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Standard Template Library Programmer's Guide
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search_n


Category: algorithms	Component type: function

Prototype

Search_n is an overloaded name; there are actually two search_n functions.

template <class ForwardIterator, class Integer, class T>
ForwardIterator search_n(ForwardIterator first, ForwardIterator last,
                         Integer count, const T& value);

template <class ForwardIterator, class Integer, 
          class T, class BinaryPredicate>
ForwardIterator search_n(ForwardIterator first, ForwardIterator last,
                         Integer count, const T& value,
                         BinaryPredicate binary_pred);

Description

Search_n searches for a subsequence of count consecutive elements in the range [first, last), all of which are equal to value. [1] It returns an iterator pointing to the beginning of that subsequence, or else last if no such subsequence exists. The two versions of search_n differ in how they determine whether two elements are the same: the first uses operator==, and the second uses the user-supplied function object binary_pred.

The first version of search returns the first iterator i in the range [first, last - count) [2] such that, for every iterator j in the range [i, i + count), *j == value. The second version returns the first iterator i in the range [first, last - count) such that, for every iterator j in the range [i, i + count), binary_pred(*j, value) is true.

Definition

Defined in the standard header algorithm, and in the nonstandard backward-compatibility header algo.h.

Requirements on types

For the first version:

ForwardIterator is a model of Forward Iterator.
Integer is an integral type.
T is a model of EqualityComparable.
ForwardIterator's value type is a model of EqualityComparable.
Objects of ForwardIterator's value type can be compared for equality with Objects of type T.

For the first version:

ForwardIterator is a model of Forward Iterator.
Integer is an integral type.
T is a model of EqualityComparable.
BinaryPredicate is a model of Binary Predicate.
ForwardIterator's value type is convertible to BinaryPredicate's first argument type.
T is convertible to BinaryPredicate's second argument type.

Preconditions

[first, last) is a valid range.
count is non-negative [1].

Complexity

Linear. Search_n performs at most last - first comparisons.

(The C++ standard permits the complexity to be O(n (last - first)), but this is unnecessarily lax. There is no reason for search_n to examine any element more than once.)

Example

bool eq_nosign(int x, int y) { return abs(x) == abs(y); }

void lookup(int* first, int* last, size_t count, int val) {
  cout << "Searching for a sequence of "
       << count
       << " '" << val << "'"
       << (count != 1 ? "s: " : ":  ");
  int* result = search_n(first, last, count, val);
  if (result == last)
    cout << "Not found" << endl;
  else
    cout << "Index = " << result - first << endl;
}

void lookup_nosign(int* first, int* last, size_t count, int val) {
  cout << "Searching for a (sign-insensitive) sequence of "
       << count
       << " '" << val << "'"
       << (count != 1 ? "s: " : ":  ");
  int* result = search_n(first, last, count, val, eq_nosign);
  if (result == last)
    cout << "Not found" << endl;
  else
    cout << "Index = " << result - first << endl;
}

int main() {
  const int N = 10;
  int A[N] = {1, 2, 1, 1, 3, -3, 1, 1, 1, 1};

  lookup(A, A+N, 1, 4);
  lookup(A, A+N, 0, 4);
  lookup(A, A+N, 1, 1);
  lookup(A, A+N, 2, 1);
  lookup(A, A+N, 3, 1);
  lookup(A, A+N, 4, 1);

  lookup(A, A+N, 1, 3);
  lookup(A, A+N, 2, 3);
  lookup_nosign(A, A+N, 1, 3);
  lookup_nosign(A, A+N, 2, 3);
}

The output is

Searching for a sequence of 1 '4':  Not found
Searching for a sequence of 0 '4's: Index = 0
Searching for a sequence of 1 '1':  Index = 0
Searching for a sequence of 2 '1's: Index = 2
Searching for a sequence of 3 '1's: Index = 6
Searching for a sequence of 4 '1's: Index = 6
Searching for a sequence of 1 '3':  Index = 4
Searching for a sequence of 2 '3's: Not found
Searching for a (sign-insensitive) sequence of 1 '3':  Index = 4
Searching for a (sign-insensitive) sequence of 2 '3's: Index = 4

Notes

[1] Note that count is permitted to be zero: a subsequence of zero elements is well defined. If you call search_n with count equal to zero, then the search will always succeed: no matter what value is, every range contains a subrange of zero consecutive elements that are equal to value. When search_n is called with count equal to zero, the return value is always first.

[2] The reason that this range is [first, last - count), rather than just [first, last), is that we are looking for a subsequence whose length is count; an iterator i can't be the beginning of such a subsequence unless last - count is greater than or equal to count. Note the implication of this: you may call search_n with arguments such that last - first is less than count, but such a search will always fail.