Question

I’m trying to find all the ranges [a,b] enclosing int values i, where a <= i <= b. I’m using set<std:pair<int,int>> for the set of ranges.

In the following, using equal range on a vector<int> yields the start and one past the end of the range.

When I do the same for a set<pair<int,int>>, the result starts and ends at one past the end of the range and therefore doesn’t include the range enclosing the value.

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

int main()
{
    int ia[] = {1,2,3,4,5,6,7,8,9,10};
    set<int> s1(begin(ia),end(ia));
    auto range1 = s1.equal_range(5);

    cout << *range1.first << " " << *range1.second << endl; //prints 5 6

    pair<int,int> p[] = {make_pair(1,10), 
                         make_pair(11,20),  
                         make_pair(21,30), 
                         make_pair(31,40)}; 
    set<pair<int,int>> s(begin(p), end(p));    

    auto range = s.equal_range(make_pair(12,12));

    cout << range.first->first << " " << range.first->second << endl; //prints 21 30, why?
    cout << range.second->first << " " << range.second->second << endl; //prints 21 30

}

prints
5 6
21 30
21 30
Why does equal_range on the set<pair<int,int>> not include the range that encloses the value (12), namely [11.20]

Answer 1

equal_range is behaving completely correctly:

assert( std::make_pair(11, 20) < std::make_pair(12, 12) );
assert( std::make_pair(12, 12) < std::make_pair(21, 30) );

[11,20] is not a range, it’s a pair. The pair [12,12] is not “within” another pair, that makes no sense to even say.

[12,12] is not “within” [11,20], it’s greater than it. The less-than operator for std::pair compares the first elements first, and only if they’re equal does it look at the second elements, so make_pair(11,x) is less than make_pair(12, y) for any x and y

So equal_range tells you that [12,12] would be inserted after [11,20] and before [21,30], which is correct.

If you want to treat pairs as ranges of values you need to write code to do that, not assume the built-in comparisons for pairs does that. You’re actually trying to find an int 12 in a range of pairs of ints, but have written code to find a pair [12,12] in a range of pairs of ints. That’s not the same thing.