How to find closest smaller number in array?

Question

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 the closest smaller number in the array, preferably from <algorithm>? Example:

Number	Result
0	0
3	0
15	12
74	60
150	89

Answer 1

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

One could make use of the already existing std::lower_bound (or std::ranges::lower_bound since C++20) for this. Check the example code given on the referenced pages.

For example, you could write a function as follows. I leave the error handling as well as corner cases perfections for you.

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

constexpr auto find_equal_or_closest(std::vector<int> const& numbers, const int query)
{
    // invalid query/ not able to find cases!
    // you could also restructure to return std::cend(numbers) from function.
    if (numbers.empty() 
        /* || !std::ranges::is_sorted(numbers) */ // optional check
        || query < *std::cbegin(numbers)
        )       
        return -1; 

    auto it = std::ranges::lower_bound(numbers, query);
    return (it != std::cend(numbers) && *it == query) ? *it : *std::prev(it);
}

See live demo