binary_search binary_search Category: algorithms Component type: function Prototype Binary_search is an overloaded name; there are actually two binary_search functions. template <class ForwardIterator, class LessThanComparable> bool binary_search(ForwardIterator first, ForwardIterator last, const LessThanComparable& value); template <class ForwardIterator, class T, class StrictWeakOrdering> bool binary_search(ForwardIterator first, ForwardIterator last, const T& value, StrictWeakOrdering comp); Description Binary_search is a version of binary search: it attempts to find the element value in an ordered range [first, last) It returns true if an element that is equivalent to [1] value is present in [first, last) and false if no such element exists. [2] The first version of binary_search uses operator< for comparison, and the second uses the function object comp. Specifically, the first version returns true if and only if there exists an iterator i in [first, last) such that *i < value and value < *i are both false. The second version returns true if and only if there exists an iterator i in [first, last) such that comp(*i, value) and comp(value, *i) are both false. 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. LessThanComparable is a model of LessThan Comparable. The ordering on objects of type LessThanComparable is a strict weak ordering, as defined in the LessThan Comparable requirements. ForwardIterator's value type is the same type as LessThanComparable. For the second version: ForwardIterator is a model of Forward Iterator. StrictWeakOrdering is a model of Strict Weak Ordering. ForwardIterator's value type is the same type as T. ForwardIterator's value type is convertible to StrictWeakOrdering's argument type. Preconditions For the first version: [first, last) is a valid range. [first, last) is ordered in ascending order according to operator<. That is, for every pair of iterators i and j in [first, last) such that i precedes j, *j < *i is false. For the second version: [first, last) is a valid range. [first, last) is ordered in ascending order according to the function object comp. That is, for every pair of iterators i and j in [first, last) such that i precedes j, comp(*j, *i) is false. Complexity The number of comparisons is logarithmic: at most log(last - first) + 2. If ForwardIterator is a Random Access Iterator then the number of steps through the range is also logarithmic; otherwise, the number of steps is proportional to last - first. [3] Example int main() { int A[] = { 1, 2, 3, 3, 3, 5, 8 }; const int N = sizeof(A) / sizeof(int); for (int i = 1; i <= 10; ++i) { cout << "Searching for " << i << ": " << (binary_search(A, A + N, i) ? "present" : "not present") << endl; } } The output is: Searching for 1: present Searching for 2: present Searching for 3: present Searching for 4: not present Searching for 5: present Searching for 6: not present Searching for 7: not present Searching for 8: present Searching for 9: not present Searching for 10: not present Notes [1] Note that you may use an ordering that is a strict weak ordering but not a total ordering; that is, there might be values x and y such that x < y, x > y, and x == y are all false. (See the LessThan Comparable requirements for a more complete discussion.) Finding value in the range [first, last), then, doesn't mean finding an element that is equal to value but rather one that is equivalent to value: one that is neither greater than nor less than value. If you're using a total ordering, however (if you're using strcmp, for example, or if you're using ordinary arithmetic comparison on integers), then you can ignore this technical distinction: for a total ordering, equality and equivalence are the same. [2] Note that this is not necessarily the information you are interested in! Usually, if you're testing whether an element is present in a range, you'd like to know where it is (if it's present), or where it should be inserted (if it's not present). The functions lower_bound, upper_bound, and equal_range provide this information. [3] This difference between Random Access Iterators and Forward Iterators is simply because advance is constant time for Random Access Iterators and linear time for Forward Iterators. See also lower_bound, upper_bound, equal_range Copyright © 1999 Silicon Graphics, Inc. All Rights Reserved. TrademarkInformation