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std::ranges::search_n

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 I, std::sentinel_for<I> S, class T,

         class Pred = ranges::equal_to, class Proj = std::identity>
  requires std::indirectly_comparable<I, const T*, Pred, Proj>
    constexpr ranges::subrange<I>
      ranges::search_n(I first, S last, std::iter_difference_t<I> count,

               const T& value, Pred pred = {}, Proj proj = {});
(1) (since C++20)
template<ranges::forward_range R, class T, class Pred = ranges::equal_to,

         class Proj = std::identity>
  requires std::indirectly_comparable<ranges::iterator_t<R>, const T*, Pred, Proj>
    constexpr ranges::borrowed_subrange_t<R>
      ranges::search_n(R&& r, ranges::range_difference_t<R> count,

               const T& value, Pred pred = {}, Proj proj = {});
(2) (since C++20)
1) Searches the range [first, last) for the first sequence of count elements whose projected values are each equal to the given value according to the binary predicate pred.
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, that is:

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

Contents

[edit] Parameters

first, last - the range of elements to examine (aka haystack)
r - the range of elements to examine (aka haystack)
count - the length of the sequence to search for
value - the value to search for (aka needle)
pred - the binary predicate that compares the projected elements with value
proj - the projection to apply to the elements of the range to examine

[edit] Return value

1) Returns std::ranges::subrange object that contains a pair of iterators in the range [first, last) that designate the found subsequence.

If no such subsequence is found, returns std::ranges::subrange{last, last}.

If count ≤ 0, returns std::ranges::subrange{first, first}.
2) same as (1) but the returned object is ranges::borrowed_subrange_t<R>.

[edit] Complexity

Linear: at most last - first applications of the predicate and the projection.

[edit] Notes

An implementation can improve efficiency of the search in average if the iterators model std::random_access_iterator.

[edit] Possible implementation

struct search_n_fn
{
  template<std::forward_iterator I, std::sentinel_for<I> S, class T,
           class Pred = ranges::equal_to, class Proj = std::identity>
    requires std::indirectly_comparable<I, const T*, Pred, Proj>
      constexpr ranges::subrange<I>
        operator()(I first, S last, std::iter_difference_t<I> count,
                   const T& value, Pred pred = {}, Proj proj = {}) const {
          if (count <= 0)
            return {first, first};
          for (; first != last; ++first) {
            if (std::invoke(pred, std::invoke(proj, *first), value)) {
              I start = first;
              std::iter_difference_t<I> n{1};
              for (;;) {
                if (n++ == count)
                  return {start, std::next(first)}; // found
                if (++first == last)
                  return {first, first}; // not found
                if (!std::invoke(pred, std::invoke(proj, *first), value))
                  break; // not equ to value
              }
            }
          }
          return {first, first};
        }
 
  template<ranges::forward_range R, class T, class Pred = ranges::equal_to,
           class Proj = std::identity>
    requires std::indirectly_comparable<ranges::iterator_t<R>, const T*, Pred, Proj>
      constexpr ranges::borrowed_subrange_t<R>
        operator()(R&& r, ranges::range_difference_t<R> count,
                   const T& value, Pred pred = {}, Proj proj = {}) const {
            return (*this)(ranges::begin(r), ranges::end(r),
                           std::move(count), value,
                           std::move(pred), std::move(proj));
        }
};
 
inline constexpr search_n_fn search_n{};

[edit] Example

#include <algorithm>
#include <iomanip>
#include <iostream>
#include <string>
 
int main()
{
    static constexpr auto nums = {1, 2, 2, 3, 4, 1, 2, 2, 2, 1};
    constexpr int count{ 3 };
    constexpr int value{ 2 };
 
    constexpr auto result1 = std::ranges::search_n(
        nums.begin(), nums.end(), count, value
    );
    static_assert( // found
        result1.size() == count &&
        std::distance(nums.begin(), result1.begin()) == 6 &&
        std::distance(nums.begin(), result1.end()) == 9
    );
 
    constexpr auto result2 = std::ranges::search_n(nums, count, value);
    static_assert( // found
        result2.size() == count &&
        std::distance(nums.begin(), result2.begin()) == 6 &&
        std::distance(nums.begin(), result2.end()) == 9
    );
 
    constexpr auto result3 = std::ranges::search_n(nums, count, /* value */ 5);
    static_assert( // not found
        result3.size() == 0 &&
        result3.begin() == result3.end() &&
        result3.end() == nums.end()
    );
 
    constexpr auto result4 = std::ranges::search_n(nums, /* count */ 0, /* value */ 1);
    static_assert( // not found
        result4.size() == 0 &&
        result4.begin() == result4.end() &&
        result4.end() == nums.begin()
    );
 
    constexpr char symbol{'B'};
    std::cout << std::boolalpha << "Find a sub-sequence: "
              << std::quoted(std::string(count, symbol)) << '\n';
 
    auto result5 = std::ranges::search_n(nums, count, symbol,
        [](const char x, const char y) { // binary predicate
            const bool o{ x == y };
            std::cout << "bin_op(" << x << ", " << y << ") == " << o << "\n";
            return o;
        },
        [](const int z) { return 'A' + z - 1; } // projects nums -> ASCII
    );
 
    std::cout << "Found: " << !result5.empty() << '\n';
}

Output:

Find a sub-sequence: "BBB"
bin_op(A, B) == false
bin_op(B, B) == true
bin_op(B, B) == true
bin_op(C, B) == false
bin_op(D, B) == false
bin_op(A, B) == false
bin_op(B, B) == true
bin_op(B, B) == true
bin_op(B, B) == true
Found: true

[edit] See also

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]
finds the last sequence of elements in a certain range
(niebloid) [edit]
searches for any one of a set of elements
(niebloid) [edit]
returns true if one sequence is a subsequence of another
(niebloid) [edit]
finds the first position where two ranges differ
(niebloid) [edit]
searches for a range of elements
(niebloid) [edit]
searches a range for a number of consecutive copies of an element
(function template) [edit]