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Block swap algorithm for array rotation

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  • Difficulty Level : Medium
  • Last Updated : 01 Jul, 2022

Write a function rotate(arr[], d, n) that rotates arr[] of size n by d elements. 

Array

Rotation of the above array by 2 will make array

ArrayRotation1

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.

Complete Interview Preparation - GFG 2) Finally when A and B are of equal size, block swap them.

Recursive Implementation:

C++




#include <bits/stdc++.h>
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 more than array size*/ 
      if(d > n)
      d = d % n;
    /*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


C




#include<stdio.h>
  
/*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++)
    printf("%d ", arr[i]);
  printf("\n ");
  
/*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 program to test above functions */
int main()
{
   int arr[] = {1, 2, 3, 4, 5, 6, 7};
   leftRotate(arr, 2, 7);
   printArray(arr, 7);
   getchar();
   return 0;
}    


Java




import java.util.*;
  
class GFG 
{
    // Wrapper over the recursive function leftRotateRec()
    // It left rotates arr[] by d.
    public static void leftRotate(int arr[], int d, 
                                                int n)
    {
        leftRotateRec(arr, 0, d, n);
    }
  
    public static void leftRotateRec(int arr[], int i, 
                                  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, i, n - d + i, d); 
            return
        
          
        /* If A is shorter*/    
        if(d < n - d) 
        
            swap(arr, i, n - d + i, d); 
            leftRotateRec(arr, i, d, n - d);     
        
        else /* If B is shorter*/    
        
            swap(arr, i, d, n - d); 
            leftRotateRec(arr, n - d + i, 2 * d - n, d); /*This is tricky*/
        
    }
  
/*UTILITY FUNCTIONS*/
/* function to print an array */
public static void printArray(int arr[], int size) 
    int i; 
    for(i = 0; i < size; i++) 
        System.out.print(arr[i] + " "); 
    System.out.println(); 
  
/*This function swaps d elements 
starting at index fi with d elements
starting at index si */
public static 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
public static void main (String[] args) 
{
    int arr[] = {1, 2, 3, 4, 5, 6, 7}; 
    leftRotate(arr, 2, 7); 
    printArray(arr, 7);     
}
}
  
// This code is contributed by codeseeker


Python3




# Wrapper over the recursive function leftRotateRec()
# It left rotates arr by d.
def leftRotate(arr, d, n):
    leftRotateRec(arr, 0, d, n);
  
def leftRotateRec(arr, i, d, n):
    '''
     * Return If number of elements to be 
     rotated is zero or equal to array size
     '''
    if (d == 0 or d == n):
        return;
  
    '''
     * If number of elements to be rotated 
     is exactly half of array size
     '''
    if (n - d == d):
        swap(arr, i, n - d + i, d);
        return;
  
    ''' If A is shorter '''
    if (d < n - d):
        swap(arr, i, n - d + i, d);
        leftRotateRec(arr, i, d, n - d);
        ''' If B is shorter '''
    else:
        swap(arr, i, d, n - d);
          
        ''' This is tricky '''
        leftRotateRec(arr, n - d + i, 2 * d - n, d); 
  
''' UTILITY FUNCTIONS '''
''' function to print an array '''
def printArray(arr, size):
    for i in range(size):
        print(arr[i], end = " ");
    print();
  
'''
 * This function swaps d elements starting at 
 * index fi with d elements starting at index si
 '''
def swap(arr, fi, si, d):
    for i in range(d):
        temp = arr[fi + i];
        arr[fi + i] = arr[si + i];
        arr[si + i] = temp;
  
# Driver Code
if __name__ == '__main__':
    arr = [1, 2, 3, 4, 5, 6, 7];
    leftRotate(arr, 2, 7);
    printArray(arr, 7);
  
# This code is contributed by Rohit_ranjan


C#




using System;
  
class GFG{
      
// Wrapper over the recursive function
// leftRotateRec() 
// It left rotates []arr by d.
public static void leftRotate(int []arr, 
                              int d, int n)
{
    leftRotateRec(arr, 0, d, n);
}
  
public static void leftRotateRec(int []arr, int i, 
                                 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, i, n - d + i, d); 
        return
    
      
    // If A is shorter
    if(d < n - d) 
    
        swap(arr, i, n - d + i, d); 
        leftRotateRec(arr, i, d, n - d);     
    
      
    // If B is shorter
    else 
    
        swap(arr, i, d, n - d); 
          
        // This is tricky
        leftRotateRec(arr, n - d + i, 
                       2 * d - n, d); 
    
}
  
// UTILITY FUNCTIONS
// Function to print an array 
public static void printArray(int []arr,
                              int size) 
    int i; 
    for(i = 0; i < size; i++) 
        Console.Write(arr[i] + " "); 
          
    Console.WriteLine(); 
  
// This function swaps d elements 
// starting at index fi with d elements
// starting at index si 
public static 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
public static void Main(String[] args) 
{
    int []arr = { 1, 2, 3, 4, 5, 6, 7 }; 
      
    leftRotate(arr, 2, 7); 
    printArray(arr, 7);     
}
}
  
// This code is contributed by amal kumar choubey


Javascript




<script>
  
let leftRotate = (arr, d, 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) { 
        arr = swap(arr, 0, n - d, d); 
        return
    
          
    /* If A is shorter*/        
    if(d < n - d) { 
        arr = swap(arr, 0, n - d, d); 
        leftRotate(arr, d, n - d);     
    
    else{             
    /* If B is shorter*/  
        arr = swap(arr, 0, d, n - d); 
        /*This is tricky*/
        leftRotate(arr + n - d, 2 * d - n, d); 
    
  
/*UTILITY FUNCTIONS*/
/* function to print an array */
let printArray = (arr, size) =>{ 
    ans = ''
    for(let i = 0; i < size; i++) 
        ans += arr[i]+" ";
    document.write(ans)
  
/*This function swaps d elements 
starting at index fi 
with d elements starting at index si */
let swap = (arr, fi, si, d) =>{  
    for(let i = 0; i < d; i++) { 
        let temp = arr[fi + i]; 
        arr[fi + i] = arr[si + i]; 
        arr[si + i] = temp; 
    
    return arr
  
// Driver Code
arr = [1, 2, 3, 4, 5, 6, 7]; 
leftRotate(arr, 2, 7); 
printArray(arr, 7); 
  
</script>


Output

3 4 5 6 7 1 2 

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;
    
    /* If number of elements to be rotated is more than array size*/ 
      if(d > n)
      d = d % n;
    
    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


C




// C code for above implementation
void leftRotate(int arr[], int d, int n)
{
  int i, j;
  if(d == 0 || d == n)
    return;
  /* If number of elements to be rotated is more than array size*/ 
  if(d > n)
    d = d % n;
  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);
}


Java




//Java code for above implementation
static void leftRotate(int arr[], int d, int n) 
int i, j; 
if(d == 0 || d == n) 
    return
/* If number of elements to be rotated is more than array size*/ 
 if(d > n)
   d = d % n;
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); 


Python3




# Python3 code for above implementation
  
def leftRotate(arr, d, n): 
    if(d == 0 or d == n): 
        return
    i =
    j = n -
    while (i != j): 
          
        if(i < j): # A is shorter
            swap(arr, d - i, d + j - i, i) 
            j -=
          
        else: # B is shorter
            swap(arr, d - i, d, j) 
            i -=
      
    swap(arr, d - i, d, i)
  
# This code is contributed 
# by Adarsh_Verma


C#




// C# code for above implementation 
static void leftRotate(int []arr, int d, int n) 
    int i, j; 
    if(d == 0 || d == n) 
        return
   /* If number of elements to be rotated is more than array size*/ 
      if(d > n)
      d = d % n;
    
    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; 
        
          
    
      
    /*Finally, block swap A and B*/
    swap(arr, d-i, d, i); 
  
// This code is contributed by Rajput-Ji


Javascript




<script>
// JavaScript code for above implementation
function leftRotate(arr, d, n)
{
    if(d == 0 || d == n)
        return
    let i = d;
    let j = n - d;
    while (i != j)
    {
        if(i < j) 
        {
          
        // A is shorter
            arr = swap(arr, d - i, d + j - i, i);
            j -= i;
        }
        else{ // B is shorter
            arr = swap(arr, d - i, d, j);
            i -= j;
        }
    }
    arr = swap(arr, d - i, d, i);
      
    // This code is contributed by rohitsingh04052.
}
</script>


Time Complexity: O(n)

Auxiliary Space: O(1)
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.


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