Demonstrates how to rotate a sqaure 2D array by 90 degrees clockwise. Uses concept of geometric transformation.

rotate.c

#include <stdio.h>

void rotateRight90(int* sourceArray, int* destArray, int side)
{
	int srcY, srcX, destY, destX, elem;
	
	for (srcY = 1; srcY <= side; srcY++)
	{
		for (srcX = 1; srcX <= side; srcX++)
		{
			elem = sourceArray[(srcY - 1) * side + srcX - 1];
			destX = 1 + side - srcY;
			destY = srcX;
			destArray[(destY - 1) * side + destX - 1] = elem;
		}
	}
}

void printSquareArray(int* array, int side)
{
	int r, c;
	for (r = 0; r < side; r++)
	{
		for (c = 0; c < side; c++)
		{
			printf("%2d ", array[r * side + c]);
		}
		printf("\n");
	}
}

int main()
{
	int sourceArray[25] = 
		{1, 2, 3, 4, 5, 
		6, 7, 8, 9, 10, 
		11, 12, 13, 14, 15, 
		16, 17, 18, 19, 20, 
		21, 22, 23, 24, 25};
		
	int destArray[25];
	
	rotateRight90(sourceArray, destArray, 5);
	
	printf("Source array:\n\n");
	printSquareArray(sourceArray, 5);
	printf("\n\n\nRotated right by 90 degrees:\n\n");
	printSquareArray(destArray, 5);
	
	return 0;
}