Write a program to rotate an array. (Method 2)

Question: Write a program to rotate an array arr[], having a size ‘n’ by ‘d’ elements?

Input: arr[] = { 4, 8, 15, 16, 23, 42, 99 }, d = 3
Output: 16, 23, 42, 99, 4, 8, 15

Let us suppose our array is in this way:

On rotating the array by 3 elements, it should look like:

We can do this by rotating the array one element at a time. We discussed it in this post.

We can also use the REVERSAL Algorithm to rotate the array. This can be done as:-

rotate(arr[], d, n)
{
    reverse(arr[], 1, d) ;
    reverse(arr[], d + 1, n);
    reverse(arr[], l, n);
}

Let AB are the two parts of the input array where A = arr[0..d-1] and B = arr[d..n-1]. The idea of the algorithm is:
Reverse A to get ArB. /* Ar is reverse of A */
Reverse B to get ArBr. /* Br is reverse of B */
Reverse all to get (ArBr) r = BA.

For arr[] = [1, 2, 3, 4, 5, 6, 7], d =2 and n = 7
A = [1, 2] and B = [3, 4, 5, 6, 7]
Reverse A, we get ArB = [2, 1, 3, 4, 5, 6, 7]
Reverse B, we get ArBr = [2, 1, 7, 6, 5, 4, 3]
Reverse all, we get (ArBr)r = [3, 4, 5, 6, 7, 1, 2]

Here is the implementation of the same:-

//Function to reverse arr[] from index start to end
void reverseArray(int arr[], int start, int end)
{
	int i;
	int temp;
	while(start < end)
	{
		temp = arr[start];
		arr[start] = arr[end];
		arr[end] = temp;
		start++;
		end--;
	}
}

//Function to left rotate arr[] of size n by d
void leftRotate(int arr[], int d, int n)
{
	reverseArray(arr, 0, d-1);
	reverseArray(arr, d, n-1);
	reverseArray(arr, 0, n-1);
}

// Function to print an array
void printArray(int arr[], int size)
{
	int i;
	for(i = 0; i < size; i++)
		printf("%d ", arr[i]);
	printf("%\n ");
}

//Driver program to test above functions
int main(void)
{
	int arr[] = {4, 8, 15, 16, 23, 42, 99};

	leftRotate(arr, 3, 7);
	printArray(arr, 7);

	return 0;
}