How to find closest smaller number in array with C++?

Consider this array:

std::vector<int> numbers = { 0, 4, 12, 60, 89 };

It is sorted and only has positive numbers.

What is the easiest way to find closest smaller number in the array, preferably from <algorithm>? Example:

>Solution :

What is the easiest way to find closest smaller number in the array, preferably from <algorithm>?

You are looking for the std::lower_bound (or std::ranges::lower_bound since C++20). Check the example code given in the referenced page.

For instance, you could write a function as follows:

#include <algorithm>  // std::ranges::lower_bound

auto find_closest(std::vector<int> const& numbers, int query)
{
    auto it = std::ranges::lower_bound(numbers, query);
    // Handle negative values
    return (it == std::cbegin(numbers)) ? *it : *(--it);
}

See live demo

I leave the error handling for you.