# std::includes

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class InputIt1, class InputIt2 > bool includes( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2 ); |  | (until C++20)
template< class InputIt1, class InputIt2 > constexpr bool includes( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2 ); |  | (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)
| (3) |
template< class InputIt1, class InputIt2, class Compare > bool includes( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, Compare comp ); |  | (until C++20)
template< class InputIt1, class InputIt2, class Compare > constexpr bool includes( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, Compare comp ); |  | (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) Both ranges must be sorted with operator<.
3) Both ranges must be sorted with the given comparison function comp.
2,4) Same as (1,3), but executed according to policy. These overloads do not participate in overload resolution unless

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.

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

1,2) At most \(\scriptsize 2 \cdot (N_1+N_2-1)\)2·(N1+N2-1) comparisons using operator<.
3,4) At most \(\scriptsize 2 \cdot (N_1+N_2-1)\)2·(N1+N2-1) 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 (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

### See also

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

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/algorithm/includes&oldid=155883"