CODEWITHSUNDEEP
c
L.I.B L.I.B
L.I.B L.I.B

Reputation: 77

"Enter a character to quit" program in C

I try to write a program in C using the loop that repeated until a specific character is inputted from the keyboard. Here's my code:

#include <stdio.h>
main ()
{
    char option;
    do
        {
            printf("Enter q to quit: ");
            option = getchar ();
        }
    while (option != 'q');
}

I've also tried with the scanf () but the result always the same. Here the output after I tried to test the program:

Enter q to quit: 3
Enter q to quit: Enter q to quit: 2
Enter q to quit: Enter q to quit: 1
Enter q to quit: Enter q to quit: q

Can anyone explain to me why the "Enter q to quit : " always appear twice and how can I fix it?

Upvotes: 1

Views: 8569

Answers (5)

Samidamaru
Samidamaru

Reputation: 1081

When you enter q, you press q followed by enter (a new line character as far as C is concerned which is \n).

So when your loop returns to the beginning, '\n' is still in your input buffer and getch() automatically reads this and checks whether that equals q before returning to the beginning of your loop again (hence your text looks like it's printed twice).

Try using fgets like this: fgets (option , 4 , stdin)

You have to make sure you have a character array big enough to hold this though so you should define char option [3]; to hold 'q', the newline character '\n' and the termination character '\0';

fgets is quite a nice solution because it will only store up the the second argument worth of characters and throw away the rest. This means 1) you don't overflow your variables/arrays and 2) you don't have junk left in the input buffer :)

Upvotes: 3

jkd
jkd

Reputation: 90

getchar() function first reads from the buffer.If buffer is empty then and then it will read from standard input(i.e.screen).

Upvotes: 0

J. Nutt
J. Nutt

Reputation: 71

"Enter q to quit: " appears twice because your input buffer still has the new line character in it when it runs a second time.

Fix:

#include <stdio.h>
int main ()
{
    char option;
    do
    {
        printf("Enter q to quit: ");
        option = getchar ();
        while(getchar() != '\n'); //Enter this line here.
    }
    while (option != 'q');
}

Upvotes: 4

Lundin
Lundin

Reputation: 215330

You get it printed twice because when you hit enter, a line feed character \n is appended to stdin.

You can discard that line feed character by adding an extra getchar:

do
{
    printf("Enter q to quit: ");
    option = getchar();
    getchar(); // discard line feed
}while (option != 'q');

Upvotes: 2

David Schwartz
David Schwartz

Reputation: 182883

If you hit two keys, you'll read two characters. If you want to read characters, call getchar. If you want to read lines, call some function that reads lines.

Upvotes: 1

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