NumPy prod()

Summary: in this tutorial, you’ll learn how to use the numpy prod() function to calculate the product of numbers in an array.

Introduction to to the NumPy prod() function

Suppose you have three numbers n, m, and k. The product of the three numbers is nxmxk. For example, the product of 2, 3, and 4 is 2x3x4 = 24.

To calcualte the products of numbers in an aray, you use the numpy prod() function:

numpy.prod(a, axis=None, dtype=None, out=None, keepdims=<no value>, initial=<no value>, where=<no value>)Code language: Python (python)

NumPy prod() function examples

Let’s take some example of using the numpy prod() function.

1) Using the numpy prod() function with 1-D array example

The following example uses the prod() to calculate the products of numbers in a 1-D array:

import numpy as np

a = np.arange(1, 5)
result = np.prod(a)

print(a)
print(f'result={result}')Code language: Python (python)

Output:

[1 2 3 4]
result=24Code language: Python (python)

How it works.

First, create an array that has 4 numbers from 1 to 4 using the arange() function.

a = np.arange(1, 5)Code language: Python (python)

Second, calculate the products of all the numbers in the array a:

result = np.prod(a)Code language: Python (python)

Third, display the numbers of the array a and their product:

print(a)
print(f'result={result}')Code language: Python (python)

Note that you can pass an array like object to the prod() function e.g., a list. For example:

import numpy as np

result = np.prod([1, 2, 3, 4, 5])

print(f'result={result}')Code language: Python (python)

Output:

result=120Code language: Python (python)

2) Using the numpy prod() function with multidimensional array examples

The following example uses the prod() to calcualte products of all numbers in a 2-D array:

import numpy as np

result = np.prod([
    [1, 2],
    [3, 4]
])

print(f'result={result}')Code language: Python (python)

Output:

result=24Code language: Python (python)

To calcualte product of numbers of an axis, you can specify the axis argument. For example, the following uses the prod() to calculate the product of numbers on axis 0:

import numpy as np

result = np.prod([
    [1, 2],
    [3, 4]
], axis=0)

print(f'result={result}')Code language: Python (python)

Output:

result=[3 8]Code language: Python (python)

Similarly, you can calculate the product of numbers on axis 1:

import numpy as np

result = np.prod([
    [1, 2],
    [3, 4]
], axis=1)

print(f'result={result}')Code language: Python (python)

Output:

result=[ 2 12]Code language: Python (python)

3) Selecting numbers to include in the product

To select specific number to include in the product, you use the where argument. For example:

import numpy as np

a = np.array([np.nan, 3, 4])
result = np.prod(a, where=[False, True, True])
print(result)Code language: Python (python)

Output:

12.0Code language: Python (python)

In this example, the array contains three elements np.nan, 3, and 4.

The where argument uses a boolean list to specify which element in the array a should be included in the product.

If the value of the where list is True, the corresponding element of the input array will be included in the product.

4) Special cases

Note that if you pass an array of integers to the prod() function that causes an overflow, the prod() won’t raise the error. For example:

import numpy as np

result = np.prod(np.arange(1, 100))
print(f'result={result}')Code language: Python (python)

Output:

result=0Code language: Python (python)

The prod() function returns 1 if the array is empty. For example:

import numpy as np

result = np.prod(np.array([]))

print(f'result={result}')Code language: Python (python)

Output:

1.0Code language: Python (python)

Summary

• Use numpy prod() function to calculate the product of numbers in an array.