#include <algorithm>
#include <iostream>
#include <vector>
 
std::size_t getIndexFor(const std::vector<int>& v, int input)
{
    auto it = std::lower_bound(v.begin(), v.end(), input);
    if (it != v.begin() && (it == v.end() || *it != input)) {
        --it;
    }
    return std::distance(v.begin(), it);
}
 
int main()
{
    const std::vector<int> v = { 0, 2, 4, 5};
 
    for (int i : {0, 1, 2, 3, 4, 5}) {
        std::cout << getIndexFor(v, i) << ":" << i << std::endl;
    }
}
 

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