function used to calculate the outer product of two given vectors. If we have two vectors A [ a,a1,a2,..an] and B [ b0,b1,b2,…bn], the outer product of these two vectors will be:

[ [ a0*b0 a0*b1 a0*b2 … a0*bn]

  [ a1*b0 a0*b1 a1*b2 … a1*bn]

  [ ……………………………….] ] 

Syntax

numpy.outer(arr1, arr2, out = None)

Parameters

The outer() function takes two main parameters, which are:

1. arr1: This is the first array.
2. arr2: This is the second array.

Also, there is one optional parameter :

1. out: This is the location where the result is stored.

Return Value

The outer() function returns a vector containing the given vectors’ outer product.

Programming Example

Program to find outer() of two numeric vectors

# Program to find outer() of two numeric vectors
import numpy as np

# Declaring the first array
arr1 = np.array([-2, -1, 0, 1, 2, 3, 4, 5])
arr2 = np.array([0, 1, 2, 3, 4, 5, 6, 7])

print("First array is :\n", arr1)
print("Second array is :\n", arr2)

# Calculating the outer product
ans = np.outer(arr1, arr2)

print("Outer Product of these vectors are:\n", ans)

Output

First array is :
 [-2 -1  0  1  2  3  4  5]
Second array is :
 [0 1 2 3 4 5 6 7]
Outer Product of these vectors are:
 [[  0  -2  -4  -6  -8 -10 -12 -14]
 [  0  -1  -2  -3  -4  -5  -6  -7]
 [  0   0   0   0   0   0   0   0]
 [  0   1   2   3   4   5   6   7]
 [  0   2   4   6   8  10  12  14]
 [  0   3   6   9  12  15  18  21]
 [  0   4   8  12  16  20  24  28]
 [  0   5  10  15  20  25  30  35]]

Explanation

First, we have created two arrays. Then we printed those two arrays. Then we called numpy.outer() to get the outer vector product. The output is produced using the np.outer() method.

Find the product of characters using an outer() method

What if we take two arrays, one has characters, and one has integers?

# Program to find outer() when the given product has characters:
import numpy as np

# Declaring the first array
arr1 = np.array(['a', 'b', 'c', 'd'], dtype=object)
arr2 = np.array([1, 2, 3, 4])

print("First array is :\n", arr1)
print("Second array is :\n", arr2)

# Calculating the outer product
ans = np.outer(arr1, arr2)

print("Outer Product of these vectors are:\n", ans)

Output

First array is :
 ['a' 'b' 'c' 'd']
Second array is :
 [1 2 3 4]
Outer Product of these vectors are:
 [['a' 'aa' 'aaa' 'aaaa']
 ['b' 'bb' 'bbb' 'bbbb']
 ['c' 'cc' 'ccc' 'cccc']
 ['d' 'dd' 'ddd' 'dddd']]

Explanation

In this example, we first created a character-type vector and one vector containing numeric values.

In the next step, we want to calculate the outer vector product of these two different variable-type vectors.

When we call this, we can see a different result. As per the string rule, when we multiply any character/string with any number, we get many copies of that character or string. As we can see, a*1=a, a*2=aa, and so on.

That is it for the np.outer() method.

See also

np.inner()

np.dot()

np.cbrt()