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std::ranges::search_n

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< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
(C++17)
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations Copy operations
(C++11)
(C++11)
Swap operations Transformation operations Generation operations Removing operations Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
Sorting operations Binary search operations
(on partitioned ranges) Set operations (on sorted ranges) Merge operations (on sorted ranges) Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations Permutation operations


C library
Numeric operations
 
Constrained algorithms
All names in this menu belong to namespace std::ranges
Non-modifying sequence operations
Modifying sequence operations
Partitioning operations
Sorting operations
       
Binary search operations (on sorted ranges)
       
       
    
Set operations (on sorted ranges)
     
        
Heap operations
     
         
Minimum/maximum operations
       
       
Permutation operations
Fold operations
(C++23)
(C++23)  
(C++23)
(C++23)  
Defined in header <algorithm>
Call signature
(1)
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 = {} );
(since C++20)
(until C++26)
template< std::forward_iterator I, std::sentinel_for <I> S,

          class Pred = ranges::equal_to, class Proj = std::identity,
          class T = std::projected_value_t<I, Proj> >
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 = {} );
(since C++26)
(2)
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 = {} );
(since C++20)
(until C++26)
template< ranges::forward_range R,

          class Pred = ranges::equal_to, class Proj = std::identity,
          class T = std::projected_value_t<ranges::iterator_t <R>, Proj> >
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 = {} );
(since C++26)
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 algorithm function objects (informally known as niebloids), that is:

Contents

[edit] Parameters

first, last - the iterator-sentinel pair defining 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 return type is ranges::borrowed_subrange_t <R>.

[edit] Complexity

Linear: at most ranges::distance (first, last) 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 .

Feature-test macro Value Std Feature
__cpp_lib_algorithm_default_value_type 202403 (C++26) List-initialization for algorithms

[edit] Possible implementation

struct search_n_fn
{
 template<std::forward_iterator I, std::sentinel_for <I> S,
 class Pred = ranges::equal_to, class Proj = std::identity,
 class T = std::projected_value_t<I, Proj>>
 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 Pred = ranges::equal_to, class Proj = std::identity,
 class T = std::projected_value_t<ranges::iterator_t <R>, Proj>>
 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

Run this code
#include <algorithm>
#include <cassert>
#include <complex>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <string>
#include <vector>
 
int main()
{
 namespace ranges = std::ranges;
 
 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 = 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 = 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 = 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 = 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 = ranges::search_n(nums, count, symbol, is_equ, to_ascii);
 if (not result5.empty())
 std::cout << "Found at position "
 << ranges::distance (nums.begin(), result5.begin()) << '\n';
 
 std::vector <std::complex <double>> nums2{{4, 2}, {4, 2}, {1, 3}};
 #ifdef __cpp_lib_algorithm_default_value_type
 auto it = ranges::search_n(nums2, 2, {4, 2});
 #else
 auto it = ranges::search_n(nums2, 2, std::complex <double>{4, 2});
 #endif
 assert (it.size() == 2);
}

Output: 

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

[edit] See also

finds the first two adjacent items that are equal (or satisfy a given predicate)
(algorithm function object)[edit]
finds the first element satisfying specific criteria
(algorithm function object)[edit]
finds the last sequence of elements in a certain range
(algorithm function object)[edit]
searches for any one of a set of elements
(algorithm function object)[edit]
returns true if one sequence is a subsequence of another
(algorithm function object)[edit]
finds the first position where two ranges differ
(algorithm function object)[edit]
searches for the first occurrence of a range of elements
(algorithm function object)[edit]
searches for the first occurrence of a number consecutive copies of an element in a range
(function template) [edit]

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