Converting sets of integers into ranges

Question

What's the most idiomatic way to convert a set of integers into a set of ranges?

E.g. given the set {0, 1, 2, 3, 4, 7, 8, 9, 11} I want to get { {0,4}, {7,9}, {11,11} }.

Let's say we are converting from std::set<int> into std::vector<std::pair<int, int>>. I treat Ranges as inclusive on both sides, since it's more convenient in my case, but I can work with open-ended ranges too if necessary.

I've written the following function, but I feel like reinventing the wheel. Please tell maybe there's something in STL or boost for this.

typedef std::pair<int, int> Range;

void setToRanges(const std::set<int>& indices, std::vector<Range>& ranges)
{
    Range r = std::make_pair(-INT_MAX, -INT_MAX);

    BOOST_FOREACH(int i, indices)
    {
           if (i != r.second + 1)
           {
            if (r.second >= 0) ranges.push_back(r);
            r.first = i;                    
           }

           r.second = i;
    }

    ranges.push_back(r);
}

Answer 1

Now one can use interval_set from Boost.ICL (Boost > 1.46)

#include <set>
#include <iostream>
#include <algorithm>

#include <boost/icl/discrete_interval.hpp>
#include <boost/icl/closed_interval.hpp>
#include <boost/icl/interval_set.hpp>

typedef std::set<int> Set;
typedef boost::icl::interval_set<int> IntervalSet;

void setToInterval(const Set& indices, IntervalSet& intervals)
{
    Set::const_iterator pos;
    for(pos = indices.begin(); pos != indices.end(); ++pos)
    {
        intervals.insert(boost::icl::construct<boost::icl::discrete_interval<int> >(*pos, *pos, boost::icl::interval_bounds::closed()));
    }
}

int main()
{
    std::cout << ">>Interval Container Library Rocks! <<\n";
    std::cout << "----------------------------------------------------\n";

    Set indices = {0, 1, 2, 3, 4, 7, 8, 9, 11};
    IntervalSet intervals;

    setToInterval(indices, intervals);

    std::cout << "  intervals joined:    " << intervals  << "\n";

    return 0;
}

Output:

  intervals joined:    {[0,4][7,9][11,11]}

Answer 2

I don't think there's anything in the STL or Boost that does this.

One thing you can do is to make your algorithm a little bit more general:

template<class InputIterator, class OutputIterator>
void setToRanges(InputIterator first, InputIterator last, OutputIterator dest)
{
    typedef std::iterator_traits<InputIterator>::value_type item_type;
    typedef typename std::pair<item_type, item_type> pair_type;
    pair_type r(-std::numeric_limits<item_type>::max(), 
                -std::numeric_limits<item_type>::max());

    for(; first != last; ++first)
    {
        item_type i = *first;
        if (i != r.second + 1)
        {
            if (r.second >= 0) 
                *dest = r;
            r.first = i;                    
        }
        r.second = i;
    }
    *dest = r;
}

Usage:

std::set<int> set;
// insert items

typedef std::pair<int, int> Range;
std::vector<Range> ranges;

setToRanges(set.begin(), set.end(), std::back_inserter(ranges));

You should also consider using the term interval instead of range, because the latter in STL parlance means "any sequence of objects that can be accessed through iterators or pointers" (source).

Finally, you should probably take at look at the Boost Interval Arithmetic Library, which is currently under review for Boost inclusion.