cin.fail() not working?

[Rave]

Hey,

i have a small problem, what i want to do is, have the user input an integer (Ie: 1,45,65) or a real number (Ie. 3.4, 6.5), if the input is an integer program in suppose to output, "you entered an integer", if its a real number, "you entered a real number".

#include <iostream>
using namespace std;



int main()
{

    int i;



    cin >>i;

    if(cin.fail()==1)
    {
        cout <<"you entered a real number"<<endl;


    }
    else
    {
         cout <<"you entered a integer number"<<endl;

    }
cout<<cin.fail();


}

the cin>>i should fail if i enter 12.12, it should fail when it tries to read the ".", but its not happening. your help is appreciated.

[Duthomhas]

The cin.fail() should only be true if the user tried to enter something other than an integer.

Given the input "12abc" then you have a valid integer input: "12", leaving "abc" in the stream. (Whitespace is not significant.)

The way to check, then, is to peek() at the next character in the stream. If it is a period, then you have a real number. If it is not, then you have an integer.

So: (1) read an int (2) peek() to see what is next (3a) if it was a period read a float (or double) and add your int to it.


A more robust method is to read the entire line of user input, and validate its form yourself. Then you can read things like "42e7" and complain about things like "-3abc".

Here is a link about it. As noted in the link, you can ignore the fancy template stuff. The important thing is checking eof().
http://www.cplusplus.com/forum/beginner/13044/page1.html#msg62827

Remember, once you have the entire line of user's input, you can try to convert it to as many different things as you like. A simple method would be to simply >> int and >> float and check to see if their values match. If they do, look to see if the input contains a period or an 'E' or 'e' to decide between integer or real.

Hope this helps.

[Rave]

well in the exercise i am not suppose to use peek(),
so cin>>i ,where i is an integer, it won't fail when it encounters a "."?

[Rave]

ok then i tried re-writing::

#include <iostream>
using namespace std;



int main()
{

    int i;
    double j;



    cin >>i;
    if(cin.eof())
    {

        cout <<"you entred a integer number\n";


    }
    else
    {
           cout <<"you entred a real number"<<endl;

    }


return 0;

}

i still don't get it, if i enter an integer, it will fully be read in and nothing will be left in the stream, thus end of file will be encountered.

if i inter a real number, after the integer takes in there will be something left in the stream, thus it won't be the end of file.

[Duthomhas]

You have mixed the methods I gave you. The cin will not signal EOF if all you have done is pressed ENTER.

#include <iostream>
#include <sstream>
#include <string>
using namespace std;

int main()
  {
  double d;
  string s;

  getline( cin, s );
  istringstream iss( s );
  iss >> d;
  if (!iss.eof())  // this works because the end of the iss stream is the same as the end of the string s
    {
    cout << "Hey, that was not a number!\n";
    }
  else if (s.find_first_of( ".Ee" ) != string::npos)  // ok, so we got a valid number. Did it have anything that makes it a real number: -1.2e3 ?
    {
    cout << "That was a real number.\n";
    }
  else
    {
    cout << "That there was an integer.\n";
    }

  return 0;
  }

Good job for trying.

[Rave]

thanks, but is there anyway to check if something is still in the buffer?

ex, if i put 6.45,
.45 will be left in the buffer.