Set of tuples (i, v) such that all tuples have different values of i

Question

I'm trying to construct a set of tuples (i, v), where i is an int and v is a double. The set cannot contain two tuples with the same value of i.

To do this, I think I should use a std::set of std:tuples. Something like:

#include <tuple>
#include <set>
using namespace std;

set<tuple<int, double>> mySet;

The std::set class allows me to specify a comparator, and I think I should use this to avoid different tuples with the same value of i, but I don't know how to do it??

See http://stackoverflow.com/questions/2620862/using-custom-stdset-comparator — LeartS, Feb 21 '14 at 19:51
@IgorTandetnik You're right! I didn't know about `map`. But I'd still like to understand how to do this with `set` — a06e, Feb 22 '14 at 21:31

Mark Ransom · Answer 1 · 2014-02-23T03:31:08.560

3

std::tuple already provides a operator< that you can use for comparisons, but it compares all elements of the tuple. You simply need to provide a comparator that compares the first element of the tuple and ignores the second.

struct comparator
{
    bool operator()(const tuple<int, double> & lhs, const tuple<int, double> &  rhs)
    {
        return get<0>(lhs) < get<0>(rhs);
    }
};

edited Feb 23 '14 at 03:31

answered Feb 21 '14 at 19:54

Mark Ransom

299,747
42
398
622

but how do I make `set` use the comparator I implemented? – a06e Feb 21 '14 at 22:04
@becko you just add a second template parameter to the set: `set,comparator> mySet`. – Mark Ransom Feb 21 '14 at 22:21
when I do this, I get an 'Error: function template "comparator" is not a type name'. – a06e Feb 22 '14 at 21:28
@becko, sorry - every time I've needed a functor a plain function was able to work. I've made an edit that should work better. – Mark Ransom Feb 23 '14 at 03:31

score 2 · Answer 2 · answered Feb 21 '14 at 19:55

2

A map may be a better choice std::map<int, double>. The interesting thing is that the value_type of the map is std::pair<const int, double> which can be used like a tuple : std::get ...

answered Feb 21 '14 at 19:55

Kiwi

2,698
16
15

Set of tuples (i, v) such that all tuples have different values of i

2 Answers2