# C++ Program for Block swap algorithm for array rotation

• Difficulty Level : Medium
• Last Updated : 13 Dec, 2021

Write a function rotate(ar[], d, n) that rotates arr[] of size n by d elements. Rotation of the above array by 2 will make array Algorithm :

```Initialize A = arr[0..d-1] and B = arr[d..n-1]
1) Do following until size of A is equal to size of B

a)  If A is shorter, divide B into Bl and Br such that Br is of same
length as A. Swap A and Br to change ABlBr into BrBlA. Now A
is at its final place, so recur on pieces of B.

b)  If A is longer, divide A into Al and Ar such that Al is of same
length as B Swap Al and B to change AlArB into BArAl. Now B
is at its final place, so recur on pieces of A.

2)  Finally when A and B are of equal size, block swap them.```

Recursive Implementation:

## C++

 `#include ``using` `namespace` `std;`` ` `/*Prototype for utility functions */``void` `printArray(``int` `arr[], ``int` `size); ``void` `swap(``int` `arr[], ``int` `fi, ``int` `si, ``int` `d); `` ` `void` `leftRotate(``int` `arr[], ``int` `d, ``int` `n) ``{ ``    ``/* Return If number of elements to be rotated  ``    ``is zero or equal to array size */``    ``if``(d == 0 || d == n) ``        ``return``; ``         ` `    ``/*If number of elements to be rotated ``    ``is exactly half of array size */``    ``if``(n - d == d) ``    ``{ ``        ``swap(arr, 0, n - d, d); ``        ``return``; ``    ``} ``         ` `    ``/* If A is shorter*/`        `    ``if``(d < n - d) ``    ``{ ``        ``swap(arr, 0, n - d, d); ``        ``leftRotate(arr, d, n - d);     ``    ``} ``    ``else` `/* If B is shorter*/`        `    ``{ ``        ``swap(arr, 0, d, n - d); ``        ``leftRotate(arr + n - d, 2 * d - n, d); ``/*This is tricky*/``    ``} ``} `` ` `/*UTILITY FUNCTIONS*/``/* function to print an array */``void` `printArray(``int` `arr[], ``int` `size) ``{ ``    ``int` `i; ``    ``for``(i = 0; i < size; i++) ``        ``cout << arr[i] << ``" "``; ``    ``cout << endl; ``} `` ` `/*This function swaps d elements starting at index fi ``with d elements starting at index si */``void` `swap(``int` `arr[], ``int` `fi, ``int` `si, ``int` `d) ``{ ``    ``int` `i, temp; ``    ``for``(i = 0; i < d; i++) ``    ``{ ``        ``temp = arr[fi + i]; ``        ``arr[fi + i] = arr[si + i]; ``        ``arr[si + i] = temp; ``    ``} ``} `` ` `// Driver Code``int` `main() ``{ ``    ``int` `arr[] = {1, 2, 3, 4, 5, 6, 7}; ``    ``leftRotate(arr, 2, 7); ``    ``printArray(arr, 7); ``    ``return` `0; ``} `` ` `// This code is contributed by rathbhupendra`

Iterative Implementation:
Here is iterative implementation of the same algorithm. Same utility function swap() is used here.

## C++

 `// C++ code for above implementation``void` `leftRotate(``int` `arr[], ``int` `d, ``int` `n)``{``    ``int` `i, j;``    ``if` `(d == 0 || d == n)``        ``return``;``         ` `    ``i = d;``    ``j = n - d;``    ``while` `(i != j)``    ``{``        ``if` `(i < j) ``/*A is shorter*/``        ``{``            ``swap(arr, d - i, d + j - i, i);``            ``j -= i;``        ``}``        ``else` `/*B is shorter*/``        ``{``            ``swap(arr, d - i, d, j);``            ``i -= j;``        ``}``         ` `        ``// printArray(arr, 7);``    ``}``     ` `    ``/*Finally, block swap A and B*/``    ``swap(arr, d - i, d, i);``}`` ` `// This code is contributed by Shivani`

Time Complexity: O(n)
Please see following posts for other methods of array rotation:
https://www.geeksforgeeks.org/array-rotation/
https://www.geeksforgeeks.org/program-for-array-rotation-continued-reversal-algorithm/

References:
http://www.cs.bell-labs.com/cm/cs/pearls/s02b.pdf
Please write comments if you find any bug in the above programs/algorithms or want to share any additional information about the block swap algorithm.

Please refer complete article on Block swap algorithm for array rotation for more details!

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