Error in program if a put a comma ", " in a name

#include <iostream>
#include <string>

using namespace std;

int main()
{
 string name[20];
 int grade[20];
 int index[20];
 int i, j;

 for(i=0;i<=20;i++)
 {
  cout << "Please enter name: ";
  cin >> name[i];
  cout << "Please enter grade: ";
  cin >> grade[i];
 }

 for(i=0;i<=20;i++)
 {
  index[i]=i;
 }

 for(i=0;i<=20;i++)
 {
  for(j=i+1;j<=20;j++)
  {
   int temp;
   
   if(name[index[i]] > name[index[j]])
   {
    temp = index[i];
    index[i] = index[j];
    index[j] = temp;
   }
  }
 }
 cout << endl;

 for(i=0;i<=20;i++)
 { 
  cout << name[index[i]] << "        "
   << grade[index[i]] << endl;
 }

 cin.ignore();
 cin.get();

 
}