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std::search_n

From cppreference.com
< cpp‎ | algorithm
 
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Defined in header <algorithm>
(1)
template< class ForwardIt, class Size, class T >

ForwardIt search_n( ForwardIt first, ForwardIt last,

                    Size count, const T& value );
 (constexpr since C++20)
(until C++26)
template< class ForwardIt, class Size,

          class T = typename std::iterator_traits
                        <ForwardIt>::value_type >
constexpr ForwardIt search_n( ForwardIt first, ForwardIt last,

                              Size count, const T& value );
 (since C++26)
(2)
template< class ExecutionPolicy,

          class ForwardIt, class Size, class T >
ForwardIt search_n( ExecutionPolicy&& policy,
                    ForwardIt first, ForwardIt last,

                    Size count, const T& value );
 (since C++17)
(until C++26)
template< class ExecutionPolicy,

          class ForwardIt, class Size,
          class T = typename std::iterator_traits
                        <ForwardIt>::value_type >
ForwardIt search_n( ExecutionPolicy&& policy,
                    ForwardIt first, ForwardIt last,

                    Size count, const T& value );
 (since C++26)
(3)
template< class ForwardIt, class Size, class T, class BinaryPred >

ForwardIt search_n( ForwardIt first, ForwardIt last,

                    Size count, const T& value, BinaryPred p );
 (constexpr since C++20)
(until C++26)
template< class ForwardIt, class Size,

          class T = typename std::iterator_traits
                        <ForwardIt>::value_type,
          class BinaryPred >
constexpr ForwardIt search_n( ForwardIt first, ForwardIt last,

                    Size count, const T& value, BinaryPred p );
 (since C++26)
(4)
template< class ExecutionPolicy, class ForwardIt, class Size,

          class T, class BinaryPred >
ForwardIt search_n( ExecutionPolicy&& policy,
                    ForwardIt first, ForwardIt last,

                    Size count, const T& value, BinaryPred p );
 (since C++17)
(until C++26)
template< class ExecutionPolicy, class ForwardIt, class Size,

          class T = typename std::iterator_traits
                        <ForwardIt>::value_type,
          class BinaryPred >
ForwardIt search_n( ExecutionPolicy&& policy,
                    ForwardIt first, ForwardIt last,

                    Size count, const T& value, BinaryPred p );
 (since C++26)

Searches the range [first, last) for the first sequence of count identical elements, each equal to the given value.

1) Elements are compared using operator==.
3) Elements are compared using the given binary predicate p.
2,4) Same as (1,3), but executed according to policy.
These overloads participate in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.

(until C++20)

std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.

(since C++20)

Contents

[edit] Parameters

first, last - the pair of iterators defining the range of elements to examine
count - the length of the sequence to search for
value - the value of the elements to search for
policy - the execution policy to use
p - binary predicate which returns ​true if the elements should be treated as equal.

The signature of the predicate function should be equivalent to the following:

 bool pred(const Type1 &a, const Type2 &b);

While the signature does not need to have const &, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1 & is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy(since C++11)).
The type Type1 must be such that an object of type ForwardIt can be dereferenced and then implicitly converted to Type1. The type Type2 must be such that an object of type T can be implicitly converted to Type2. ​

Type requirements
-
ForwardIt must meet the requirements of LegacyForwardIterator.
-
BinaryPred must meet the requirements of BinaryPredicate.
-
Size must be convertible to an integral type.

[edit] Return value

If count is positive, returns an iterator to the beginning of the first sequence found in the range [first, last). Each iterator it in the sequence should satisfy the following condition:

1,2) *it == value is true.
3,4) p(*it, value) != false is true.

If no such sequence is found, last is returned.

If count is zero or negative, first is returned.

[edit] Complexity

Given N as std::distance(first, last):

1,2) At most N comparisons using operator==.
3,4) At most N applications of the predicate p.

[edit] Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

  • If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
  • If the algorithm fails to allocate memory, std::bad_alloc is thrown.

[edit] Possible implementation

search_n (1) 
template<class ForwardIt, class Size,
         class T = typename std::iterator_traits<ForwardIt>::value_type>
ForwardIt search_n(ForwardIt first, ForwardIt last, Size count, const T& value)
{
    if (count <= 0)
        return first;
 
    for (; first != last; ++first)
    {
        if (!(*first == value))
            continue;
 
        ForwardIt candidate = first;
 
        for (Size cur_count = 1; true; ++cur_count)
        {
            if (cur_count >= count)
                return candidate; // success
 
            ++first;
            if (first == last)
                return last; // exhausted the list
 
            if (!(*first == value))
                break; // too few in a row
        }
    }
    return last;
}
search_n (3) 
template<class ForwardIt, class Size,
         class T = typename std::iterator_traits<ForwardIt>::value_type,
         class BinaryPred>
ForwardIt search_n(ForwardIt first, ForwardIt last, Size count, const T& value,
                   BinaryPred p)
{
    if (count <= 0)
        return first;
 
    for (; first != last; ++first)
    {
        if (!p(*first, value))
            continue;
 
        ForwardIt candidate = first;
 
        for (Size cur_count = 1; true; ++cur_count)
        {
            if (cur_count >= count)
                return candidate; // success
 
            ++first;
            if (first == last)
                return last; // exhausted the list
 
            if (!p(*first, value))
                break; // too few in a row
        }
    }
    return last;
}

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