Chrono Time Counter Issue in C++ Error: No match for operator

Question

#include <iostream>
#include <chrono>
#include <unistd.h>
using namespace std;

void SieveOfEratosthenes (int n)
{
    bool prime[n+1], flag=true;
    int counter=0, ct=0;
    for (int i =0;i<n+1;i++){
        prime[i]=true;
    }
    for (int p=2; p*p<=n; p++)
    {
        if (prime[p]==true)
        {
            for (int i=p*2; i<=n; i += p)
            {
                prime[i] = false; 
            }
        }
    }
   // Print all prime numbers 
    for (int p=2; p<=n; p++)
    {
        if (flag)
        {
              auto begin=chrono::high_resolution_clock::now();
        }
        if (prime[p])
        {
              cout << p << " ";
              counter+=1;
              flag=false;
        }
        if(counter==10)
        {
              auto end=chrono::high_resolution_clock::now();
              auto duration=chrono::nanoseconds(end-begin);
              cout<<"Time elapsed:"<<duration.count();
              counter=0;
              flag=true;
        }
    }
    cout<<endl;
}


int main()
{
     int n;
     cout<<"Type a number:";
     cin>>n;
     cout<<endl<<"Following are the prime numbers smaller or equal to:"<<n<<endl;
     SieveOfEratosthenes(n);
     return 0;
}
// Driver Program to test above function

这是一种查找小于 n 的素数（从键盘输入）的算法，我想得到每 10 个素数的时间 found.I 出现运算符错误并且它在（结束-开始）。我无法理解错误在哪里 is.And 我已经尝试将函数写在一个单独的 .o 文件中，但仍然得到 nothing.Any 帮助，将不胜感激。！

enter image description here

Answer 1

begin 定义在 if 块内，无法被 end-begin 访问。一个简单的解决方法是将定义移出 for-loop:

      decltype(chrono::high_resolution_clock::now()) begin;                                                                                                                                    
     // Print all prime numbers                                                                                                                                                                
      for (int p=2; p<=n; p++)                                                                                                                                                                 
      {                                                                                                                                                                                        
          if (flag)                                                                                                                                                                            
          {                                                                                                                                                                                    
                begin=chrono::high_resolution_clock::now();                                                                                                                                    
          }
          // ...
      }

注意全局 using namespace std 通常不是一个好主意。由于 begin 与 std::begin 冲突，错误消息变得复杂。如果改为写 namespace chrono = std::chrono; using std::cout;，错误会变得更清楚：

test.cpp: In function ‘void SieveOfEratosthenes(int)’:
test.cpp:41:53: error: ‘begin’ was not declared in this scope; did you mean ‘std::begin’?
   41 |               auto duration=chrono::nanoseconds(end-begin);
      |                                                     ^~~~~
      |                                                     std::begin

Chrono Time Counter Issue in C++ Error: No match for operator

Chrono Time Counter Issue in C++ Error: No match for operator

c++

time

operator-keyword

chrono