C++ equivalent of fflush(stdin); ?

nigor · #1 (**permalink**) November 19th, 2003, 11:29 PM

This is a newbie program we had to write for our college test. The program asks for user to input a string, and then tells the user the number of characters in the string.

Here is the program

Code:

#include <iostream>
#include <string>
#include <conio.h>
#include <iomanip>
#include <stdio.h>

using namespace std;

struct text
{
    string input_text;
    int num_letters;
};

int main()
{
    text count;
    char responce = 0;

    do
    {
        cout << "Please enter a string: ";
        getline(cin, count.input_text);

        count.num_letters = count.input_text.length();

        cout << "The string " << count.input_text << " had "
            << count.num_letters << " characters." << endl << endl;


        cout << "Would you like to enter another string (y or n) ?: ";
                cin >> responce;
        cout << endl << endl;

    } while(tolower(responce) != 'n');

    getch();
    return 0;
}

The program works on the first run. But after the question "Do you want to enter another string ..." the program no longer works properly. The program does not give me a chance to enter another string if I answer 'y'. It's like the program automatically enters the string for me and gives me the answer. Here is what I am talking about:

http://server4.uploadit.org/files/191103-error.png

I first thought that a newline character was stuck somewhere in the input buffer. I rewrote the program as follows:

Code:

#include <iostream>
#include <string>
#include <conio.h>
#include <iomanip>
#include <stdio.h>

using namespace std;

struct text
{
    string input_text;
    int num_letters;
};

int main()
{
    text count;
    char responce = 0;

    do
    {
        cout << "Please enter a string: ";
        getline(cin, count.input_text);

        count.num_letters = count.input_text.length();

        cout << "The string " << count.input_text << " had "
            << count.num_letters << " characters." << endl << endl;


        cout << "Would you like to enter another string (y or n) ?: ";
        scanf("%c", &responce);
        fflush(stdin);
        cout << endl << endl;

    } while(tolower(responce) != 'n');

    getch();
    return 0;
}

================================================== =================
Works like a charm.

http://server4.uploadit.org/files/191103-no_error.png

'fflush(stdin);' did all the magic. Something did get stuck in the input buffer. My questions are: Can I replace fflush(stdin); or whatever else with something of C++ so that the program works? Why does this problem occur in the first place?

Thanks!

7stud · #2 (**permalink**) December 17th, 2003, 05:47 AM

Quote:

quote:I first thought that a newline character was stuck somewhere in the input buffer.

That's exactly right. This is a problem that everyone encounters at some point along the C++ learning curve. The problem occurs when you use getline() after you use cin>> to read input. When you answer yes to continue, you type this:

y\n

The '\n' is entered into the input stream when you hit return. cin>> is defined to skip all leading "whitespace characters" and stop reading input when it encounters a whitespace character. Whitespace characters are spaces, as well as carriage returns and tabs. cin>> actually stops before reading in a trailing whitespace character.

[edit]
The getline() function, unlike cin>>, does not skip leading whitespace characters, but similar to cin>> it stops when it encounters a whitespace character, but unlike with cin>> when it encounters a whitespace character, like a '\n', the '\n' is read in before getline() stops.
[/edit]

So, after reading the input:

y\n

cin>> stops at the '\n', and the '\n' is next up to be read, and your subsequent use of getline() doesn't cause the program to pause for input because there is still a '\n' to be read in, and the getline() function is happy to do that. That makes it seem like those lines weren't executed.

To solve the problem, all you have to do is use cin.ignore() after using cin>> and before using getline().

cin.ignore(); //will skip one character of the input.

Other forms of cin.ignore():

cin.ignore(10); // will skip ten characters(1 is the default if you don't specify a size)

cin.ignore(50, '\n'); // will skip 50 characters or until it encounters a '\n', whichever occurs first.

nigor · #3 (**permalink**) December 17th, 2003, 12:03 PM

Ahh, very nice explanation. Thanks!

sudobash · #4 (**permalink**) January 9th, 2008, 03:55 PM

this seems to be working for me right now in Ubuntu 7.10:
void flush()
{
char nextChar;
while( nextChar != '\n' && nextChar != EOF)
{nextChar = cin.get();}
}
may have to play around with where you call the function...