std::ranges::find_last_if() algorithm

since C++20

Simplified

// (1)
constexpr ranges::subrange<I>
    find_last_if( I first, S last, Pred pred, Proj proj = {} );

// (2)
constexpr ranges::borrowed_subrange_t<R>
    find_last_if( R&& r, Pred pred, Proj proj = {} );

The type of arguments are generic and have the following constraints:

• I - std::input_iterator
• S - std::sentinel_for<I>
• Proj - (none)
• Pred:
  • (1) - std::indirect_unary_predicate<std::projected<I, Proj>>
  • (2) - std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>>
• (2) - R - std::ranges::input_range

The Proj template argument has a default type of std::identity for all overloads.

Detailed

// (1)
template<
  std::forward_iterator I,
  std::sentinel_for<I> S,
  class Proj = std::identity,
  std::indirect_unary_predicate<std::projected<I, Proj>> Pred
>
constexpr ranges::subrange<I>
    find_last_if( I first, S last, Pred pred, Proj proj = {} );

// (2)
template<
  ranges::forward_range R,
  class Proj = std::identity,
  std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>> Pred
>
constexpr ranges::borrowed_subrange_t<R>
    find_last_if( R&& r, Pred pred, Proj proj = {} );

Returns the last element in the range [first; last) that satisfies specific criteria:

• (1) Searches for an element for which predicate pred returns true.

• (2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are niebloids.

Parameters

first last	The range of elements to examine.
r	The range of elements to examine.
pred	Predicate to apply to the projected elements.
proj	Projection to apply to the elements.

Return value

• (1) Let i be the last iterator in the range [first; last) for which *i == value is true. Returns a value of type ranges::subrange<I> initialized as follows:

  {
    i,
    last
  }

  or { last, last } if no such iterator is found.

• (2) Same as (1) but the return type is ranges::borrowed_subrange_t<I>.

Complexity

At most last - first applications of the predicate and projection.

Exceptions

(none)

Possible implementation

find_last_if(1)

struct find_last_if_fn
{
    template<std::forward_iterator I, std::sentinel_for<I> S,
             class Proj = std::identity,
             std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
    constexpr ranges::subrange<I>
        operator()(I first, S last, Pred pred, Proj proj = {}) const
    {
        // Note: if I is mere forward_iterator, we may only go from begin to end.
        I found {};
        for (; first != last; ++first)
            if (std::invoke(pred, std::invoke(proj, *first)))
                found = first;

        if (found == I {})
            return {first, first};

        return {found, std::ranges::next(found, last)};
    }

    template<ranges::forward_range R, class Proj = std::identity,
             std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>>
                 Pred>
    constexpr ranges::borrowed_subrange_t<R>
        operator()(R&& r, Pred pred, Proj proj = {}) const
    {
        return this->operator()(ranges::begin(r), ranges::end(r),
                                std::ref(pred), std::ref(proj));
    }
};

inline constexpr find_last_if_fn find_last_if;

Examples

Main.cpp

#include <algorithm>
#include <forward_list>
#include <iomanip>
#include <iostream>
#include <string_view>

int main()
{
    constexpr static auto v = {1, 2, 3, 1, 2, 3, 1, 2};

    {
        constexpr auto i1 = std::ranges::find_last(v.begin(), v.end(), 3);
        constexpr auto i2 = std::ranges::find_last(v, 3);
        static_assert(std::ranges::distance(v.begin(), i1.begin()) == 5);
        static_assert(std::ranges::distance(v.begin(), i2.begin()) == 5);
    }
    {
        constexpr auto i1 = std::ranges::find_last(v.begin(), v.end(), -3);
        constexpr auto i2 = std::ranges::find_last(v, -3);
        static_assert(i1.begin() == v.end());
        static_assert(i2.begin() == v.end());
    }

    auto abs = [](int x) { return x < 0 ? -x : x; };

    {
        auto pred = [](int x) { return x == 3; };
        constexpr auto i1 = std::ranges::find_last_if(v.begin(), v.end(), pred, abs);
        constexpr auto i2 = std::ranges::find_last_if(v, pred, abs);
        static_assert(std::ranges::distance(v.begin(), i1.begin()) == 5);
        static_assert(std::ranges::distance(v.begin(), i2.begin()) == 5);
    }
    {
        auto pred = [](int x) { return x == -3; };
        constexpr auto i1 = std::ranges::find_last_if(v.begin(), v.end(), pred, abs);
        constexpr auto i2 = std::ranges::find_last_if(v, pred, abs);
        static_assert(i1.begin() == v.end());
        static_assert(i2.begin() == v.end());
    }

    {
        auto pred = [](int x) { return x == 1 or x == 2; };
        constexpr auto i1 = std::ranges::find_last_if_not(v.begin(), v.end(), pred, abs);
        constexpr auto i2 = std::ranges::find_last_if_not(v, pred, abs);
        static_assert(std::ranges::distance(v.begin(), i1.begin()) == 5);
        static_assert(std::ranges::distance(v.begin(), i2.begin()) == 5);
    }
    {
        auto pred = [](int x) { return x == 1 or x == 2 or x == 3; };
        constexpr auto i1 = std::ranges::find_last_if_not(v.begin(), v.end(), pred, abs);
        constexpr auto i2 = std::ranges::find_last_if_not(v, pred, abs);
        static_assert(i1.begin() == v.end());
        static_assert(i2.begin() == v.end());
    }

    using P = std::pair<std::string_view, int>;
    std::forward_list<P> list
    {
        {"one", 1}, {"two", 2}, {"three", 3},
        {"one", 4}, {"two", 5}, {"three", 6},
    };
    auto cmp_one = [](const std::string_view &s) { return s == "one"; };

    // find latest element that satisfy the comparator, and projecting pair::first
    const auto subrange = std::ranges::find_last_if(list, cmp_one, &P::first);

    // print the found element and the "tail" after it
    for (P const& e : subrange)
        std::cout << '{' << std::quoted(e.first) << ", " << e.second << "} ";
    std::cout << '\n';
}

Output

{"one", 4} {"two", 5} {"three", 6}