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# std::search_n

From cppreference.com
< cpp‎ | algorithm

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 ForwardIt, class Size, class T, class BinaryPredicate > ForwardIt search_n( ForwardIt first, ForwardIt last, Size count, const T& value, BinaryPredicate p ); | (2) |

Searches the range `[first, last)` for the first sequence of count identical elements, each equal to the given value value. The first version uses `operator==` to compare the elements, the second version uses the given binary predicate `p`.

### [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
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] 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

Run this code

    #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

find_end | finds the last sequence of elements in a certain range (function template) [edit]
findfind_iffind_if_not (C++11) | finds the first element satisfying specific criteria (function template) [edit]
search | searches for a range of elements (function template) [edit]
std::experimental::parallel::search_n (parallelism TS) | parallelized version of `std::search_n` (function template) [edit]

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