std::set_union

Defined in header <algorithm>
template< class InputIt1, class InputIt2, class OutputIt >
OutputIt set_union( InputIt1 first1, InputIt1 last1,
                    InputIt2 first2, InputIt2 last2,
                    OutputIt d_first );
(1)
template< class InputIt1, class InputIt2,
          class OutputIt, class Compare >
OutputIt set_union( InputIt1 first1, InputIt1 last1,
                    InputIt2 first2, InputIt2 last2,
                    OutputIt d_first, Compare comp );
(2)

Constructs a sorted range beginning at d_first consisting of all elements present in one or both sorted ranges [first1, last1) and [first2, last2).

1) Expects both input ranges to be sorted with operator<
2) Expects them to be sorted with the given comparison function comp

If some element is found m times in [first1, last1) and n times in [first2, last2), then all m elements will be copied from [first1, last1) to d_first, preserving order, and then exactly std::max(n-m, 0) elements will be copied from [first2, last2) to d_first, also preserving order.

The resulting range cannot overlap with either of the input ranges.

Parameters

first1, last1 - the first input sorted range
first2, last2 - the second input sorted range
comp - comparison function object (i.e. an object that satisfies the requirements of Compare) which returns ​true if the first argument is less than (i.e. is ordered before) the second.

The signature of the comparison function should be equivalent to the following:

bool cmp(const Type1 &a, const Type2 &b);

The signature does not need to have const &, but the function object must not modify the objects passed to it.
The types Type1 and Type2 must be such that objects of types InputIt1 and InputIt2 can be dereferenced and then implicitly converted to both Type1 and Type2. ​

Type requirements
- InputIt1 must meet the requirements of InputIterator.
- InputIt2 must meet the requirements of InputIterator.
- OutputIt must meet the requirements of OutputIterator.

Return value

Iterator past the end of the constructed range.

Complexity

At most 2·(N1+N2-1) comparisons, where N1= std::distance(first1, last1) and N2= std::distance(first2, last2).

Notes

This algorithm performs a similar task as std::merge does. Both consume two sorted input ranges and produce a sorted output with elements from both inputs. The difference bewteen these two algorithms is with handling values from both input ranges which compare equivalent (see notes on LessThanComparable). If any equivalent values appeared n times in the first range and m times in the second, std::merge would output all n+m occurrences whereas std::set_union would output std::max(n, m) ones only. So std::merge outputs exactly std::distance(first1, last1) + std::distance(first2, last2) values and std::set_union may produce less.

Possible implementation

First version
template<class InputIt1, class InputIt2, class OutputIt>
OutputIt set_union(InputIt1 first1, InputIt1 last1,
                   InputIt2 first2, InputIt2 last2,
                   OutputIt d_first)
{
    for (; first1 != last1; ++d_first) {
        if (first2 == last2)
            return std::copy(first1, last1, d_first);
        if (*first2 < *first1) {
            *d_first = *first2++;
        } else {
            *d_first = *first1;
            if (!(*first1 < *first2))
                ++first2;
            ++first1;
        }
    }
    return std::copy(first2, last2, d_first);
}
Second version
template<class InputIt1, class InputIt2,
         class OutputIt, class Compare>
OutputIt set_union(InputIt1 first1, InputIt1 last1,
                   InputIt2 first2, InputIt2 last2,
                   OutputIt d_first, Compare comp)
{
    for (; first1 != last1; ++d_first) {
        if (first2 == last2)
            return std::copy(first1, last1, d_first);
        if (comp(*first2, *first1)) {
            *d_first = *first2++;
        } else {
            *d_first = *first1;
            if (!comp(*first1, *first2))
                ++first2;
            ++first1;
        }
    }
    return std::copy(first2, last2, d_first);
}

Example

Example with vectors :

#include <vector>
#include <set>
#include <iostream>
#include <algorithm>
#include <iterator>
 
int main()
{
    std::vector<int> v1 = {1, 2, 3, 4, 5}; 
    std::vector<int> v2 = {      3, 4, 5, 6, 7}; 
    std::vector<int> dest1;
 
    std::set_union(v1.begin(), v1.end(),
                   v2.begin(), v2.end(),                  
                   std::back_inserter(dest1));
 
    for (const auto &i : dest1) {
        std::cout << i << ' ';
    }   
    std::cout << '\n';
}

Output:

1 2 3 4 5 6 7

See also

returns true if one set is a subset of another
(function template)
merges two sorted ranges
(function template)
computes the difference between two sets
(function template)
computes the intersection of two sets
(function template)
computes the symmetric difference between two sets
(function template)
std::experimental::parallel::set_union
(parallelism TS)
parallelized version of std::set_union
(function template)
doc_CPP
2016-10-11 10:06:32
Comments
Leave a Comment

Please login to continue.