在 std::vector 中搜索值

Question

std::vector<int> v1 = { 0,48000 };
e1 = left(v1,0); // should return first item's iterator
e1 = left(v1,10000); // should return first item's iterator
e1 = left(v1,20000); // should return first item's iterator
e1 = left(v1,48000); // should return the second item's iterator

e1 = right(v1,0); // should return first
e1 = right(v1,10000); // should return second
e1 = right(v1,48000); // should return second
e1 = right(v1,60000); // should return end()

My problem is how to implement functions left and right . std::upper_bound and std::lower_bound don't seem to help:

auto l1 = std::lower_bound(v1.begin(), v1.end(), 0); // 0
l1 = std::lower_bound(v1.begin(), v1.end(), 14000); // 48000
l1 = std::lower_bound(v1.begin(), v1.end(), 38000);
l1 = std::lower_bound(v1.begin(), v1.end(), 48000);
l1 = std::lower_bound(v1.begin(), v1.end(), 58000); // end()

auto l1 = std::upper_bound(v1.begin(), v1.end(), 0); // 48000
l1 = std::upper_bound(v1.begin(), v1.end(), 14000); // 48000
l1 = std::upper_bound(v1.begin(), v1.end(), 38000); // 48000
l1 = std::upper_bound(v1.begin(), v1.end(), 48000); // end()
l1 = std::upper_bound(v1.begin(), v1.end(), 58000); // end()

Basically, in a container which contains some sorted values, I want, by specifying an value that does not exist in the container, to take what's left and what's right on it (without of course searching manually in a loop if possible).

Answer 1

Your right() function finds an iterator pointing to the first element greater than or equal to a given value while the left() function returns an iterator pointing to the first element lesser than or equal to a given value.

So, your right() function should use std::lower_bound() while your left() function should do the same but with a greater-than comparator and with reverse iterators.

Try the following:

#include <algorithm>
#include <utility>
#include <iterator>
#include <vector>

template <typename T>
typename std::vector<T>::const_iterator left(std::vector<T> const& v, T const& val) {
    return v.empty() || val < v.front() || val > v.back()
        ? v.end()
        : std::next(std::lower_bound(v.rbegin(), v.rend(), val, std::greater<T>())).base();
}

template <typename T>
typename std::vector<T>::const_iterator right(std::vector<T> const& v, T const& val) {
    return v.empty() || val < v.front()
        ? v.end()
        : std::lower_bound(v.begin(), v.end(), val);
}