std::ranges::search_n

Defined in header <algorithm>

Call signature

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> 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> search_n( R&& r, ranges::range_difference_t<R> count, const T& value, Pred pred = {}, Proj proj = {} );`	(2)	(since C++20)

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>
    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>
    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:

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.

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

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 return type is ranges::borrowed_subrange_t<R>.

Complexity

Linear: at most ranges::distance(first, last) applications of the predicate and the projection.

Notes

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

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 {};

Example

#include <algorithm>
#include <iomanip>
#include <iostream>
#include <iterator>
#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};
    typedef int count_t, value_t;
 
    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_t{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_t{0}, value_t{1});
    static_assert( // not found
        result4.size() == 0 &&
        result4.begin() == result4.end() &&
        result4.end() == nums.begin()
    );
 
    constexpr char symbol{'B'};
    auto to_ascii = [](const int z) -> char { return 'A' + z - 1; };
    auto is_equ = [](const char x, const char y) { return x == y; };
 
    std::cout << "Find a sub-sequence " << std::string(count, symbol) << " in the ";
    std::ranges::transform(nums, std::ostream_iterator<char>(std::cout, ""), to_ascii);
    std::cout << '\n';
 
    auto result5 = std::ranges::search_n(nums, count, symbol, is_equ, to_ascii);
    if (not result5.empty())
        std::cout << "Found at position "
                  << std::ranges::distance(nums.begin(), result5.begin()) << '\n';
}

Output:

Find a sub-sequence BBB in the ABBCDABBBA
Found at position 6

ranges::adjacent_find (C++20)	finds the first two adjacent items that are equal (or satisfy a given predicate) (niebloid)
ranges::findranges::find_ifranges::find_if_not (C++20)(C++20)(C++20)	finds the first element satisfying specific criteria (niebloid)
ranges::find_end (C++20)	finds the last sequence of elements in a certain range (niebloid)
ranges::find_first_of (C++20)	searches for any one of a set of elements (niebloid)
ranges::includes (C++20)	returns `true` if one sequence is a subsequence of another (niebloid)
ranges::mismatch (C++20)	finds the first position where two ranges differ (niebloid)
ranges::search (C++20)	searches for a range of elements (niebloid)
search_n	searches a range for a number of consecutive copies of an element (function template)

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