Книга: Standard Template Library Programmer

find_end

find_end

Category: algorithms

Component type: function

find_end is an overloaded name; there are actually two find_end functions.

template<class ForwardIterator1, class ForwardIterator2>
ForwardIterator1 find_end(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2);
template<class ForwardIterator1, class ForwardIterator2, class BinaryPredicate>
ForwardIterator1 find_end(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2, BinaryPredicate comp);

Find_end is misnamed: it is much more similar to search than to find, and a more accurate name would have been search_end.

Like search, find_end attempts to find a subsequence within the range [first1, last1) that is identical to [first2, last2). The difference is that while search finds the first such subsequence, find_end finds the last such subsequence. Find_end returns an iterator pointing to the beginning of that subsequence; if no such subsequence exists, it returns last1.

The two versions of find_end differ in how they determine whether two elements are the same: the first uses operator==, and the second uses the user-supplied function object comp.

The first version of find_end returns the last iterator i in the range [first1, last1 – (last2 – first2)) such that, for every iterator j in the range [first2, last2), *(i + (j – first2)) == *j. The second version of find_end returns the last iterator i in [first1, last1 – (last2 – first2)) such that, for every iterator j in [first2, last2), binary_pred(*(i + (j – first2)), *j) is true. These conditions simply mean that every element in the subrange beginning with i must be the same as the corresponding element in [first2, last2).

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

For the first version:

• ForwardIterator1 is a model of Forward Iterator.

• ForwardIterator2 is a model of Forward Iterator.

• ForwardIterator1's value type is a model of EqualityComparable.

• ForwardIterator2's value type is a model of EqualityComparable.

• Objects of ForwardIterator1's value type can be compared for equality with Objects of ForwardIterator2's value type.

For the second version:

• ForwardIterator1 is a model of Forward Iterator.

• ForwardIterator2 is a model of Forward Iterator.

• BinaryPredicate is a model of Binary Predicate.

• ForwardIterator1's value type is convertible to BinaryPredicate's first argument type.

• ForwardIterator2's value type is convertible to BinaryPredicate's second argument type.

• [first1, last1) is a valid range.

• [first2, last2) is a valid range.

The number of comparisons is proportional to (last1 – first1) * (last2 – first2). If both ForwardIterator1 and ForwardIterator2 are models of Bidirectional Iterator, then the average complexity is linear and the worst case is at most (last1 – first1) * (last2 – first2) comparisons.

int main() {
 char* s = "executable.exe";
 char* suffix = "exe";
 const int N = strlen(s);
 const int N_suf = strlen(suffix);
 char* location = find_end(s, s + N, suffix, suffix + N_suf);
 if (location != s + N) {
  cout << "Found a match for " << suffix << " within " << s << endl;
  cout << s << endl;
  int i;
  for (i = 0; i < (location – s); ++i) cout << ' ';
  for (i = 0; i < N_suf; ++i) cout << '^';
  cout << endl;
 } else cout << "No match for " << suffix << " within " << s << endl;
}

[1] The reason that this range is [first1, last1 – (last2 – first2)), instead of simply [first1, last1), is that we are looking for a subsequence that is equal to the complete sequence [first2, last2). An iterator i can't be the beginning of such a subsequence unless last1 – i is greater than or equal to last2 – first2. Note the implication of this: you may call find_end with arguments such that last1 – first1 is less than last2 – first2, but such a search will always fail.

search

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