Question

I get some strange behavior when sorting a c++ vector with at custom comparator class.

I want to sort a array of indexes based on some values in an other array. But as far as I can see the sort function just reverses my indexes. I’ve done some logging of the compare function, at it seems to be working just fine.

Is there anyone who can spot what I’m doing wrong?

My code:

template<class T>
class Comparator {
   vector<T> & data;
public:
  bool operator()(int a, int b) {
    return data.at(a) < data.at(b) ? -1 : (data.at(a) > data.at(b) ? 1 : 0);
  }

  Comparator(vector<T> & data) : data(data) {}
};

void sortTest2() {
  //SETUP
  int n = 5;
  vector<int> indexes(n);
  newIdAr(indexes, n); //init to {0,1,2,3,4}
  vector<double> data(n);
  randomData(data, n); //Init to radom data
  Comparator<double> comparator(data);

  //TEST
  print(indexes, data, n); // Prints [0.00125126, 0.563585, 0.193304, 0.808741]
  sort(indexes.begin(), indexes.end(),comparator);
  print(indexes, data, n); // Prints [0.808741, 0.193304, 0.563585, 0.00125126]
  sort(indexes.begin(), indexes.end(),comparator);
  print(indexes, data, n); // Prints [0.00125126, 0.563585, 0.193304, 0.808741]
  cout << "Shuffle" << endl;
  random_shuffle(indexes.begin(), indexes.end());
  print(indexes, data, n);
  sort(indexes.begin(), indexes.end(), comparator);
  print(indexes, data, n);
}

My output:

[0.00125126, 0.563585, 0.193304, 0.808741, 0.585009]
[0.585009, 0.808741, 0.193304, 0.563585, 0.00125126]
[0.00125126, 0.563585, 0.193304, 0.808741, 0.585009]
Shuffle
[0.193304, 0.00125126, 0.585009, 0.563585, 0.808741]
[0.808741, 0.563585, 0.585009, 0.00125126, 0.193304]

Code for the auxiliary functions:

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

void print(vector<int> & sort, vector<double> & data, int N) {
  cout << "[";
  for (int i = 0; i < N - 1; ++i) {
    cout << data[sort[i]] << ", ";
  }
  cout << data[sort[N - 1]] << "]" << endl;
}

void newIdAr(vector<int> & ar, int N) {
  for (int i = 0; i < N; ++i) {
    ar[i] = i;
  }
}

void randomData(vector<double> & data, int n) {
  for (int i = 0; i < n; ++i) {
    data[i] = ((double) rand()) / RAND_MAX;
  }
}

Answer 1

Comparators should return true if the first argument is lower than the second one, or false otherwise. Returning 0, -1 and 1 is invalid in your code.

You could realize this by analizing the comparator’s operator() signature:

bool operator()(int a, int b)

I believe your implementation should be:

bool operator()(int a, int b) {
    return data.at(a) < data.at(b);
}