std::includes(3)

💖 std::includes 💖

Okay bestie, welcome to the std::includes(3) page from the C++ Standard Libary (yes, the manpage header spells it Libary 😭 and honestly it's iconic). This is the algorithm that answers: “is this sorted range hiding inside that other sorted range?” Let's get into it, fr fr. ✨

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🎀 NAME 🎀

std::includes — std::includes. That's the name. That's the whole name. We love a self-referential moment. 💅

🌈 Synopsis 🌈

Defined in header <algorithm>.

template< class InputIt1, class InputIt2 >

bool includes( InputIt1 first1, InputIt1 last1,          (1)
(constexpr since C++20)

               InputIt2 first2, InputIt2 last2 );
template< class ExecutionPolicy,

          class ForwardIt1, class ForwardIt2 >
bool includes( ExecutionPolicy&& policy,                 (2) (since
C++17)
               ForwardIt1 first1, ForwardIt1 last1,

               ForwardIt2 first2, ForwardIt2 last2 );
template< class InputIt1, class InputIt2, class Compare
>

bool includes( InputIt1 first1, InputIt1 last1,          (3)
(constexpr since C++20)

               InputIt2 first2, InputIt2 last2, Compare
comp );
template< class ExecutionPolicy,

          class ForwardIt1, class ForwardIt2, class
Compare >
bool includes( ExecutionPolicy&& policy,                 (4) (since
C++17)
               ForwardIt1 first1, ForwardIt1 last1,

               ForwardIt2 first2, ForwardIt2 last2,
Compare comp );

So what does it do? Returns true if the sorted range [first2, last2) is a subsequence of the sorted range [first1, last1) — and bestie, a subsequence need not be contiguous. It can be skipping around, main-character style. 💃

• 1) If [first1, last1) or [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) or [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. Tiny empty-icon behavior, but make it make sense. 💖

🔥 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 \(\scriptsize 2 \cdot (N_1+N_2)-1\)2⋅(N 1+N 2)-1 comparisons using operator< (until C++20) / std::less{} (since C++20).
• 3,4) At most \(\scriptsize 2 \cdot (N_1+N_2)-1\)2⋅(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 LWG 1205 — applied to C++98
  • Behavior as published: the return value was unclear if [first2, last2) is empty
  • Correct behavior: 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 — returns true if one sequence is a subsequence of another (C++20) (niebloid)

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That's the whole vibe check, bestie. Catch you later — source: http://cppreference.com, version/date: 2024.06.10. 💖✨