c++(8):c++ function lower_bound(), upper_bound()

std::lower_bound()

1) Function:

Returns an iterator over the first element not less than the given value

2) Links:

C++ official website: https://en.cppreference.com/w/cpp/algorithm/lower_bound
is defined in the header file:middle

3) Definition:

The second form of the definition is as follows, and the other forms are similar

(1)
template< class ForwardIt, class T >
ForwardIt lower_bound( ForwardIt first, ForwardIt last, const T& value );

(2)
template< class ForwardIt, class T, class Compare >
ForwardIt lower_bound( ForwardIt first, ForwardIt last,
                       const T& value, Compare comp );

Perform a binary search in the range [first, last), find the iterator of the first element that does not satisfy element < value [or comp(element, value)) (that is, greater than or equal to)], if no such element is found, Then return last.

4) Parameters:

first, last - iterators defining the partial order ranges to check
value - the value of the compared elements
comp - a binary predicate that returns true if the first argument is less than (ie comes before the second).

5) Return value:

Iterator 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.

6) The algorithm of the function:

first edition

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;
}

second edition

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;
}

7) Example of function:

#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';
    }
}

printout:

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

std::upper_bound()

std::lower_bound()Similar to
the official website link: https://en.cppreference.com/w/cpp/algorithm/upper_bound
Function: Return an iterator pointing to the first element in the range [first, last), so that value < element [or comp(value , element)] is true (i.e. strictly greater than ), if no such element is found, last is returned.
definition:

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