Sorting an array according to another array using pair in STL

We are given two arrays. We need to sort one array according to another. Examples:

Input : 2 1 5 4 9 3 6 7 10 8
        A B C D E F G H I J
Output : 1 2 3 4 5 6 7 8 9 10 
         B A F D C G H J E I 
Here we are sorting second array
(a character array) according to 
the first array (an integer array).

We have discussed different ways in below post. Sort an array according to the order defined by another array In this post we are focusing on using the pair container present in STL of C++. To achieve our task we are going to make pairs of respective elements from both the arrays.Then simply use the sort function. The important thing to note is, the first element in the pairs should be from the array according to which the sorting is to be performed. 

Here are the steps to solve this problem :

1. Construct a pair array pairt[] of type pair<int, char> having size n (here n is the size of the input arrays)
2. Fill the pair array with the respective elements from the input arrays a[] and b[].
3. Using the sort() method from the STL sort the pair array pairt[]. Sorting will be based on the array of integers a[] which is the first element in the pair.
4. Use the components in the sorted pair array pairt[] to modify the initial input arrays a[] and b[].
5. At last, print the sorted arrays a[] and b[].

CPP

// Sort an array according to
// other using pair in STL.
#include <bits/stdc++.h>
using namespace std;

// Function to sort character array b[]
// according to the order defined by a[]
void pairsort(int a[], char b[], int n)
{
    pair<int, char> pairt[n];

    // Storing the respective array
    // elements in pairs.
    for (int i = 0; i < n; i++)
    {
        pairt[i].first = a[i];
        pairt[i].second = b[i];
    }

    // Sorting the pair array.
    sort(pairt, pairt + n);

    // Modifying original arrays
    for (int i = 0; i < n; i++)
    {
        a[i] = pairt[i].first;
        b[i] = pairt[i].second;
    }
}

// Driver function
int main()
{
    int a[] = {2, 1, 5, 4, 9, 3, 6, 7, 10, 8};
    char b[] = {'A', 'B', 'C', 'D', 'E', 'F',
                         'G', 'H', 'I', 'J'};

    int n = sizeof(a) / sizeof(a[0]);

    // Function calling
    pairsort(a, b, n);

    for (int i = 0; i < n; i++)
        cout << a[i] << " ";
    cout << endl;

    for (int i = 0; i < n; i++)
        cout << b[i] << " ";

    return 0;
}

Python3

# Function to sort character array b[]
# according to the order defined by a[]
def pairsort(a, b, n):
    pairt = [(a[i], b[i]) for i in range(n)]

    # Sorting the pair array.
    pairt.sort()

    # Modifying original arrays
    for i in range(n):
        a[i] = pairt[i][0]
        b[i] = pairt[i][1]

# Driver function
if __name__ == "__main__":
    a = [2, 1, 5, 4, 9, 3, 6, 7, 10, 8]
    b = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J']
    n = len(a)

    # Function calling
    pairsort(a, b, n)

    for i in range(n):
        print(a[i], end=" ")
    print()

    for i in range(n):
        print(b[i], end=" ")

Output:

1 2 3 4 5 6 7 8 9 10 
B A F D C G H J E I 

Complexity Analysis :

• Time complexity : O(nlogn)  overall time complexity of the program is dominated by the sort() function
• Space complexity : O(n) because the program creates a pair array pairt[] of size n to store the elements from the input arrays.

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