c + +输入两次直到按Enter键

Question

I have this code that adds up doubles entered by the user and stops when the user enters a negative number. I want to change it so that it will stop when the user presses the ENTER key and doesn't enter a number, is this possible? And if so, how?

double sum = 0, n;

cout << endl;

do
{
    cout << "Enter an amount <negative to quit>: ";
    cin >> n;

    if(n >= 0)
    {
        sum += n;
    }
}while(n >= 0);

return sum;

Answer 1

Use getline() as below:

#include <iostream>
#include <string>
using namespace std;
int main()
{
    string s;
    double sum=0.0;
    while (1)
    {
        cout<<"Enter Number:";
        getline(cin, s);
        if (s.empty())
        {
            cout <<"Sum is: " <<sum;
            return 0;
        }
        else
        {
          sum=sum+ stod( s );
        }
    }    
    return 0;
}

An example output:

  Enter Number:89
  Enter Number:89.9
  Enter Number:
  Sum is: 178.9 

Answer 2

I usually never do >= because this can get messy especially when you need to find the median or mode. For the code above this is how I would go about doing it.

  double sum =0;
  double n =0;

  while(cin >> n) // this will keep going as long as you either enter a letter or just enter
  {
      sum += n; // this will take any input that is good 

      if(!cin.good()) // this will break if anything but numbers are entered as long as you enter anything other then enter or a number
        break;

  }