CODEWITHSUNDEEP
cscanf
WhiteEyeTree
WhiteEyeTree

Reputation: 73

C loop prints string twice? (using scanf("%c"))

Sorry for the probably dumb question, but i wanted to practice loops a bit and came out with this idea.Basically it ask you to enter or not in a loop and when you're in, it ask you for something to do.The problem is that just after i enter the loop it prints two times the printf string before passing to the scanf one and waiting for an input. I can't figure it out. All help is welcome! Here's the code:

#include <stdio.h>

int main() 
{
    char check = 'a';
    char int_check = 'a';
    int b = 0;
    printf("want to go in? [y or n]\n");
    scanf("%c",&check);
    if ( check == 'y') {
        while (1){
            printf("Waiting: \n");
            scanf("%c",&int_check);
            if ( int_check == 'q'){
                printf("You're out, bye!\n");
                break;
            };
        };
    } else if ( check == 'n'){
        printf("You're not in, see ya!\n");
    }else {
        printf("Please, type 'y' or 'n'\n");
    };
    return 0;
}

Upvotes: 3

Views: 1672

Answers (2)

gcbenison
gcbenison

Reputation: 11963

You can change the program to respond to keyboard immediately, i.e. without waiting for the user to press enter. It requires changing the properties of the input terminal, and is generally messier and less portable-seeming than line-oriented input. This page describes how to do it, and here is the code modified to work that way:

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <termios.h>

struct termios saved_settings;

void
reset_term_mode(void)
{
  tcsetattr (STDIN_FILENO, TCSANOW, &saved_settings);
}

int main() 
{
    tcgetattr(STDIN_FILENO, &saved_settings);
    atexit(reset_term_mode);

    struct termios term_settings;

    tcgetattr(STDIN_FILENO, &term_settings);
    term_settings.c_lflag &= ~(ICANON|ECHO);
    term_settings.c_cc[VMIN] = 1;
    term_settings.c_cc[VTIME] = 0;
    tcsetattr(STDIN_FILENO, TCSAFLUSH, &term_settings);

    char check = 'a';
    char int_check = 'a';
    int b = 0;
    printf("want to go in? [y or n]\n");
    scanf("%c",&check);
    if ( check == 'y') {
        while (1){
            printf("Waiting: \n");
            scanf("%c", &int_check);
            if ( int_check == 'q'){
                printf("You're out, bye!\n");
                break;
            };
        };
    } else if ( check == 'n'){
        printf("You're not in, see ya!\n");
    }else {
        printf("Please, type 'y' or 'n'\n");
    };
    return 0;
}

Upvotes: 2

Bill Lynch
Bill Lynch

Reputation: 81936

If you input onto a terminal the following:

x

The first loop will see an x

The second loop will see a newline character.

The easiest way to work around this is to use sscanf and getline.

Upvotes: 4

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