C++ - 前两个元素之间的差异

Question

I've recently began to work with C++ std::sets , so there is a question I did not find answer to in Google.

I have a std::set of some int values (eg, let it be 1, 2, 3, 4, 5 ). The task is to calculate difference between two first elements. Is it possible to do with C++?

I'm using data structure std::set ; And I know that first element can be got like that:

int diff = *arSeq.begin();

Where arSeq is mentioned set .

Is there any way to get the second element?

Answer 1

Yes it is possible. Access the first element via the std::set::begin iterator and the next one using the std::next function.

#include <iostream>
#include <set>

int main() {
    std::set<int> s = { 1, 2, 3, 4, 5 };
    auto result = *s.begin() - *std::next(s.begin());
    std::cout << result;
}

Please note that you will again get the same result of -1 even if you defined your set as:

std::set<int> s = { 5, 3, 1, 4, 2 }; 

because the std::set is a container that:

contains a sorted set of unique objects...

Answer 2

You can do it the following way

#include <iostream>
#include <set>
#include <iterator>

int main()
{
    std::set<int> s({ 1, 2, 3, 4, 5 });

    long long int diff = 0;

    if (not (s.size() < 2))
    {
        auto first = s.begin();
        auto second = std::next(first);

        diff = *first < *second ? ( long long int )*second - *first : ( long long int )*first - *second;
    }

    std::cout << "difference = " << diff << std::endl;

    return 0;
}

Answer 3

What did you mean by first two elements. First two element inserted? then its not possible. Set elements are sorted with given comparator. By default primitive elements are sorted in ascending order. See the output of following program.

// set::begin/end
#include <iostream>
#include <set>
#include<functional>

int main ()
{
  int myints[] = {75,23,65,42,13};
  std::set<int, std::greater<int>> myset (myints,myints+5);

  auto first = myset.begin();
  auto second = std::next(first);

  std::cout <<*first<< ", " <<*second<<"\n";

  return 0;
}`enter code here`