std::ranges::find_first_of(3)

std::ranges::find_first_of(3) — live from the C++ Standard Libary manual ✨ (yes, that’s the header’s spelling, preserving it fr fr 💀)

🌸 NAME 🌸

std::ranges::find_first_of - std::ranges::find_first_of

Okay bestie, the NAME section said it twice, so I’m saying it twice too 🫶 iconic behavior.

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✨ Synopsis ✨

Defined in header <algorithm>

Call signature

(1) since C++20:

template< std::input_iterator I1, std::sentinel_for<I1> S1,
          std::forward_iterator I2, std::sentinel_for<I2> S2,
          class Pred = ranges::equal_to,
          class Proj1 = std::identity,
          class Proj2 = std::identity >
    requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
constexpr I1
    find_first_of( I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {},
                   Proj1 proj1 = {}, Proj2 proj2 = {} );

(2) since C++20:

template< ranges::input_range R1, ranges::forward_range R2,
          class Pred = ranges::equal_to,
          class Proj1 = std::identity,
          class Proj2 = std::identity >
    requires std::indirectly_comparable<ranges::iterator_t<R1>,
                                        ranges::iterator_t<R2>,
                                        Pred, Proj1, Proj2>
constexpr ranges::borrowed_iterator_t<R1>
    find_first_of( R1&& r1, R2&& r2, Pred pred = {},
                   Proj1 proj1 = {}, Proj2 proj2 = {} );

(1) searches the range [first1, last1) for any of the elements in the range [first2, last2), after projecting the ranges with proj1 and proj2 respectively. The projected elements are compared using the binary predicate pred.

(2) same as (1), but uses r1 as the first source range and r2 as the second source 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.

The function-like entities described on this page are niebloids, that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

In practice, they may be implemented as function objects, or with special compiler extensions. no cap, that’s the lore 🫡

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🎀 Parameters 🎀

• first1, last1 — the range of elements to examine (aka haystack)
• first2, last2 — the range of elements to search for (aka needles)
• r1 — the range of elements to examine (aka haystack)
• r2 — the range of elements to search for (aka needles)
• pred — binary predicate to compare the elements
• proj1 — projection to apply to the elements in the first range
• proj2 — projection to apply to the elements in the second range

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🦋 Return value 🦋

Iterator to the first element in the range [first1, last1) that is equal to an element from the range [first2, last2) after projection. If no such element is found, an iterator comparing equal to last1 is returned. simple as that, periodt 💅

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💅 Complexity 💅

At most S * N applications of the predicate and each projection, where:

• (1) S = ranges::distance(first2, last2) and N = ranges::distance(first1, last1);
• (2) S = ranges::distance(r2) and N = ranges::distance(r1).

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🧚 Possible implementation 🧚

struct find_first_of_fn {
    template<std::input_iterator I1, std::sentinel_for<I1> S1,
             std::forward_iterator I2, std::sentinel_for<I2> S2,
             class Pred = ranges::equal_to,
             class Proj1 = std::identity,
             class Proj2 = std::identity>
    requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
    constexpr I1 operator()(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {},
                            Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        for (; first1 != last1; ++first1)
            for (auto i = first2; i != last2; ++i)
                if (std::invoke(pred, std::invoke(proj1, *first1), std::invoke(proj2, *i)))
                    return first1;
        return first1;
    }

    template<ranges::input_range R1, ranges::forward_range R2,
             class Pred = ranges::equal_to,
             class Proj1 = std::identity,
             class Proj2 = std::identity>
    requires std::indirectly_comparable<ranges::iterator_t<R1>,
                                        ranges::iterator_t<R2>,
                                        Pred, Proj1, Proj2>
    constexpr ranges::borrowed_iterator_t<R1>
        operator()(R1&& r1, R2&& r2, Pred pred = {},
                   Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        return (*this)(ranges::begin(r1), ranges::end(r1),
                       ranges::begin(r2), ranges::end(r2),
                       std::move(pred), std::move(proj1),
                       std::move(proj2));
    }
};

inline constexpr find_first_of_fn find_first_of {};

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🔥 Example 🔥

// Run this code

#include <algorithm>
#include <iostream>
#include <iterator>

int main()
{
    namespace rng = std::ranges;

    constexpr static auto haystack = {1, 2, 3, 4};
    constexpr static auto needles  = {0, 3, 4, 3};

    constexpr auto found1 = rng::find_first_of(haystack.begin(), haystack.end(),
                                               needles.begin(), needles.end());
    static_assert(std::distance(haystack.begin(), found1) == 2);

    constexpr auto found2 = rng::find_first_of(haystack, needles);
    static_assert(std::distance(haystack.begin(), found2) == 2);

    constexpr static auto negatives = {-6, -3, -4, -3};
    constexpr auto not_found = rng::find_first_of(haystack, negatives);
    static_assert(not_found == haystack.end());

    constexpr auto found3 = rng::find_first_of(haystack, negatives,
        [](int x, int y) { return x == -y; }); // uses a binary comparator
    static_assert(std::distance(haystack.begin(), found3) == 2);

    struct P { int x, y; };
    constexpr static auto p1 = {P{1, -1}, P{2, -2}, P{3, -3}, P{4, -4}};
    constexpr static auto p2 = {P{5, -5}, P{6, -3}, P{7, -5}, P{8, -3}};

    // Compare only P::y data members by projecting them:
    const auto found4 = rng::find_first_of(p1, p2, {}, &P::y, &P::y);
    std::cout << "First equivalent element {" << found4->x << ", " << found4->y
              << "} was found at position " << std::distance(p1.begin(), found4)
              << ".\n";
}

Output:

First equivalent element {3, -3} was found at position 2.

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🌈 See also 🌈

• find_first_of — searches for any one of a set of elements (function template)
• ranges::adjacent_find — finds the first two adjacent items that are equal (or satisfy a given predicate) (C++20) (niebloid)
• ranges::find — finds the first element satisfying specific criteria (C++20) (niebloid)
• ranges::find_if — finds the first element satisfying specific criteria (C++20) (niebloid)
• ranges::find_if_not — finds the first element satisfying specific criteria (C++20) (niebloid)
• ranges::find_end — finds the last sequence of elements in a certain range (C++20) (niebloid)
• ranges::search — searches for a range of elements (C++20) (niebloid)
• ranges::search_n — searches for a number consecutive copies of an element in a range (C++20) (niebloid)

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