Книга: Standard Template Library Programmer

search_n

search_n

Category: algorithms

Component type: function

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);

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.

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

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 second 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.

• [first, last) is a valid range.

• count is non-negative [1].

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.)

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

[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.

search, find_end, find, find_if

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