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std::lower_bound(3) C++ Standard Libary std::lower_bound(3) NAME std::lower_bound - std::lower_bound Synopsis Defined in header <algorithm> template< class ForwardIt, class T > (constexpr since ForwardIt lower_bound( ForwardIt first, C++20) ForwardIt last, (until C++26) const T& value ); template< class ForwardIt, class T = typename std::iterator_traits <ForwardIt>::value_type > (since C++26) constexpr ForwardIt lower_bound( ForwardIt first, ForwardIt last, const T& value ); template< class ForwardIt, class T, class Compare > (1) (con- stexpr since ForwardIt lower_bound( ForwardIt first, C++20) ForwardIt last, (until C++26) const T& value, Compare comp ); template< class ForwardIt, class T = typename std::iterator_traits (2) <ForwardIt>::value_type, class Compare > (since C++26) constexpr ForwardIt lower_bound( ForwardIt first, ForwardIt last, const T& value, Compare comp ); Searches for the first element in the partitioned range [first, last) which is not ordered before value. 1) The order is determined by operator<: Returns the first iterator iter in [first, last) where bool(*iter < value) is false, or last if no such iter exists. (un- til C++20) If the elements elem of [first, last) are not partitioned with re- spect to the expression bool(elem < value), the behavior is undefined. Equivalent to std::lower_bound(first, last, value, std::less{}). (since C++20) 2) The order is determined by comp: Returns the first iterator iter in [first, last) where bool(comp(*iter, value)) is false, or last if no such iter exists. If the elements elem of [first, last) are not partitioned with re- spect to the expression bool(comp(elem, value)), the behavior is undefined. Parameters first, last - the partitioned range of elements to examine value - value to compare the elements to binary predicate which returns true if the first ar- gument 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); comp - 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) Type1 and Type2 re- gardless of value category (thus, Type1 & is not allowed , nor is Type1 unless for Type1 a move is equivalent to a copy (since C++11)). The type Type1 must be such that an object of type ForwardIt can be dereferenced and then implicitly converted to Type1. The type Type2 must be such that an object of type T can be implic- itly converted to Type2. Type requirements - ForwardIt must meet the requirements of LegacyForwardIterator. - Compare must meet the requirements of BinaryPredicate. It is not re- quired to satisfy Compare. Return value Iterator to the first element of the range [first, last) not ordered before value, or last if no such element is found. Complexity Given \(\scriptsize N\)N as std::distance(first, last): 1) At most \(\scriptsize \log_{2}(N)+O(1)\)log 2(N)+O(1) comparisons with value using operator< (until C++20) std::less{} (since C++20). 2) At most \(\scriptsize \log_{2}(N)+O(1)\)log 2(N)+O(1) applications of the comparator comp. However, if ForwardIt is not a LegacyRandomAccessIterator, the num- ber of iterator increments is linear in \(\scriptsize N\)N. 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) template<class ForwardIt, class T = typename std::iterator_traits<For- wardIt>::value_type> ForwardIt lower_bound(ForwardIt first, ForwardIt last, const T& value) { return std::lower_bound(first, last, value, std::less{}); } lower_bound (2) template<class ForwardIt, class T = typename std::iterator_traits<For- wardIt>::value_type, 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; } Notes Although std::lower_bound only requires [first, last) to be parti- tioned, this algorithm is usually used in the case where [first, last) is sorted, so that the binary search is valid for any value. Unlike std::binary_search, std::lower_bound does not require opera- tor< or comp to be asymmetric (i.e., a < b and b < a always have different results). In fact, it does not even require value < *iter or comp(value, *iter) to be well- formed for any iterator iter in [first, last). Feature-test macro Value Std Feature __cpp_lib_algorithm_default_value_type 202403 (C++26) List-initial- ization for algorithms (1,2) Example // Run this code #include <algorithm> #include <cassert> #include <complex> #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::dis- tance(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'; } using CD = std::complex<double>; std::vector<CD> nums{{1, 0}, {2, 2}, {2, 1}, {3, 0}}; auto cmpz = [](CD x, CD y) { return x.real() < y.real(); }; #ifdef __cpp_lib_algorithm_default_value_type auto it = std::lower_bound(nums.cbegin(), nums.cend(), {2, 0}, cmpz); #else auto it = std::lower_bound(nums.cbegin(), nums.cend(), CD{2, 0}, cmpz); #endif assert((*it == CD{2, 2})); } 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 retroac- tively to previously published C++ standards. DR Applied to Behavior as published Cor- rect behavior Compare was required to satisfy only a partitioning is LWG 270 C++98 Compare and T was required required; to be LessThanComparable (strict weak heteroge- neous comparisons ordering required) permitted at most \(\scriptsize corrected to \(\scriptsize LWG 384 C++98 \log(N)+1\)log(N)+1 comparisons were \log_{2}(N)+O(1)\)log allowed 2(N)+1 if any iterator iter exists in [first, last) such that LWG 2150 C++98 bool(comp(*iter, value)) is false, no itera- tor after std::lower_bound iter can be returned could return any iterator in [iter, last) See also equal_range returns range of elements matching a specific key (function template) partition divides a range of elements into two groups (function template) partition_point locates the partition point of a partitioned range (C++11) (function template) returns an iterator to the first element greater than a certain upper_bound value (function template) returns an iterator to the first element not less than the given lower_bound key (public member function of std::set<Key,Com- pare,Allocator>) returns an iterator to the first element not less than the given lower_bound key (public member function of std::multi- set<Key,Compare,Allocator>) ranges::lower_bound returns an iterator to the first element not less than the given (C++20) value (niebloid) http://cppreference.com 2024.06.10 std::lower_bound(3)
NAME | Synopsis | Parameters | Type requirements | Return value | Complexity | Possible implementation | Notes | Example | Output: | See also
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