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std::search_n

From cppreference.com
< cpp‎ | algorithm
 
 
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
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
 
Defined in header <algorithm>
template< class ForwardIt, class Size, class T >

ForwardIt search_n( ForwardIt first, ForwardIt last,

                    Size count, const T& value );
(1) (constexpr since C++20)
template< class ExecutionPolicy,

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

                    Size count, const T& value );
(2) (since C++17)
template< class ForwardIt, class Size, class T, class BinaryPred >

ForwardIt search_n( ForwardIt first, ForwardIt last,

                    Size count, const T& value, BinaryPred p );
(3) (constexpr since C++20)
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 );
(4) (since C++17)

Searches the range [firstlast) 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

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 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. See execution policy for details.
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 [firstlast). 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>
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, 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;
}

[edit] Example

#include <algorithm>
#include <iostream>
#include <iterator>
 
template<class Container, class Size, class T>
constexpr bool consecutive_values(const Container& c, Size count, const T& v)
{
    return std::search_n(std::begin(c), std::end(c), count, v) != std::end(c);
}
 
int main()
{
    constexpr char sequence[] = "1001010100010101001010101";
 
    static_assert(consecutive_values(sequence, 3, '0'));
 
    std::cout << std::boolalpha
              << "Has 4 consecutive zeros: "
              << consecutive_values(sequence, 4, '0') << '\n'
              << "Has 3 consecutive zeros: "
              << consecutive_values(sequence, 3, '0') << '\n';
}

Output:

Has 4 consecutive zeros: false
Has 3 consecutive zeros: true

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 283 C++98 T was required to be EqualityComparable, but
the value type of InputIt is not always T
removed the requirement
LWG 426 C++98 the complexity upper limit was N·count,
it is negative if count is negative
the upper limit is 0
if count is non-positive
LWG 714 C++98 if count > 0, the complexity upper limit was N·count, but in
the worst case the number of comparisons/operations is always N
changed the upper
limit to N in this case
LWG 2150 C++98 the condition of “sequence occurence” was incorrect corrected

[edit] See also

finds the last sequence of elements in a certain range
(function template) [edit]
finds the first element satisfying specific criteria
(function template) [edit]
searches for a range of elements
(function template) [edit]
searches for a number consecutive copies of an element in a range
(niebloid)[edit]