是否有像lower_bound这样的函数返回最后一个值而不是第一个值？

Question

For example, if I have a sorted array

{1,1,1,1,1,4,5}

and I want to know the rightmost index of 1 , is there a function that will allow me to do that? (Apart from reverse sorting the array)

Answer 1

This should work:

auto p = std::equal_range( std::begin(v), std::end(v), 1 );
if( p.first != p.second ) {
    auto it = p.second - 1;
    //...
}

live example

Answer 2

There's none so you should craft one on your own.

template<class Ctr, class Elem> auto rightmost(Ctr &&c, Elem &&e) {
    using std::begin;
    using std::end;
    auto b{begin(c)};
    auto retVal{std::upper_bound(b, end(c), e)};
    return retVal == b? b : --retVal;
}

Answer 3

#include <iostream>
#include <array>
#include <algorithm>
#include <iterator>

int main()
{
    std::array<int, 6> data({2,2,2,2,4,7});

    auto it = std::upper_bound(data.begin(), data.end(), 2);
    int index = std::distance(data.begin(), it) - 1;

    std::cout << "index for last '2' is " << index << std::endl;
}

output:
index for last '2' is 3