Defined in header
template<classForwardIt,classT>ForwardItlower_bound(ForwardItfirst,ForwardItlast,constT&value); (1)(constexpr since C++20)
(until C++26)template<classForwardIt,classT=typenamestd::iterator_traits<ForwardIt>::value_type>constexprForwardItlower_bound(ForwardItfirst,ForwardItlast,constT&value);(since C++26)template<classForwardIt,classT,classCompare>ForwardItlower_bound(ForwardItfirst,ForwardItlast,constT&value,Comparecomp); (2)(constexpr since C++20)
(until C++26)template<classForwardIt,classT=typenamestd::iterator_traits<ForwardIt>::value_type,classCompare>constexprForwardItlower_bound(ForwardItfirst,ForwardItlast,constT&value,Comparecomp);(since C++26)Searches for the first element in the partitioned source range [first, last) which is not ordered before value.
1) The order is determined by operator<(until C++20)std::less{}(since C++20).
If the elements e of the source range are not
with respect to the expression bool(e<value)(until C++20)bool(std::less{}(e,value))(since C++20), the behavior is undefined.
2) The order is determined by comp.
If the elements e of the source range are not partitioned with respect to the expression bool(comp(e,value)), the behavior is undefined.
Parameters
first, last - the pair of iterators defining the source
value - value to compare the elements to comp - binary predicate which returns true if the first argument is ordered before the second. The signature of the predicate function should be equivalent to the following:
boolpred(constType1&a,constType2&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) Type1 and Type2 regardless of
(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 implicitly converted to Type2.
Type requirements -ForwardIt must meet the requirements of
. -Compare must meet the requirements of
. It is not required to satisfy
. Return value
Iterator to the first element which is not ordered before value, or last if no such element is found.
Complexity
Given N as std::distance(first,last):
1) At most log2(N)+𝓞(1) comparisons with value using operator<(until C++20)std::less{}(since C++20).
2) At most log2(N)+𝓞(1) applications of the comparator comp.
Notes
Although std::lower_bound only requires the source range to be partitioned, this algorithm is usually used in the case where the source range is sorted, so that the binary search is valid for any value.
Unlike
, std::lower_bound does not require operator< 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 every iterator iter in the source range.
If ForwardIt is not a
, the number of iterator increments is linear in std::distance(first,last). Notably,
,
,
, and
iterators are not random access, and so their member lower_bound functions should be preferred.
macroValueStdFeature
__cpp_lib_algorithm_default_value_type
(C++26)
for algorithms (
)Possible implementation
See also the implementations in
and
.
template<classForwardIt,classT=typenamestd::iterator_traits<ForwardIt>::value_type>ForwardItlower_bound(ForwardItfirst,ForwardItlast,constT&value){returnstd::lower_bound(first,last,value,std::less{});}
template<classForwardIt,classT=typenamestd::iterator_traits<ForwardIt>::value_type,classCompare>ForwardItlower_bound(ForwardItfirst,ForwardItlast,constT&value,Comparecomp){ForwardItit;typenamestd::iterator_traits<ForwardIt>::difference_typecount,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;}elsecount=step;}returnfirst;}Example
Run this code
#include<algorithm>#include<cassert>#include<complex>#include<iostream>#include<vector>structPriceInfo{doubleprice;};intmain(){conststd::vector<int>data{1,2,4,5,5,6};for(inti=0;i<8;++i){// Search for first element x such that i ≤ xautolower=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(constdoubleto_find:{102.5,110.2}){autoprc_info=std::lower_bound(prices.begin(),prices.end(),to_find,[](constPriceInfo&info,doublevalue){returninfo.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';}usingCD=std::complex<double>;std::vector<CD>nums{{1,0},{2,2},{2,1},{3,0}};autocmpz=[](CDx,CDy){returnx.real()<y.real();};#ifdef __cpp_lib_algorithm_default_value_typeautoit=std::lower_bound(nums.cbegin(),nums.cend(),{2,0},cmpz);#elseautoit=std::lower_bound(nums.cbegin(),nums.cend(),CD{2,0},cmpz);#endifassert((*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 retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
C++98 Compare was required to satisfy
and T was required
to be
(strict weak ordering required) only a partitioning is required;
heterogeneous comparisons permitted
C++98 at most log(N)+1 comparisons were allowed corrected to log2(N)+𝓞(1)
C++98 if any iterator iter exists in the source range such that
bool(comp(*iter,value)) is false, std::lower_bound
could return any iterator in [iter, last)no iterator after
iter can be returned See also
(C++20)
finds the first element not less than the given value using binary search
(algorithm function object)
finds the range of elements matching the given value using binary search
(function template & algorithm function object)
(C++20)
divides a range of elements into two groups
(function template & algorithm function object)
(C++20)
(C++11)
locates the partition point of a partitioned range
(function template & algorithm function object)
(C++20)
finds the first element greater than the given value using binary search
(function template & algorithm function object)
(C++20)
returns an iterator to the first element not less than the given key
(public member function of std::set<Key,Compare,Allocator>)
returns an iterator to the first element not less than the given key
(public member function of std::multiset<Key,Compare,Allocator>)