Difference between revisions of "cpp/algorithm/set union"

Latest revision as of 21:55, 6 April 2024

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

If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, then all m elements will be copied from [first1, last1) to the output range, preserving order, and then the final std::max(n - m, 0) elements will be copied from [first2, last2) to the output range, also preserving order.

If the output range overlaps with [first1, last1) or [first2, last2), the behavior is undefined.

[edit] Parameters

[edit] Return value

Iterator past the end of the constructed range.

[edit] Complexity

Given \(\scriptsize N_1\)N1 as std::distance(first1, last1) and \(\scriptsize N_2\)N2 as std::distance(first2, last2):

[edit] Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

• If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

[edit] Possible implementation

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);
}

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)
            // Finished range 2, include the rest of range 1:
            return std::copy(first1, last1, d_first);
 
        if (comp(*first2, *first1))
            *d_first = *first2++;
        else
        {
            *d_first = *first1;
            if (!comp(*first1, *first2)) // Equivalent => don't need to include *first2.
                ++first2;
            ++first1;
        }
    }
    // Finished range 1, include the rest of range 2:
    return std::copy(first2, last2, d_first);
}

[edit] 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 between 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 fewer.

[edit] Example

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
 
void println(const std::vector<int>& v)
{
    for (int i : v)
        std::cout << i << ' ';
    std::cout << '\n';
}
 
int main()
{
    std::vector<int> v1, v2, dest;
 
    v1 = {1, 2, 3, 4, 5};
    v2 = {3, 4, 5, 6, 7};
 
    std::set_union(v1.cbegin(), v1.cend(),
                   v2.cbegin(), v2.cend(),
                   std::back_inserter(dest));
    println(dest);
 
    dest.clear();
 
    v1 = {1, 2, 3, 4, 5, 5, 5};
    v2 = {3, 4, 5, 6, 7};
 
    std::set_union(v1.cbegin(), v1.cend(),
                   v2.cbegin(), v2.cend(),
                   std::back_inserter(dest));
    println(dest);
}

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[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also