Difference between revisions of "cpp/algorithm/ranges/find end"

Latest revision as of 06:21, 22 September 2023

Defined in header `<algorithm>`
Call signature
template< std::forward_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 ranges::subrange<I1> find_end( I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );	(1)	(since C++20)
template< ranges::forward_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_subrange_t<R1> find_end( R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );	(2)	(since C++20)

1) Searches for the last occurrence of the sequence [first2, last2) in the range [first1, last1), after projection 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.

first1, last1	-	the range of elements to examine (aka haystack)
first2, last2	-	the range of elements to search for (aka needle)
r1	-	the range of elements to examine (aka haystack)
r2	-	the range of elements to search for (aka needle)
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

[edit] Return value

1) ranges::subrange<I1>{} value initialized with expression {i, i + (i == last1 ? 0 : ranges::distance(first2, last2))} that denotes the last occurrence of the sequence [first2, last2) in range [first1, last1) (after projections with proj1 and proj2). If [first2, last2) is empty or if no such sequence is found, the return value is effectively initialized with {last1, last1}.

2) Same as (1), except that the return type is ranges::borrowed_subrange_t<R1>.

[edit] Complexity

At most $\scriptsize S\cdot(N-S+1)$ $S\cdot(N-S+1)$ applications of the corresponding predicate and each projection, where $\scriptsize S$ $S$ is ranges::distance(first2, last2) and $\scriptsize N$ $N$ is ranges::distance(first1, last1) for (1), or $\scriptsize S$ $S$ is ranges::distance(r2) and $\scriptsize N$ $N$ is ranges::distance(r1) for (2).

[edit] Notes

An implementation can improve efficiency of the search if the input iterators model std::bidirectional_iterator by searching from the end towards the begin. Modelling the std::random_access_iterator may improve the comparison speed. All this however does not change the theoretical complexity of the worst case.

[edit] Possible implementation

struct find_end_fn
{
    template<std::forward_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 ranges::subrange<I1>
        operator()(I1 first1, S1 last1,
                   I2 first2, S2 last2, Pred pred = {},
                   Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        if (first2 == last2)
        {
            auto last_it = ranges::next(first1, last1);
            return {last_it, last_it};
        }
        auto result = ranges::search(
            std::move(first1), last1, first2, last2, pred, proj1, proj2);
 
        if (result.empty())
            return result;
 
        for (;;)
        {
            auto new_result = ranges::search(
                std::next(result.begin()), last1, first2, last2, pred, proj1, proj2);
            if (new_result.empty())
                return result;
            else
                result = std::move(new_result);
        }
    }
 
    template<ranges::forward_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_subrange_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_end_fn find_end {};

[edit] Example

Run this code

#include <algorithm>
#include <array>
#include <cctype>
#include <iostream>
#include <ranges>
#include <string_view>
 
void print(const auto haystack, const auto needle)
{
    const auto pos = std::distance(haystack.begin(), needle.begin());
    std::cout << "In \"";
    for (const auto c : haystack)
        std::cout << c;
    std::cout << "\" found \"";
    for (const auto c : needle)
        std::cout << c;
    std::cout << "\" at position [" << pos << ".." << pos + needle.size() << ")\n"
        << std::string(4 + pos, ' ') << std::string(needle.size(), '^') << '\n';
}
 
int main()
{
    using namespace std::literals;
    constexpr auto secret{"password password word..."sv};
    constexpr auto wanted{"password"sv};
 
    constexpr auto found1 = std::ranges::find_end(
        secret.cbegin(), secret.cend(), wanted.cbegin(), wanted.cend());
    print(secret, found1);
 
    constexpr auto found2 = std::ranges::find_end(secret, "word"sv);
    print(secret, found2);
 
    const auto found3 = std::ranges::find_end(secret, "ORD"sv,
        [](const char x, const char y) { // uses a binary predicate
            return std::tolower(x) == std::tolower(y);
        });
    print(secret, found3);
 
    const auto found4 = std::ranges::find_end(secret, "SWORD"sv, {}, {},
        [](char c) { return std::tolower(c); }); // projects the 2nd range
    print(secret, found4);
 
    static_assert(std::ranges::find_end(secret, "PASS"sv).empty()); // => not found
}

Output:

In "password password word..." found "password" at position [9..17)
             ^^^^^^^^
In "password password word..." found "word" at position [18..22)
                      ^^^^
In "password password word..." found "ord" at position [19..22)
                       ^^^
In "password password word..." found "sword" at position [12..17)
                ^^^^^

[edit] See also

ranges::find_lastranges::find_last_ifranges::find_last_if_not (C++23)(C++23)(C++23)	finds the last element satisfying specific criteria (niebloid)[edit]
ranges::findranges::find_ifranges::find_if_not (C++20)(C++20)(C++20)	finds the first element satisfying specific criteria (niebloid)[edit]
ranges::find_first_of (C++20)	searches for any one of a set of elements (niebloid)[edit]
ranges::adjacent_find (C++20)	finds the first two adjacent items that are equal (or satisfy a given predicate) (niebloid)[edit]
ranges::search (C++20)	searches for the first occurrence of a range of elements (niebloid)[edit]
ranges::search_n (C++20)	searches for the first occurrence of a number consecutive copies of an element in a range (niebloid)[edit]
find_end	finds the last sequence of elements in a certain range (function template) [edit]

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Difference between revisions of "cpp/algorithm/ranges/find end"

Latest revision as of 06:21, 22 September 2023

Contents

[edit] Parameters