Thursday, July 12, 2012

Write a program to rotate a matrix by 90 degrees

e.g)
Input:-
1 2 3
4 5 6
7 8 9

Output:-
7 4 1
8 5 2
9 6 3

Approach to rotate a matrix by 90 degrees

First we transpose the matrix and swap the columns to rotate the matrix by 90 degrees.
1) Transpose the matrix.
1 4 7
2 5 8
3 6 9

2) Swap the columns.
7 4 1
8 5 2
9 6 3

C++ program to rotate a matrix by 90 degress

#include <iostream>
#include <cstdlib>
#include <ctime>
using namespace std;

int main() {
    srand(time(0));

    int m = 3;
    int n = 3;
    int** matrix = (int**) calloc(sizeof(int), m);
    for ( int i = 0; i < m; i++ ) {
        matrix[i] = (int*) calloc(sizeof(int), n);
    }

    // Fill random values in the matrix
    for ( int i = 0; i < m; i++ ) {
        for ( int j = 0; j < n; j++ ) {
            matrix[i][j] = rand() % 100;
        }
    }

    // Print the matrix
    for ( int i = 0; i < m; i++ ) {
        for ( int j = 0; j < n; j++ ) {
            cout << matrix[i][j] << " ";
        }
        cout << endl;
    }

    cout << endl;

    // Transpose the matrix
    for ( int i = 0; i < m; i++ ) {
        for ( int j = i + 1; j < n; j++ ) {
            int tmp = matrix[i][j];
            matrix[i][j] = matrix[j][i];
            matrix[j][i] = tmp;
        }
    }

    // Swap the columns
    for ( int i = 0; i < m; i++ ) {
        for ( int j = 0; j < n/2; j++ ) {
            int tmp = matrix[i][j];
            matrix[i][j] = matrix[i][n-1-j];
            matrix[i][n-1-j] = tmp;
        }
    }

    // Print the rotated matrix
    for ( int i = 0; i < m; i++ ) {
        for ( int j = 0; j < n; j++ ) {
            cout << matrix[i][j] << " ";
        }
        cout << endl;
    }
}

Output:-
28 66 73
67 21 63
87 54 59

87 67 28
54 21 66
59 63 73

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