如何将一个向量插入到一个集合中？ [英] How insert OF a vector into a set works?

问题描述

我真的很困惑这个程序中的do-while循环如何工作，特别是向量的插入操作到一个

I am really confused how the do-while loop in this program works especially the insert operation of a vector into a set

s.insert(v)

。有没有人可以解释这个循环中发生了什么？

. Do anyone can explain what happens in this loop?

#include <iostream>     // std::cout
#include <algorithm>    // std::next_permutation, std::sort
#include <vector>
#include <set>

using namespace std;

int main () {
  int myints[] = {0,1,2,3,4,5,6,7,8,9};
  
  int  N = 2;
  
  set<vector<int>> s;
  
  do {
      vector<int> v;
      for (int i = 0; i < N; i++) 
      v.push_back(myints[i]);
      s.insert(v);
  } while ( std::next_permutation(myints,myints+10) );
  
  cout << "number of permutations = " << s.size() << endl;

  for(auto c: s){
    for (auto i: c)
        cout << i << " ";
    cout << endl;
  }
  
  return 0; 
}

推荐答案

您的问题可能是您不知道集合是什么。集合是像vector这样的容器类，除了你不能两次插入相同的元素。在您的情况下，s是一组vector< int>类型的对象。因此，s.insert（v）将向量v插入到集合中。



要了解集合的工作原理，您可能需要阅读：

http://www.cplusplus.com/reference/set/set/ [ ^ ]



其中一个重要的事情是，一个集合使元素按排序顺序排列。插入新元素时，使用二进制搜索来查找元素必须插入的位置。



参见http://www.cplusplus.com/reference/set/set/insert/ [ ^ ]了解有关插入操作的更多信息。



希望让它更清晰一点。



[已修改]



这是对所发生情况的逐行描述：

Your problem might be that you don't know what a set is. A set is container class like vector, except that you cannot insert the same element twice. In your case s is a set of objects of type vector<int>. Hence, s.insert(v) inserts the vector v into the set s.

To understand how a set works you might want to read:
http://www.cplusplus.com/reference/set/set/[^]

One of the important things is that a set keeps its elements in sorted order. When you insert a new element, a binary search is used to find the place where the element has to be inserted.

See http://www.cplusplus.com/reference/set/set/insert/[^] to read more about the insert operation.

Hope that made it a little clearer.

[AMENDED]

Here is a line by line description of what happens:

do {
    vector<int> v; // declares a vector of ints

    // this loop copies the values of the myints array
    // into v (note that the call to next_permutation will rearrange
    // the values in myints at the end of each iteration)
    for (int i = 0; i < N; i++)
       v.push_back(myints[i]);

    // inserts the vector v into the set s; as a set keeps its elements
    // in an ordered sequence, the insert function has to compare the new
    // element (v) with the elements already stored in the set. This comparison
    // is performed by the compare function specified in the sets
    // template parameters, and as no such parameter is specified, the
    // operation< member function of vector<int> is used, which compares two
    // vectors according to their lexicographical order. (Perhaps this is
    // the part you didn't understand)
    s.insert(v);

// calls std::next_permutations, which rearranges the values in
// the myints array; once the original permutation (0, ...9) has been reached,
// next_permutation will return false and the loop ends
} while ( std::next_permutation(myints,myints+10) );

[第2修订]

[2nd AMENDMENT]

// loop over all elements of set s and assign each one in turn to a variable c
// (note: the auto keyword lets c assume the type that is the element type of
// set s, which is vector<int>)
for(auto c: s){

  // so now we have in c one of the vector<int> elements stored in s; in
  // the inner loop, we walk over all elements of that vector<int> and each
  // is in turn assigned to variable i
  for (auto i: c)
      // we output that element i and a space
      cout << i << " ";

  // after the inner loop has output all elements of c, we terminate
  // the output line
  cout << endl;
}

我希望能清楚说明嵌套循环的作用。

I hope that makes it clear what the nested loops do.

do {
    vector<int> v;  // create a new empty vector called v
    for (int i = 0; i < N; i++) // start a loop of N iterations
        v.push_back(myints[i]);  // add the integer at myints[i] to the vector v
    s.insert(v);  // on completion of the inner loop, add the vector v to the set s
  } while ( std::next_permutation(myints,myints+10) ); // repeat until this expression is false
</int>

set :: insert 有详细记录，例如参见std :: set :: insert [ ^ ]。在你的情况下插入一个（ 2 ）整数的向量到s。更不用说了。

set::insert is well documented, see, for instance "std::set::insert"[^]. In your case is inserting a vector of (2) integers into s. Nothing more, nothing less.

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