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std::search_n

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
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
 
Defined in header <algorithm>
template< class ForwardIt, class Size, class T >
ForwardIt search_n( ForwardIt first, ForwardIt last, Size count, const T& value );
(1)
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 BinaryPredicate >

ForwardIt search_n( ForwardIt first, ForwardIt last, Size count, const T& value,

                     BinaryPredicate p );
(3)
template< class ExecutionPolicy, class ForwardIt, class Size, class T, class BinaryPredicate >

ForwardIt search_n( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, Size count, const T& value,

                     BinaryPredicate p );
(4) (since C++17)

Searches the range [first, last) for the first sequence of count identical elements, each equal to the given value 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 do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

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);

The signature does not need to have const &, but the function must not modify the objects passed to it.
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 ForwardIterator.

[edit] Return value

Iterator to the beginning of the found sequence in the range [first, last). If no such sequence is found, last is returned.

[edit] Complexity

At most last - first applications of the predicate.

[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,
  • if policy is std::parallel_vector_execution_policy, std::terminate is called
  • if policy is std::sequential_execution_policy or std::parallel_execution_policy, the algorithm exits with an std::exception_list containing all uncaught exceptions. If there was only one uncaught exception, the algorithm may rethrow it without wrapping in std::exception_list. It is unspecified how much work the algorithm will perform before returning after the first exception was encountered.
  • if policy is some other type, the behavior is implementation-defined
  • If the algorithm fails to allocate memory (either for itself or to construct an std::exception_list when handling a user exception), std::bad_alloc is thrown.

[edit] Possible implementation

First version
template<class ForwardIt, class Size, class T>
ForwardIt search_n(ForwardIt first, ForwardIt last,
                    Size count, const T& value)
{
    for(; first != last; ++first) {
        if (!(*first == value)) {
            continue;
        }
 
        ForwardIt candidate = first;
        Size cur_count = 0;
 
        while (true) {
            ++cur_count;
            if (cur_count == count) {
                // success
                return candidate;
            }
            ++first;
            if (first == last) {
                // exhausted the list
                return last;
            }
            if (!(*first == value)) {
                // too few in a row
                break;
            }
        }
    }
    return last;
}
Second version
template<class ForwardIt, class Size, class T, class BinaryPredicate>
ForwardIt search_n(ForwardIt first, ForwardIt last,
                    Size count, const T& value, BinaryPredicate p)
{
    for(; first != last; ++first) {
        if (!p(*first, value)) {
            continue;
        }
 
        ForwardIt candidate = first;
        Size cur_count = 0;
 
        while (true) {
            ++cur_count;
            if (cur_count == count) {
                // success
                return candidate;
            }
            ++first;
            if (first == last) {
                // exhausted the list
                return last;
            }
            if (!p(*first, value)) {
                // too few in a row
                break;
            }
        }
    }
    return last;
}

[edit] Example

#include <iostream>
#include <algorithm>
#include <iterator>
 
template <class Container, class Size, class T>
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()
{
   const char sequence[] = "1001010100010101001010101";
 
   std::cout << std::boolalpha;
   std::cout << "Has 4 consecutive zeros: "
             << consecutive_values(sequence,4,'0') << '\n';
   std::cout << "Has 3 consecutive zeros: "
             << consecutive_values(sequence,3,'0') << '\n';
}

Output:

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

[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]
parallelized version of std::search_n
(function template) [edit]