Rotate Matrix by 90 Degrees in C

In this article, we are given an N x N matrix, and our task is to rotate it by 90 degrees clockwise in C.

Example

Input:
[1 2 3], [4 5 6], [7 8 9]

Output:
[7 4 1], [8 5 2], [9 6 3]

Below are different approaches to rotate a matrix by 90 degrees in the clockwise direction:

• Using Brute Force Approach
• Using In-Place Rotation

Using Brute Force Approach

This is the simple and direct approach to understand and implement. In this approach, we create a new matrix and manually place each element at its correct rotated position.

• Define a function and take the given matrix and its size as input parameters.
• Now, create a new matrix of the same size.Iterate through the elements and place them in their new correct rotated positions.
• Copy the rotated matrix back to the original matrix. Return the rotated matrix.

Example

Here is an example code implementing above steps to rotate a matrix by 90 degrees in the clockwise direction using brute force approach.

#include <stdio.h>
#define N 3
void rotateMatrix(int matrix[N][N])
{
    int rotated[N][N];
    for (int i = 0; i < N; i++)
    {
        for (int j = 0; j < N; j++)
        {
            rotated[j][N - 1 - i] = matrix[i][j];
        }
    }
    for (int i = 0; i < N; i++)
    {
        for (int j = 0; j < N; j++)
        {
            matrix[i][j] = rotated[i][j];
        }
    }
}
void printMatrix(int matrix[N][N])
{
    for (int i = 0; i < N; i++)
    {
        for (int j = 0; j < N; j++)
        {
            printf("%d ", matrix[i][j]);
        }
        printf("\n");
    }
}
int main()
{
    int matrix[N][N] = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
    printf("Original Matrix: \n");
    printMatrix(matrix);
    rotateMatrix(matrix);
    printf("Rotated Matrix: \n");
    printMatrix(matrix);
    return 0;
}

Using In-Place Rotation

This is a space-optimized approach. We make modifications in the input matrix without taking a new matrix. In this approach, we will rotate the matrix in place by first transposing the matrix and then reversing the row.

• Define a function and take the matrix and its size as input.
• Now, transpose the matrix (Transposing of the matrix is converting the rows into columns and vice-versa.)
• Reverse each row to obtain the required rotated matrix. Print the rotated Matrix.

Example

The following example code uses in-place rotation to rotate a matrix by 90 degrees in the clockwise direction.

#include <stdio.h>
#define N 3
void rotateMatrixInPlace(int matrix[N][N])
{
    for (int i = 0; i < N; i++)
    {
        for (int j = i + 1; j < N; j++)
        {
            int temp = matrix[i][j];
            matrix[i][j] = matrix[j][i];
            matrix[j][i] = temp;
        }
    }
    for (int i = 0; i < N; i++)
    {
        for (int j = 0; j < N / 2; j++)
        {
            int temp = matrix[i][j];
            matrix[i][j] = matrix[i][N - 1 - j];
            matrix[i][N - 1 - j] = temp;
        }
    }
}
void printMatrix(int matrix[N][N])
{
    for (int i = 0; i < N; i++)
    {
        for (int j = 0; j < N; j++)
        {
            printf("%d ", matrix[i][j]);
        }
        printf("\n");
    }
}
int main()
{
    int matrix[N][N] = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
    printf("Original Matrix: \n");
    printMatrix(matrix);
    rotateMatrixInPlace(matrix);
    printf("Rotated Matrix: \n");
    printMatrix(matrix);
    return 0;
}

Complexity Comparison

Here is a comparison of time and space complexity of all the above approaches.