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std::ranges::set_difference, std::ranges::set_difference_result

From cppreference.com
< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms: 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
Operations on uninitialized storage
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
C library
 
Constrained algorithms
Non-modifying sequence operations
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
ranges::set_difference
Heap operations
Minimum/maximum operations
Permutations
 
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 ranges::set_difference_result<I1, O>
ranges::set_difference( 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 ranges::set_difference_result<
          ranges::borrowed_iterator_t<R1>, O>
ranges::set_difference( R1&& r1, R2&& r2, O result, Comp comp = {},

                        Proj1 proj1 = {}, Proj2 proj2 = {} );
(2) (since C++20)
Helper types
template<class I, class O>
using set_difference_result = ranges::in_out_result<I, O>;
(3) (since C++20)

Copies the elements from the sorted input range [first1, last1) which are not found in the sorted input range [first2, last2) to the output range beginning at result.

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

An object equal to {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] Possible implementation

struct set_difference_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_difference_result<I1, O>
    operator()( I1 first1, S1 last1, I2 first2, S2 last2,
                O result, Comp comp = {},
                Proj1 proj1 = {}, Proj2 proj2 = {} ) const {
        while (!(first1 == last1 or first2 == last2)) {
            if (std::invoke(comp, std::invoke(proj1, *first1), std::invoke(proj2, *first2)))
                *result = *first1, ++first1, ++result;
            else if (std::invoke(comp, std::invoke(proj2, *first2), std::invoke(proj1, *first1)))
                ++first2;
            else
                ++first1, ++first2;
        }
        return ranges::copy(std::move(first1), std::move(last1), std::move(result));
    }
 
    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_difference_result<
              ranges::borrowed_iterator_t<R1>, 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_difference_fn set_difference{};

[edit] Example

#include <algorithm>
#include <iostream>
#include <iterator>
#include <string_view>
#include <vector>
 
auto print = [](const auto& v, std::string_view end = "") {
    std::cout << "{ ";
    for (auto i : v) std::cout << i << ' ';
    std::cout << "} " << end;
};
 
struct Order // a struct with some interesting data
{
    int order_id;
 
    friend std::ostream& operator<<(std::ostream& os, const Order& ord) {
        return os << "{" << ord.order_id << "},";
    }
};
 
int main()
{
    const auto v1 = {1, 2, 5, 5, 5, 9};
    const auto v2 = {2, 5, 7};
    std::vector<int> diff;
 
    std::ranges::set_difference(v1, v2, std::back_inserter(diff));
    print(v1, "∖ ");
    print(v2, "= ");
    print(diff, "\n");
 
    // we want to know which orders "cut" between old and new states:
    const std::vector<Order> old_orders { {1}, {2}, {5}, {9}, };
    const std::vector<Order> new_orders { {2}, {5}, {7}, };
    std::vector<Order> cut_orders;
 
    std::ranges::set_difference(old_orders, new_orders,
                                std::back_inserter(cut_orders), {},
                                &Order::order_id, &Order::order_id);
 
    std::cout << "old orders = "; print(old_orders, "\n");
    std::cout << "new orders = "; print(new_orders, "\n");
    std::cout << "cut orders = "; print(cut_orders, "\n");
}

Output:

{ 1 2 5 5 5 9 } ∖ { 2 5 7 } = { 1 5 5 9 } 
old orders = { {1}, {2}, {5}, {9}, } 
new orders = { {2}, {5}, {7}, } 
cut orders = { {1}, {9}, }

[edit] See also

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