Sort one array based on values of another array

[speedster123]

Hi friends,
I am trying to sort the values in one array based on the sorting sequence of another array. I use sort(A) for sorting A.
For ex,
A=[9,6,3,5,7];
B=[1,2,3,4,5];
Sorted values of A : [3,5,6,7,9]
B sorted w.r.t A : [3,4,2,5,1]

My code is as follows. I am not able to figure out how to use the sort command to get the sorting done.

int main(){
        
  vector <int> A(5),B(5);

  
  for (int d=0;d<5;d++){
    A[d]=rand()%10;
    B[d]=d;
    }
    sort(B.begin(),B.end(),sort_vector());// What should be the sort_vector()
                                          // so as to sort the values of B[] 
                                          // based on the sorted order of A[]?

  getchar();
  return 0;
}

[jsmith]

You're not going to be able to do that using std::sort() unless you combine A and B into a struct and sort based on B.

struct Pair {
    int A;
    int B;

    bool operator<( const Pair& rhs ) const
        { return B < rhs.B; }
};

std::vector<Pair> v;

std::sort( v.begin(), v.end() );

[speedster123]

Hi jsmith,

I dont understand yet... if A and B are vector<ints> will it still work?
Also, I am not able to understand the logic...
My problem was to be able to sort an array, and obtain the sorted array of indices too.
Is there an easier way to do it?

Thanks.

[creekist]

Array sorting thing:

#include<iostream>
using namespace std;

int main() {
	
	int array[5] = {3, 4, 6, 7, 1}; //original 5 element array that needs sorting
	int newarray[5]; //new sorted array that will be constructed based on old array

	// \/ Finds the maximum number of the array and sets the first element of the new array = to the max

	int firstmax = array[0]; 
	int secondmax = 0;

	for (int i = 0; i < 5; i++) {
		secondmax = array[i];
		if (secondmax > firstmax) {
			firstmax = secondmax;}}

	newarray[0] = firstmax;

	// /\ Finds the maximum number of the array and sets the first element of the new array = to the max

	// \/ Sorts remaining elements of the array in descending order

	int positron = 1;
	int difference = 1;
	int counter = 0;

	while (true) {
		for (int i = 0; i < 5; i++) {
			if (array[i] == firstmax - difference) {
				newarray[positron] = firstmax - difference;
				firstmax -= difference;
				difference = 0;
				positron++;
				counter++;}}
		difference++;
		if (counter == 4) {break;}}

	// /\ Sorts remaining elements of the array in descending order

	// \/ Prints the array

	for (int i = 0; i < 5; i++) {
		cout << newarray[i];}

	// /\ Prints the array

	while (true) {}
	return 0;}
		

[m4ster r0shi]

Check this out:

#include <algorithm>
#include <iostream>
#include <vector>
#include <cstdlib>
#include <ctime>

using namespace std;

struct MyComparator
{
    const vector<int> & value_vector;

    MyComparator(const vector<int> & val_vec):
        value_vector(val_vec) {}

    bool operator()(int i1, int i2)
    {
        return value_vector[i1] < value_vector[i2];
    }
};

void print(const vector<int> & v, const char * msg)
{
    int size = v.size();

    for (int i = 0; i < size; ++i)
        cout << v[i] << " ";

    cout << msg << endl;
}

int main()
{
    srand(time(0));

    vector<int> A(5), B(5);

    for (int i = 0; i < 5; ++i)
    {
        A[i] = rand() % 10;
        B[i] = i;
    }

    print(A, "<- A");
    print(B, "<- B");

    sort(B.begin(), B.end(), MyComparator(A));

    print(B, "<- B (sorted)");

    cout << "\n(hit enter to quit)";
    cin.get();

    return 0;
}

[speedster123]

Thanks m4ster r0shi,

Exactly what I was looking for!