NumPy: Multiply an array of dimension by an array with dimensions

Last update on August 29 2025 12:48:49 (UTC/GMT +8 hours)

Multiply arrays of compatible dimensions.

Write a NumPy program to multiply an array of dimension (2,2,3) by an array with dimensions (2,2).

Sample Solution:

Python Code:

# Importing the NumPy library
import numpy as np

# Creating a NumPy array 'nums1' consisting of ones with shape (2, 2, 3)
nums1 = np.ones((2, 2, 3))

# Creating another NumPy array 'nums2' consisting of threes with shape (2, 2)
nums2 = 3 * np.ones((2, 2))

# Displaying the original array 'nums1'
print("Original array:")
print(nums1)

# Performing element-wise multiplication of 'nums1' and 'nums2' along the second axis using None to add a new axis
new_array = nums1 * nums2[:, :, None]

# Displaying the new array obtained after the multiplication
print("\nNew array:")
print(new_array)

Sample Output:

Original array:
[[[1. 1. 1.]
  [1. 1. 1.]]

 [[1. 1. 1.]
  [1. 1. 1.]]]

New array:
[[[3. 3. 3.]
  [3. 3. 3.]]

 [[3. 3. 3.]
  [3. 3. 3.]]]

Explanation:

nums1 = np.ones((2,2,3)): Creates a 3D NumPy array nums1 of shape (2, 2, 3) filled with ones.

nums2 = 3*np.ones((2,2)): Creates a 2D NumPy array nums2 of shape (2, 2) filled with threes.

new_array = nums1 * nums2[:,:,None]

In the above code -

nums2[:,:,None]: Adds a new axis to nums2 to make it a 3D array with shape (2, 2, 1). This is done to match the shape of nums1 for broadcasting.
new_array = nums1 * nums2[:,:,None]: Performs element-wise multiplication of nums1 and the reshaped nums2, using broadcasting. Since nums1 contains ones and nums2 contains threes, the resulting new_array will be a 3D array of shape (2, 2, 3) filled with threes.

For more Practice: Solve these Related Problems:

Write a NumPy program to multiply a 3D array of shape (2,2,3) with a 2D array of shape (2,2) using broadcasting.
Create a function that scales each 2D sub-array by corresponding elements of a smaller array.
Implement a solution that reshapes the 2D array to allow element-wise multiplication with the 3D array.
Test the multiplication on arrays with random values to ensure that broadcasting rules are correctly applied.

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