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std::ranges::set_union, std::ranges::set_union_result

From cppreference.com
< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. std::ranges::copy, std::ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
Operations on uninitialized storage
(C++17)
(C++17)
(C++17)
C library
 
Constrained algorithms
Non-modifying sequence operations
Modifying sequence operations
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
ranges::set_union
Heap operations
Minimum/maximum operations
Permutations
Operations on uninitialized storage
Return types
 
Defined in header <algorithm>
Call signature
template< std::input_iterator I1, std::sentinel_for<I1> S1,

          std::input_iterator I2, std::sentinel_for<I2> S2,
          std::weakly_incrementable O, class Comp = ranges::less,
          class Proj1 = std::identity, class Proj2 = std::identity >
requires  std::mergeable<I1, I2, O, Comp, Proj1, Proj2>
constexpr set_union_result<I1, I2, O>
          set_union( I1 first1, S1 last1, I2 first2, S2 last2,
                     O result, Comp comp = {},

                     Proj1 proj1 = {}, Proj2 proj2 = {} );
(1) (since C++20)
template< ranges::input_range R1, ranges::input_range R2,

          std::weakly_incrementable O, class Comp = ranges::less,
          class Proj1 = std::identity, class Proj2 = std::identity >
requires  std::mergeable<ranges::iterator_t<R1>, ranges::iterator_t<R2>,
          O, Comp, Proj1, Proj2>
constexpr set_union_result<ranges::borrowed_iterator_t<R1>,
                           ranges::borrowed_iterator_t<R2>, O>
          set_union( R1&& r1, R2&& r2, O result, Comp comp = {},

                     Proj1 proj1 = {}, Proj2 proj2 = {} );
(2) (since C++20)
Helper types
template<class I1, class I2, class O>
using set_union_result = ranges::in_in_out_result<I1, I2, O>;
(3) (since C++20)

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

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 result, preserving order, and then exactly max(n-m, 0) elements will be copied from [first2, last2) to result, also preserving order.

1) Elements are compared using the given binary comparison function comp.
2) Same as (1), but uses r1 as the first range and r2 as the second range, as if using ranges::begin(r1) as first1, ranges::end(r1) as last1, ranges::begin(r2) as first2, and ranges::end(r2) as last2.

Preconditions:

  • The input ranges must be sorted with respect to comp and proj1 or proj2, respectively.
  • The resulting range cannot overlap with either of the input ranges.


The function-like entities described on this page are niebloids, that is:

In practice, they may be implemented as function objects, or with special compiler extensions.

Contents

[edit] Parameters

first1, last1 - the first input sorted range
first2, last2 - the second input sorted range
result - the beginning of the output range
comp - comparison to apply to the projected elements
proj1 - projection to apply to the elements in the first range
proj2 - projection to apply to the elements in the second range.

[edit] Return value

{last1, last2, result_last}, where result_last is the end of the constructed range.

[edit] Complexity

At most 2·(N₁+N₂)-1 comparisons and applications of each projection, where
N₁ = ranges::distance(first1, last1), and N₂ = ranges::distance(first2, last2).

[edit] Notes

This algorithm performs a similar task as ranges::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, ranges::merge would output all n+m occurrences whereas ranges::set_union would output std::max(n, m) ones only. So ranges::merge outputs exactly ranges::distance(first1, last1) + ranges::distance(first2, last2) values and ranges::set_union may produce less.

[edit] Possible implementation

struct set_union_fn {
    template< std::input_iterator I1, std::sentinel_for<I1> S1,
              std::input_iterator I2, std::sentinel_for<I2> S2,
              std::weakly_incrementable O, class Comp = ranges::less,
              class Proj1 = std::identity, class Proj2 = std::identity >
    requires std::mergeable<I1, I2, O, Comp, Proj1, Proj2>
    constexpr ranges::set_union_result<I1, I2, O>
    operator()( I1 first1, S1 last1, I2 first2, S2 last2,
                O result, Comp comp = {},
                Proj1 proj1 = {}, Proj2 proj2 = {} ) const {
        for (; !(first1 == last1 or first2 == last2); ++result) {
            if (std::invoke(comp, std::invoke(proj1, *first1), std::invoke(proj2, *first2)))
                *result = *first1, ++first1;
            else if (std::invoke(comp, std::invoke(proj2, *first2), std::invoke(proj1, *first1)))
                *result = *first2, ++first2;
            else
                *result = *first1, ++first1, ++first2;
        }
        auto res1 = ranges::copy(std::move(first1), std::move(last1), std::move(result));
        auto res2 = ranges::copy(std::move(first2), std::move(last2), std::move(res1.out));
        return {std::move(res1.in), std::move(res2.in), std::move(res2.out)};
    }
 
    template< ranges::input_range R1, ranges::input_range R2,
              std::weakly_incrementable O, class Comp = ranges::less,
              class Proj1 = std::identity, class Proj2 = std::identity >
    requires std::mergeable<ranges::iterator_t<R1>, ranges::iterator_t<R2>,
             O, Comp, Proj1, Proj2>
    constexpr ranges::set_union_result<ranges::borrowed_iterator_t<R1>,
              ranges::borrowed_iterator_t<R2>, O>
    operator()( R1&& r1, R2&& r2, O result, Comp comp = {},
                Proj1 proj1 = {}, Proj2 proj2 = {} ) const {
        return (*this)(ranges::begin(r1), ranges::end(r1),
                       ranges::begin(r2), ranges::end(r2),
                       std::move(result), std::move(comp),
                       std::move(proj1), std::move(proj2));
    }
};
 
inline constexpr set_union_fn set_union{};

[edit] Example

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
 
void print(const auto& in1, const auto& in2, auto first, auto last) {
    std::cout << "{ ";
    for (const auto& e : in1) { std::cout << e << ' '; }
    std::cout << "} ∪ { ";
    for (const auto& e : in2) { std::cout << e << ' '; }
    std::cout << "} =\n{ ";
    while (!(first == last)) { std::cout << *first++ << ' '; }
    std::cout << "}\n\n";
}
 
int main()
{
    std::vector<int> in1, in2, out;
 
    in1 = {1, 2, 3, 4, 5};
    in2 = {      3, 4, 5, 6, 7};
    out.resize(in1.size() + in2.size());
    const auto ret = std::ranges::set_union(in1, in2, out.begin());
    print(in1, in2, out.begin(), ret.out);
 
    in1 = {1, 2, 3, 4, 5, 5, 5};
    in2 = {      3, 4, 5, 6, 7};
    out.clear();
    out.reserve(in1.size() + in2.size());
    std::ranges::set_union(in1, in2, std::back_inserter(out));
    print(in1, in2, out.cbegin(), out.cend());
}

Output:

{ 1 2 3 4 5 } ∪ { 3 4 5 6 7 } =
{ 1 2 3 4 5 6 7 }
 
{ 1 2 3 4 5 5 5 } ∪ { 3 4 5 6 7 } =
{ 1 2 3 4 5 5 5 6 7 }

[edit] See also

computes the difference between two sets
(niebloid) [edit]
computes the intersection of two sets
(niebloid) [edit]
computes the symmetric difference between two sets
(niebloid) [edit]
merges two sorted ranges
(niebloid) [edit]
returns true if one sequence is a subsequence of another
(niebloid) [edit]
computes the union of two sets
(function template) [edit]