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

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< cpp‎ | algorithm
m (→‎Complexity: insignificant fix)
m (→‎See also: + std::ranges version)
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Revision as of 11:43, 9 January 2021

 
C++
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
(C++17)
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
(C++11)
(C++11)
(C++11)
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
set_difference

Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations
(C++20)
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
 
Defined in header <algorithm>
(1)
template< class InputIt1, class InputIt2, class OutputIt >

OutputIt set_difference( InputIt1 first1, InputIt1 last1,
                         InputIt2 first2, InputIt2 last2,

                         OutputIt d_first );
 (until C++20)
template< class InputIt1, class InputIt2, class OutputIt >

constexpr OutputIt set_difference( InputIt1 first1, InputIt1 last1,
                                   InputIt2 first2, InputIt2 last2,

                                   OutputIt d_first );
 (since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class ForwardIt3 >

ForwardIt3 set_difference( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1,
                           ForwardIt2 first2, ForwardIt2 last2,

                           ForwardIt3 d_first );
 (2) (since C++17)
(3)
template< class InputIt1, class InputIt2,

          class OutputIt, class Compare >
OutputIt set_difference( InputIt1 first1, InputIt1 last1,
                         InputIt2 first2, InputIt2 last2,

                         OutputIt d_first, Compare comp );
 (until C++20)
template< class InputIt1, class InputIt2,

          class OutputIt, class Compare >
constexpr OutputIt set_difference( InputIt1 first1, InputIt1 last1,
                                   InputIt2 first2, InputIt2 last2,

                                   OutputIt d_first, Compare comp );
 (since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2,

          class ForwardIt3, class Compare >
ForwardIt3 set_difference( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1,
                           ForwardIt2 first2, ForwardIt2 last2,

                           ForwardIt3 d_first, Compare comp );
 (4) (since C++17)

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

The resulting range is also sorted. Equivalent elements are treated individually, that is, if some element is found m times in [first1, last1) and n times in [first2, last2), it will be copied to d_first exactly std::max(m-n, 0) times. The resulting range cannot overlap with either of the input ranges.

1) Elements are compared using operator< and the ranges must be sorted with respect to the same.
3) Elements are compared using the given binary comparison function comp and the ranges must be sorted with respect to the same.
2,4) Same as (1,3), but executed according to policy. These overloads participate in overload resolution only if

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.

(until C++20)

std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.

(since C++20)

Contents

Parameters

first1, last1 - the range of elements to examine
first2, last2 - the range of elements to search for
d_first - the beginning of the destination range
policy - the execution policy to use. See execution policy for details.
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);

While the signature does not need to have const&, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1& is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy(since C++11)).
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, InputIt2 must meet the requirements of LegacyInputIterator.
-
OutputIt must meet the requirements of LegacyOutputIterator.
-
ForwardIt1, ForwardIt2, ForwardIt3 must meet the requirements of LegacyForwardIterator.

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).

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.

Possible implementation

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

Example

Run this code
#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 std::vector<int> v1 {1, 2, 5, 5, 5, 9};
    const std::vector<int> v2 {2, 5, 7};
    std::vector<int> diff;
 
    std::set_difference(v1.begin(), v1.end(), v2.begin(), v2.end(),
                        std::inserter(diff, diff.begin()));
    print(v1, "∖ ");
    print(v2, "= ");
    print(diff, "\n");
 
    // we want to know which orders "cut" between old and new states:
    std::vector<Order> old_orders { {1}, {2}, {5}, {9} };
    std::vector<Order> new_orders { {2}, {5}, {7} };
    std::vector<Order> cut_orders;
 
    std::set_difference(old_orders.begin(), old_orders.end(),
                        new_orders.begin(), new_orders.end(),
                        std::back_inserter(cut_orders),
                        [](auto& a, auto& b) { return a.order_id < b.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, }

See also

 returns true if one sequence is a subsequence of another
(function template) [edit]
computes the symmetric difference between two sets
(function template) [edit]
(C++20)
 computes the difference between two sets
(niebloid)[edit]
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