Using stringstream and an int variable in C++ to verify that input is an int

Question

void get_english_input() {
    string input = " ";
    stringstream my_string(input);
    int ft;
    double in;

    while(true) {    
        cout << "Enter an integer value of feet." << endl;
        getline(cin, input);
        my_string << input;
        if(my_string >> ft)
            break;
        cout << "Invalid input! Please try again." << endl;
    }
    cout << "you entered " << ft << " as the int value for feet." << endl;
    /*while(true) {
        cout << "Enter a double value of inches." << endl;
        getline(cin, input);
        my_string << input;
            break;
    cout << "Invalid input! Please try again." << endl;
    }
    cout << "we are done entering english input" << endl;
    cout << "feet = " << ft << endl;
    cout << "inches = " << in << endl;*/
}

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This code is supposed to test if the input is an integer by trying to put the contents of my_string into ft. If I enter a letter instead of an integer I get the error message "Invalid input! Please try again," which is what is supposed to happen. The problem is, after I get that message once, I will get it for every input after that, even if the next input is valid.

---

Someone suggested that I should use std::cin.clear(); to clear the error-flags. I tried putting it before the getline() and it did not change the problem. Was I using this incorrectly?

Answer 1

You could reset the error state of my_string:

my_string.clear();
my_string.ignore( /* big number of choice */ );

But I think it would be easier here just to reinitialize it every time:

while(true) {    
    cout << "Enter an integer value of feet." << endl;
    getline(cin, input);
    stringstream my_string(input);

Answer 2

Check out lexical_cast from Boost ...

Answer 3

The brute-force solution is to dump your input into a std::string, then loop over the string and check if each character is between 0 and 9.

It's not the most elegant approach. But it's simple and stupid. :-)

bool isnum(char c)
{
   if(! ( c <= '9' && c >= '0'))
   {  
       return false;
   }
   return true;
}


bool has_int(std::string &s)
{
   for( int i = 0; i < s.length(); i++)
   {
      if( ! isnum(s[i])
      {
         return false;
      }
   }

   return true;
}

Answer 4

I think

mystring >> ft

will always evaluate to be true (maybe not if mystring was empty). The operation will still work whether or not mystring actually contains a number or not.

One idea is to

 size_t found=input.find_first_not_of("0123456789 ");
  if (found!=string::npos)
  {
   cout <<   "Invalid input! Please try again."
  }

Adapted from here.