C: Do-While Loop Repeating Too Much!

问题

I have a small program that which is confusing me. I am trying using a loop to take input from user. In case input is wrong, it is repeated again but if it is right, it exits. The code snippet is:

void main()
{
    char user_status; // Checks User Status q = Quiz Master and p = Participant
    int valid_status = '0'; // Checks If User Status Is Valid Or Not. Used In Some Loops. 0 = Invalid, 1 = Invalid.
    printf("Welcome to General Knowledge Quiz Management System.\nThis application has been designed to help you conduct a quiz or test your GK.");
    do
    {
        user_status = '0';
        printf("\n\nPlease enter your role.\nQuiz Master = \'q\'\nParticipant = \'p\'\n");
        scanf("%c", &user_status);
        if (user_status == 'q'|| user_status == 'Q')
        {
            printf("Initializing Quiz Master Segment\n\n________________________________\n");
            initiate_qm();
            valid_status = '1';
        }
        else if (user_status == 'p' || user_status == 'P')
        {
            printf("Initializing Participant Segment");
            initiate_pa();
            valid_status = '1';
        }
    }
    while (valid_status != '1')
        printf("\nProgram Will Exit Now. Press Any Key To Return To Windows.");
    getch();
}

I am expecting this output:

Please Enter Your Role 
Quiz Master = 'q' 
Participant = 'p'

Till now, it works great. When I input q/Q/p/P, it works great. But when I input something wrong, it does not give required output.

For example, if I input "abc", I should get the above text again asking me to input q or p. But instead, I get this:

 Please Enter Your Role 
 Quiz Master = 'q' 
 Participant = 'p' 
 Please Enter Your Role 
 Quiz Master = 'q' 
 Participant = 'p'
 Please Enter Your Role 
 Quiz Master = 'q' 
 Participant = 'p'
 Please Enter Your Role 
 Quiz Master = 'q' 
 Participant = 'p'
 _ (I have to input here)

Now, why is it repeating 3 extra times. One interesting thing to note is that if I input something that is 2 characters long, it repeats 2 extra times and if I leave it blank(just hit return), it does not repeat extra times.

I have to use only C. I am using Visual C++ 2010 to compile.

Thanks.

回答1:

Because you have given scanf three characters to process. It removes first first character the first time it calls scanf getting 'a', but still has 'bc' left in the stdin buffer.

You need to check for leftover stuff in your buffer before you look for input again. And I'd avoid flushing the stdin buffer because it's undefined behavior. (http://www.gidnetwork.com/b-57.html)

You can read the remaining characters and discard them with

do{  
    scanf("%c", &user_status);  
}while(user_status!='\n'); //This discards all characters until you get to a newline

right after you read the character you want.

回答2:

You want

do
{

} while (condition);

As your forgot the semicolon, you get:

do
{
    ....
}

while(condition)
    do something else;

You could have noticed that just by auto-indenting your code in an editor like I did on your question.

Also when you do some scanf you should rather include the \n in the format specification.

回答3:

First of all, # include <stdio.h> and use getc(stdin) to get a character. It'll help you to prevent cursor from moving and putting unnecessary characters to console. Secondly, write the welcome message before the loop.

来源：https://stackoverflow.com/questions/5216330/c-do-while-loop-repeating-too-much

标签

loops

do-while