Numpy.diag() method is “used to extract and construct a diagonal array.”
Syntax
numpy.diag(arr,k)
Parameters
It takes two parameters, out of which one parameter is optional.
- arr: It is an input array
- k: It is optional and takes 0 by default. If the value of this parameter is greater than 0, it means the diagonal is above the main diagonal, and vice versa if it is not.
Return Value
It returns an array with a diagonal array.
Example 1: How to Use numpy.diag() Method
import numpy as np
a = np.matrix([[1, 2, 3], [4, 5, 6], [9, 8, 7]])
print("Main Diagonal: \n", np.diag(a), "\n")
print("Above main diagonal: \n", np.diag(a, 1),
"\n") # k=1 (for above main diagonal)
print("Below main diagonal: \n", np.diag(a, -1)) # k=-1 (for below main diagonal)
Output
Main Diagonal: [1 5 7]
Above main diagonal: [2 6]
Below main diagonal: [4 8]Example 2: How does the np.diag() Method work?
import numpy as np
a = np.matrix([[1, 2, 3], [4, 5, 6], [9, 8, 7], [11, 13, 15]])
print("Main Diagonal: \n", np.diag(a), "\n")
# k=1 (for above main diagonal)
print("Above main diagonal: \n", np.diag(a, 1), "\n")
# k=-1 (for below main diagonal)
print("Below main diagonal: \n", np.diag(a, -1))Output
Main Diagonal:
[1 5 7]
Above main diagonal:
[2 6]
Below main diagonal:
[ 4 8 15]Example 3: Use a 4×4 Matrix and Apply the diag() Function
import numpy as np
# Define a 4x4 matrix
matrix = np.array([[1, 2, 3, 4],
[5, 6, 7, 8],
[9, 10, 11, 12],
[13, 14, 15, 16]])
print("Original Matrix:")
print(matrix)
# Extract the diagonal elements
diagonal = np.diag(matrix)
print("\nDiagonal Elements:")
print(diagonal)Output
Original Matrix:
[[ 1 2 3 4]
[ 5 6 7 8]
[ 9 10 11 12]
[13 14 15 16]]
Diagonal Elements:
[ 1 6 11 16]
Example 4: Use Construct Diagonal From Python NumPy Array
By using numpy.diag() function, we can also create a matrix with diagonal values.
import numpy as np
arr = np.array([11, 19, 21, 46])
arr2 = np.diag(arr)
print(arr2)
Output
[[11 0 0 0]
[ 0 19 0 0]
[ 0 0 21 0]
[ 0 0 0 46]]
That’s it.
Ankit Lathiya is a Master of Computer Application by education and Android and Laravel Developer by profession and one of the authors of this blog. He is also expert in JavaScript and Python development.
