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NumPy - Stack a 3x3 identity matrix vertically and horizontally

By IncludeHelp Last updated : December 13, 2023

Problem statement

Write a Python program to stack a NumPy 3x3 identity matrix vertically and horizontally.

Prerequisites

To understand the given solution, you should know about the following Python topics:

• Python NumPy
• Python print() Method
• Python numpy.identity() Method
• Python numpy.hstack() Method
• Python numpy.vstack() Method

Stack a 3x3 identity matrix vertically and horizontally

To stack a 3x3 identity matrix vertically, use numpy.vstack() method (which stacks the arrays in sequence vertically (row wise).), and to stack it horizontally, use numpy.hstack() method (which stacks the arrays in sequence horizontally (column wise).). To create a 3x3 identity matrix, use numpy.identity() method. In both of the methods hstack() and vstack(), you need to pass the identity matrix three times (as a tuple).

Below are the syntaxes of numpy.vstack() and numpy.hstack() methods:

numpy.vstack(tup, *, dtype=None, casting='same_kind')
numpy.hstack(tup, *, dtype=None, casting='same_kind')

Python program to stack a 3x3 identity matrix vertically and horizontally

Create a 3x3 identity matrix, and stack it vertically and horizontally.

# Importing numpy library
import numpy as np

# Creating a 3x3 identity matrix
iMatrix = np.identity(3)

# Printing the matrix
print("3x3 identity matrix (iMatrix) is:\n")
print(iMatrix)

# Stacking the iMatrix vertically
vStack = np.vstack((iMatrix, iMatrix, iMatrix))
# Stacking the iMatrix horizontally
hStack = np.hstack((iMatrix, iMatrix, iMatrix))

# Printing the stacked matrices
print("\n3x3 Identity Matrix (Vertical Stack):\n", vStack)
print("\n3x3 Identity Matrix (Horizontal Stack):\n", hStack)

Output

The output of the above program is:

3x3 identity matrix (iMatrix) is:

[[1. 0. 0.]
 [0. 1. 0.]
 [0. 0. 1.]]

3x3 Identity Matrix (Vertical Stack):
 [[1. 0. 0.]
 [0. 1. 0.]
 [0. 0. 1.]
 [1. 0. 0.]
 [0. 1. 0.]
 [0. 0. 1.]
 [1. 0. 0.]
 [0. 1. 0.]
 [0. 0. 1.]]

3x3 Identity Matrix (Horizontal Stack):
 [[1. 0. 0. 1. 0. 0. 1. 0. 0.]
 [0. 1. 0. 0. 1. 0. 0. 1. 0.]
 [0. 0. 1. 0. 0. 1. 0. 0. 1.]]

Code Explanation

• To use NumPy in the program, we imported the numpy module.
• Created a 3x3 identity matrix (iMatrix) by using the np.identity() method and printed it.
• Then, to stack this 3x3 identity matrix vertically and horizontally, we used np.vstack() and np.hstack() methods.
• Finally, printed both of the results (vStack and hStack).

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