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std::ranges::find_end

From cppreference.com
< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms: std::ranges::copy, std::ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
C library
 
Constrained algorithms
Non-modifying sequence operations
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Permutations
 
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>
    ranges::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>
    ranges::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:

In practice, they may be implemented as function objects, or with special compiler extensions.

Contents

[edit] Parameters

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> object initialized with expression {i, i + (i == last1 ? 0 : last2 - first2)} that is 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 returned object is effectivelly intialized with {last1, last1}.
2) same as (1) but the returned object is ranges::borrowed_subrange_t<R1>.

[edit] Complexity

At most S*(N-S+1) applications of the corresponding predicate and each projection, where S = last2 - first2 and N = last1 - first1 for (1), or S = ranges::size(r2) and N = ranges::size(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 hovewer 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

#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

finds the last sequence of elements in a certain range
(function template) [edit]
finds the first two adjacent items that are equal (or satisfy a given predicate)
(niebloid) [edit]
finds the first element satisfying specific criteria
(niebloid) [edit]
searches for any one of a set of elements
(niebloid) [edit]
searches for a range of elements
(niebloid) [edit]
searches for a number consecutive copies of an element in a range
(niebloid) [edit]