std:: includes

From cppreference.com

Defined in header `<algorithm>`
template < class InputIt1, class InputIt2 > bool includes ( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2 ) ;	(1)	(constexpr since C++20)
template < class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > bool includes ( ExecutionPolicy && policy, ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, ForwardIt2 last2 ) ;	(2)	(since C++17)
template < class InputIt1, class InputIt2, class Compare > bool includes ( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, Compare comp ) ;	(3)	(constexpr since C++20)
template < class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class Compare > bool includes ( ExecutionPolicy && policy, ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, ForwardIt2 last2, Compare comp ) ;	(4)	(since C++17)

Returns true if the sorted range [ first2 , last2 ) is a subsequence of the sorted range [ first1 , last1 ) (a subsequence need not be contiguous).

1) If

first1

last1

first2

last2

is not sorted with respect to operator < (until C++20) std:: less { } (since C++20) , the behavior is undefined.

3) If

first1

last1

first2

last2

is not sorted with respect to comp , the behavior is undefined.

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)

Parameters

first1, last1	-	the sorted range of elements to examine
first2, last2	-	the sorted range of elements to search for
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 an object of type InputIt can be dereferenced and then implicitly converted to both of them.
Type requirements
- `InputIt1, InputIt2` must meet the requirements of LegacyInputIterator .
- `ForwardIt1, ForwardIt2` must meet the requirements of LegacyForwardIterator .
- `Compare` must meet the requirements of Compare .

Return value

true if [ first2 , last2 ) is a subsequence of [ first1 , last1 ) ; otherwise false .

An empty sequence is a subsequence of any sequence, so true is returned if [ first2 , last2 ) is empty.

Complexity

Given $\scriptsize N_1$ $N 1$ as std:: distance ( first1, last1 ) and $\scriptsize N_2$ $N 2$ as std:: distance ( first2, last2 ) :

1,2) At most

2\cdot(N 1 +N 2)-1

comparisons using operator < (until C++20) std:: less { } (since C++20) .

3,4) At most

2\cdot(N 1 +N 2)-1

applications of the comparison function comp .

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

include (1)

template<class InputIt1, class InputIt2>
bool includes(InputIt1 first1, InputIt1 last1,
              InputIt2 first2, InputIt2 last2)
{
    for (; first2 != last2; ++first1)
    {
        if (first1 == last1 || *first2 < *first1)
            return false;
        if (!(*first1 < *first2))
            ++first2;
    }
    return true;
}

include (3)

template<class InputIt1, class InputIt2, class Compare>
bool includes(InputIt1 first1, InputIt1 last1,
              InputIt2 first2, InputIt2 last2, Compare comp)
{
    for (; first2 != last2; ++first1)
    {
        if (first1 == last1 || comp(*first2, *first1))
            return false;
        if (!comp(*first1, *first2))
            ++first2;
    }
    return true;
}

Example

Run this code

#include <algorithm>
#include <cctype>
#include <iostream>
 
template<class Os, class Co>
Os& operator<<(Os& os, const Co& v)
{
    for (const auto& i : v)
        os << i << ' ';
    return os << '\t';
}
 
int main()
{
    const auto
        v1 = {'a', 'b', 'c', 'f', 'h', 'x'},
        v2 = {'a', 'b', 'c'},
        v3 = {'a', 'c'},
        v4 = {'a', 'a', 'b'},
        v5 = {'g'},
        v6 = {'a', 'c', 'g'},
        v7 = {'A', 'B', 'C'};
 
    auto no_case = [](char a, char b) { return std::tolower(a) < std::tolower(b); };
 
    std::cout
    << v1 << "\nincludes:\n" << std::boolalpha
    << v2 << ": " << std::includes(v1.begin(), v1.end(), v2.begin(), v2.end()) << '\n'
    << v3 << ": " << std::includes(v1.begin(), v1.end(), v3.begin(), v3.end()) << '\n'
    << v4 << ": " << std::includes(v1.begin(), v1.end(), v4.begin(), v4.end()) << '\n'
    << v5 << ": " << std::includes(v1.begin(), v1.end(), v5.begin(), v5.end()) << '\n'
    << v6 << ": " << std::includes(v1.begin(), v1.end(), v6.begin(), v6.end()) << '\n'
    << v7 << ": " << std::includes(v1.begin(), v1.end(), v7.begin(), v7.end(), no_case)
          << " (case-insensitive)\n";
}

Output:

a b c f h x
includes:
a b c   : true
a c     : true
a a b   : false
g       : false
a c g   : false
A B C   : true (case-insensitive)

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
LWG 1205	C++98	the return value was unclear if `[` first2 `,` last2 `)` is empty	returns true in this case

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set_difference	computes the difference between two sets (function template)
search	searches for the first occurrence of a range of elements (function template)
ranges::includes (C++20)	returns true if one sequence is a subsequence of another (algorithm function object)