std::lower_bound
| Defined in header <algorithm>
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| (1) | ||
template< class ForwardIt, class T > ForwardIt lower_bound( ForwardIt first, ForwardIt last, const T& value ); |
(until C++20) | |
| template< class ForwardIt, class T > constexpr ForwardIt lower_bound( ForwardIt first, ForwardIt last, |
(since C++20) | |
| (2) | ||
template< class ForwardIt, class T, class Compare > ForwardIt lower_bound( ForwardIt first, ForwardIt last, |
(until C++20) | |
| template< class ForwardIt, class T, class Compare > constexpr ForwardIt lower_bound( ForwardIt first, ForwardIt last, |
(since C++20) | |
Returns an iterator pointing to the first element in the range [first, last) such that element < value (or comp(element, value)) is false, (i.e. that is greater than or equal to value), or last if no such element is found.
The range [first, last) must be partitioned with respect to the expression element < value (or comp(element, value)), i.e., all elements for which the expression is true must precede all elements for which the expression is false. A fully-sorted range meets this criterion.
The first version uses operator< to compare the elements, the second version uses the given comparison function comp.
Contents |
Parameters
| first, last | - | iterators defining the partially-ordered range to examine |
| value | - | value to compare the elements to |
| comp | - | binary predicate which returns true if the first argument is less than (i.e. is ordered before) the second. The signature of the predicate function should be equivalent to the following: bool pred(const Type1 &a, const Type2 &b); While the signature does not need to have const &, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) |
| Type requirements | ||
-ForwardIt must meet the requirements of LegacyForwardIterator.
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-Compare must meet the requirements of BinaryPredicate. It is not required to satisfy Compare.
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Return value
Iterator pointing to the first element in the range [first, last) such that element < value (or comp(element, value)) is false, or last if no such element is found.
Complexity
The number of comparisons performed is logarithmic in the distance between first and last (at most log2(last - first) + O(1) comparisons). However, for non-LegacyRandomAccessIterators, the number of iterator increments is linear. Notably, std::map, std::multimap, std::set, and std::multiset iterators are not random access, and so their member lower_bound functions should be preferred.
Possible implementation
See also the implementations in libstdc++ and libc++.
| lower_bound (1) |
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template<class ForwardIt, class T> ForwardIt lower_bound(ForwardIt first, ForwardIt last, const T& value) { ForwardIt it; typename std::iterator_traits<ForwardIt>::difference_type count, step; count = std::distance(first, last); while (count > 0) { it = first; step = count / 2; std::advance(it, step); if (*it < value) { first = ++it; count -= step + 1; } else count = step; } return first; } |
| lower_bound (2) |
template<class ForwardIt, class T, class Compare> ForwardIt lower_bound(ForwardIt first, ForwardIt last, const T& value, Compare comp) { ForwardIt it; typename std::iterator_traits<ForwardIt>::difference_type count, step; count = std::distance(first, last); while (count > 0) { it = first; step = count / 2; std::advance(it, step); if (comp(*it, value)) { first = ++it; count -= step + 1; } else count = step; } return first; } |
Example
#include <algorithm> #include <iostream> #include <vector> struct PriceInfo { double price; }; int main() { const std::vector<int> data{1, 2, 4, 5, 5, 6}; for (int i = 0; i < 8; ++i) { // Search for first element x such that i ≤ x auto lower = std::lower_bound(data.begin(), data.end(), i); std::cout << i << " ≤ "; lower != data.end() ? std::cout << *lower << " at index " << std::distance(data.begin(), lower) : std::cout << "not found"; std::cout << '\n'; } std::vector<PriceInfo> prices{{100.0}, {101.5}, {102.5}, {102.5}, {107.3}}; for (const double to_find : {102.5, 110.2}) { auto prc_info = std::lower_bound(prices.begin(), prices.end(), to_find, [](const PriceInfo& info, double value) { return info.price < value; }); prc_info != prices.end() ? std::cout << prc_info->price << " at index " << prc_info - prices.begin() : std::cout << to_find << " not found"; std::cout << '\n'; } }
Output:
0 ≤ 1 at index 0 1 ≤ 1 at index 0 2 ≤ 2 at index 1 3 ≤ 4 at index 2 4 ≤ 4 at index 2 5 ≤ 5 at index 3 6 ≤ 6 at index 5 7 ≤ not found 102.5 at index 2 110.2 not found
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 270 | C++98 | Compare was required to satisfy Compare and T was requiredto be LessThanComparable (strict weak ordering required) |
only a partitioning is required; heterogeneous comparisons permitted |
| LWG 384 | C++98 | at most log(last - first) + 1 comparisons were allowed | corrected to log2(last - first) + O(1) |
See also
| returns range of elements matching a specific key (function template) | |
| divides a range of elements into two groups (function template) | |
| (C++11) |
locates the partition point of a partitioned range (function template) |
| returns an iterator to the first element greater than a certain value (function template) | |
| returns an iterator to the first element not less than the given key (public member function of std::set<Key,Compare,Allocator>)
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| returns an iterator to the first element not less than the given key (public member function of std::multiset<Key,Compare,Allocator>)
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| (C++20) |
returns an iterator to the first element not less than the given value (niebloid) |
