Reorder array using given indexes

Given two input arrays of same size, second array is index array, we need to reorder elements in same array according to given index array.

Example

a)    Input
array : [4, 2, 1, 3]
index : [3, 1, 0, 2]

Output
array : [1, 2, 3, 4]
index : [0, 1, 2, 3]
Here, given index array is [3, 1, 0, 2],
Therefore, array[3] = 4
             array[1] = 2
             array[0] = 1
             array[2] = 3
Output will be: [1, 2, 3, 4]

b)    Input
array : [9, 5, 3, 7, 11, 1]
index : [4, 2, 1, 0, 3, 5]

Output
array : [7, 3, 5, 11, 8, 9, 1]
index : [0, 1, 2, 3, 4, 5]

Method 1

Time complexity : O(n)

Algorithm

Step 1 : create a function which takes the two input arrays array[] and index[] and reorders based on index array.

Step 2 : In the function,

a)    Create an auxiliary array temp same size of given arrays.
b)    Traverse the given array and put elements at there correct position in temp based on index[]. temp[index[i]] = array[i]
c)    Copy this temp array as given array and change index array based on indexes.

Step 3 : call this function on given input arrays and print the arrays.

Algorithm working

C++ Program

#include <bits/stdc++.h>
using namespace std;
 
//Function for reoreder elements based on index array
void Reorder(int array[], int index[], int N)
{
    int temp[N];//auxilary array    
    //array[i] should present at (index[i]) index
    for (int i=0; i<N; i++)
    {
        temp[index[i]] = array[i];
    }
    for (int i=0; i<N; i++)
    { 
       array[i] = temp[i];
       index[i] = i;
    }
}
//function to print the given array
int PrintArray(int array[],int N)
{
    for (int i=0; i<N; i++)
       {
           cout << array[i] << " ";
       }
}
//Main function
int main()
{
    int array[] = {50, 40, 70, 60, 90};
    int index[] = {3,  0,  4,  1,  2};
    int N = sizeof(array)/sizeof(array[0]);
    cout << "Input array: \n";
    PrintArray(array,N);
    cout << "\nInput index array: \n";
    PrintArray(index,N);
    //Reordering by using function 
    Reorder(array,index,N);
 
    cout << "\nOutput array: \n";
    PrintArray(array,N);
    cout << "\nOutput index array: \n";
    PrintArray(index,N);
    return 0;
}
Try It

Method 2

Time complexity : O(n)

No auxiliary array

Algorithm

Step 1 : create a function which takes the given arrays and reorders based on the index array.

Step 2 : If index[i] = i, we need not change the position so, for all elements in the input array.
a)    Store values at correct position array[i] should be placed in dummy variables.
b)    Place array[i] at its position and update index value. (array[index[i]] = array[i])
c)    Copy old values to current positions. array[i] = oldElementvalue and update index.

Step 3 : call this function input arrays, to reorder them.

Algorithm working

C++ Program

#include <bits/stdc++.h>
using namespace std;
 
//Function for reoreder elements based on index array
void Reorder(int array[], int index[], int N)
{
    for(int i=0; i<N; i++)
    {
        while (index[i] != i)
        {
            //storing correct position values
            int  oldIndexValue = index[index[i]];
            char oldElementValue   = array[index[i]];
            array[index[i]] = array[i];
            index[index[i]] = index[i];
            index[i] = oldIndexValue;
            array[i]   = oldElementValue;
        }
    }
}
//function to print the given array
int PrintArray(int array[],int N)
{
    for (int i=0; i<N; i++)
       {
           cout << array[i] << " ";
       }
}
//Main Function
int main()
{
   int array[] = {50, 40, 70, 60, 90};
    int index[] = {3,  0,  4,  1,  2};
    int N = sizeof(array)/sizeof(array[0]);
    cout << "Input array: \n";
    PrintArray(array,N);
    cout << "\nInput index array: \n";
    PrintArray(index,N);
    //Reordering by using function 
    Reorder(array,index,N);
 
    cout << "\nOutput array: \n";
    PrintArray(array,N);
    cout << "\nOutput index array: \n";
    PrintArray(index,N);
    return 0;
}
Try It


Next > < Prev
Scroll to Top