Question

我正在尝试按对中的第一个值对对向量进行排序。我已经尝试按照已经发布的关于此问题的其他问题的答案的建议，但由于某种原因我无法对矢量进行排序。我尝试过使用std :: sort和std :: stable_sort无济于事。代码编译良好，运行没有任何投诉，但数组没有排序。我的示例代码如下：

#include <iostream>
#include <vector>
#include <algorithm>

bool compare(const std::pair<int, int>&i, const std::pair<int, int>&j){
  return i.first < j.first;
}

int main(){
  std::vector<std::pair<int, char>> vec;
  vec.reserve(10); // reserve space for 10 elements
  int i;
  std::string letters = "abcdefghij";
  int randNum;

  for(i=0; i<10; i++){
    randNum = std::rand()%(10-0 + 1); // generate random numbers between 0 and 10
    vec[i].first = randNum; // assign random integer to first element of pair
    vec[i].second = letters[i]; // assign letter to second element of pair
  }

  for(i=0; i<10; i++){    // print out unsorted array
    std::cout << vec[i].first << " " << vec[i].second << "\n";
  }
  std::cout << "\n";
  std::sort(vec.begin(), vec.end(), compare);

  for(i=0; i<10; i++){   // print out sorted array
    std::cout << vec[i].first << " " << vec[i].second << "\n";
  }

  return 1;
}

结果输出如下：

10 a
1 b
0 c
6 d
8 e
3 f
2 g
0 h
9 i
4 j

10 a
1 b
0 c
6 d
8 e
3 f
2 g
0 h
9 i
4 j

Answer 1

你实际上没有调整矢量大小，你的矢量是空的。

而不是：

vec.reserve(10);

使用：

vec.resize(10);

如果您使用了基于范围的for循环，这将是显而易见的：

// This won't print anything out at all.
for (const auto &it : vec) {
  std::cout << it.first << " " << it.second << "\n";
}

Answer 2

您可以使用索引到push_back替换初始化向量的for循环。

public class Number0to9 {    
  public static String input;

  public static void main(String[] args) {        
    tryagain: while (true) {
      System.out.print("Type a number between 0 and 9 : ");

      Scanner scan = new Scanner(System.in);          
      int x = scan.nextInt();
      int output = x;
      input = "You entered number: ";

      switch (x) {
        case 0:
          input += output;
          break tryagain;
        case 1:
          input += output;
          break tryagain;
        case 2:
          input += output;
          break tryagain;
        case 3: 
          input += output;
          break tryagain;
        case 4:
          input += output;
          break tryagain;
        case 5: 
          input += output;
          break tryagain;
        case 6: 
          input += output;
          break tryagain;
        case 7:
          input += output;
          break tryagain;
        case 8:
          input += output;
          break tryagain;
        case 9:
          input += output;
          break tryagain;
        default:
          System.out.println("Your number was bigger than 9");
          break;
      }   
    }        
  } 
}

Answer 3

如果您开始学习C ++，请学习C ++ 11的功能。它将使您的代码更容易编写，并减少错误。这是编写程序，IMO的更好方法，它可以解决您遇到的错误。另外，请注意，main表示返回0表示“ok”，1表示“not ok”

#include <iostream>
#include <vector>
#include <algorithm>

int main() {
  std::vector<std::pair<int, char>> vec;
  std::string letters = "abcdefghij";

  for(auto l: letters) {
    int randNum = std::rand()%(10-0 + 1); // generate random numbers between 0 and 10
    vec.push_back({randNum, l}); // use constructor to create the pair
                                 // pushback figures it is a pair that must be 
                                 // inserted and calls corresponding constructor (pair 
                                 // in this case)
  } 

  for(auto p: vec) {
    std::cout << p.first << " " << p.second << "\n";
  }

  std::cout << "\n";

  std::sort(vec.begin(), vec.end(), [](auto a, auto b) {  // use lambda. Cleaner and easier to read
     return a.first < b.first;
  }); 

  for(auto p: vec) {
    std::cout << p.first << " " << p.second << "\n";
  }

  return 0;
}

如何通过第一个元素对对矢量进行排序？

3 个答案: