Reorder array using given indexes

June 23, 2019

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]

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.

Code:-

[code language=”cpp”]

#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;
}

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