Program to print the Diagonals of a Matrix in O(N) time

• Last Updated : 27 Apr, 2021

Given a 2D square matrix, the task is to print the Principal and Secondary diagonals of this matrix in O(N) time complexity. For O(N2) time, please refer this article

Examples:

Input:
4
1 2 3 4
4 3 2 1
7 8 9 6
6 5 4 3
Output:
Principal Diagonal: 1, 3, 9, 3
Secondary Diagonal: 4, 2, 8, 6

Input:
3
1 1 1
1 1 1
1 1 1
Output:
Principal Diagonal: 1, 1, 1
Secondary Diagonal: 1, 1, 1

Approach:

1.Consider the following 4 X 4 input matrix.

A00 A01 A02 A03
A10 A11 A12 A13
A20 A21 A22 A23
A30 A31 A32 A33

2.The primary diagonal is formed by the elements A00, A11, A22, A33.

Condition for Principal Diagonal:

The row-column condition is row = column.

3.The secondary diagonal is formed by the elements A03, A12, A21, A30.

Condition for Secondary Diagonal:

The row-column condition is row = numberOfRows - column - 1.

In this method, we use one loop i.e. a loop to find the diagonal elements as per the below formula:

principal diagonal = matrix[i][i];
secondary diagonal = matrix[i][n - i - 1];

where 0 <= i <= n

Below is the implementation of the above approach:

C++

 // C++ Program to print the Diagonals of a Matrix #include using namespace std; const int MAX = 100; // Function to print the Principal Diagonalvoid printPrincipalDiagonal(int mat[][MAX], int n){    cout << "Principal Diagonal: ";     for (int i = 0; i < n; i++) {         // Condition for principal diagonal        cout << mat[i][i] << ", ";    }    cout << endl;} // Function to print the Secondary Diagonalvoid printSecondaryDiagonal(int mat[][MAX], int n){    cout << "Secondary Diagonal: ";     for (int i = 0; i < n; i++) {         // Condition for secondary diagonal        cout << mat[i][n - i - 1] << ", ";    }     cout << endl;} // Driver codeint main(){    int n = 4;    int a[][MAX] = { { 1, 2, 3, 4 },                     { 5, 6, 7, 8 },                     { 1, 2, 3, 4 },                     { 5, 6, 7, 8 } };     printPrincipalDiagonal(a, n);    printSecondaryDiagonal(a, n);    return 0;}

Java

 // Java Program to print the Diagonals of a Matrixclass GFG{         static final int MAX = 100;         // Function to print the Principal Diagonal    static void printPrincipalDiagonal(int mat[][], int n)    {        System.out.print("Principal Diagonal: ");             for (int i = 0; i < n; i++)        {                 // Condition for principal diagonal            System.out.print(mat[i][i] + ", ");        }        System.out.println();    }         // Function to print the Secondary Diagonal    static void printSecondaryDiagonal(int mat[][], int n)    {        System.out.print("Secondary Diagonal: ");             for (int i = 0; i < n; i++)        {                 // Condition for secondary diagonal            System.out.print(mat[i][n - i - 1] + ", ");        }             System.out.println();    }         // Driver code    public static void main (String[] args)    {        int n = 4;        int a[][] = { { 1, 2, 3, 4 },                        { 5, 6, 7, 8 },                        { 1, 2, 3, 4 },                        { 5, 6, 7, 8 } };             printPrincipalDiagonal(a, n);        printSecondaryDiagonal(a, n);    }} // This code is contributed by AnkitRai01

Python3

 # Python Program to print the Diagonals of a MatrixMAX = 100; # Function to print the Principal Diagonaldef printPrincipalDiagonal(mat, n):    print("Principal Diagonal: ", end = "");     for i in range(n):         # Condition for principal diagonal        print(mat[i][i], end= ", ");         print(); # Function to print the Secondary Diagonaldef printSecondaryDiagonal(mat, n):    print("Secondary Diagonal: ", end = "");     for i in range(n):         # Condition for secondary diagonal        print(mat[i][n - i - 1], end = ", ");         print(); # Driver codeif __name__ == '__main__':    n = 4;    a = [[ 1, 2, 3, 4 ],        [ 5, 6, 7, 8 ],        [ 1, 2, 3, 4 ],        [ 5, 6, 7, 8 ]];     printPrincipalDiagonal(a, n);    printSecondaryDiagonal(a, n); # This code is contributed by PrinciRaj1992

C#

 // C# Program to print the Diagonals of a Matrixusing System; class GFG{         // Function to print the Principal Diagonal    static void printPrincipalDiagonal(int [,]mat, int n)    {        Console.Write("Principal Diagonal: ");             for (int i = 0; i < n; i++)        {                 // Condition for principal diagonal            Console.Write(mat[i, i] + ", ");        }        Console.WriteLine();    }         // Function to print the Secondary Diagonal    static void printSecondaryDiagonal(int [,]mat, int n)    {        Console.Write("Secondary Diagonal: ");             for (int i = 0; i < n; i++)        {                 // Condition for secondary diagonal            Console.Write(mat[i, n - i - 1] + ", ");        }             Console.WriteLine();    }         // Driver code    public static void Main()    {        int n = 4;        int [,]a = { { 1, 2, 3, 4 },                     { 5, 6, 7, 8 },                     { 1, 2, 3, 4 },                     { 5, 6, 7, 8 } };             printPrincipalDiagonal(a, n);        printSecondaryDiagonal(a, n);    }} // This code is contributed by AnkitRai01

Javascript


Output:
Principal Diagonal: 1, 6, 3, 8,
Secondary Diagonal: 4, 7, 2, 5,

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