std::lower_bound

template< class ForwardIt, class T >
ForwardIt lower_bound( ForwardIt first, ForwardIt last, const T& value );

template< class ForwardIt, class T >
constexpr ForwardIt lower_bound( ForwardIt first, ForwardIt last,
                                 const T& value );

template< class ForwardIt, class T, class Compare >
ForwardIt lower_bound( ForwardIt first, ForwardIt last,
                       const T& value, Compare comp );

template< class ForwardIt, class T, class Compare >
constexpr ForwardIt lower_bound( ForwardIt first, ForwardIt last,
                                 const T& value, Compare comp );

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.

Parameters

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 log
2(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++.

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;
}

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 (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';
    }
}

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.

See also