function for finding last item less-than-or-equal to, like lower_bound

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南方客 2021-01-30 12:52

Is there a function in that uses binary search, like lower_bound but that returns the last item less-than-or-equal-to according to a given predic

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梦如初夏 (楼主)

2021-01-30 13:40

std::prev: https://en.cppreference.com/w/cpp/iterator/prev

#include 
#include 

int main()
{
    std::map m{{2, 'a'}, {4, 'b'}, {6, 'c'}, {8, 'd'}, {10, 'e'}};

    int num = 3;
    auto it = m.upper_bound(num);
    auto pv = std::prev(it);

    std::cout << "upper bound of " << num << ": "
        << it->first << ", " << it->second << '\n';
    std::cout << "lower than or equal of " << num << ": "
        << pv->first << ", " << pv->second << '\n';
}

Output:

upper bound of 3: 4, b
lower than or equal than 3: 2, a

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