Hi all,
I've got a problem with a simple menu. After a question that I print to the terminal, I want to enter an answer. What actually happens is that the while loop executes twice before I can enter my answer (y or n). The first execution goes to the default switch, and the second execution allows me to enter my answer. Why does this happen, and how can I get round this problem?

thanks
Spiros

PS I'm not posting all te code as its getting near 500 lines...

Code:

        while(1) {
                printf("...question... (yes/no) ");
                ch=toupper(getchar());
                switch(ch) {
                        case('Y'): 
                                operation1;
                                break;
                        case('N'):
                                operation2
                                break;
                        default: printf("Incorrect choice, try again\n");
                }
                if (ch=='Y' || ch=='N') {
                        break;
                }
        }

**dwks** · 09-08-2006

You need to clear the input buffer; the second time around the getchar() reads the '\n' that the user entered the first time as "Y\n" or something.

http://faq.cprogramming.com/cgi-bin/...&id=1043284392

This ought to do:

Code:

int ch;
while ((ch = getchar()) != '\n' && ch != EOF);

**s_siouris** · 09-08-2006

thank you. I did have a look at this before you posted, but I cannot understand what is happening in the which statement. I don't see any operation being performed, just a conditional, so how is the buffer cleared?

I did what you suggested and it works now, but I don't understand what's going on. Can you please provide a brief explanation?

thank you

Spiros

**dwks** · 09-08-2006

Code:

int c;
while ((ch = getchar()) != '\n' && ch != EOF);

is the same as

Code:

int c;
for(;;) {
    ch = getchar();  /* get a character from the buffer */
    if(ch == '\n' || ch == EOF) {  /* if it's the last character in the line */
        break;  /* stop reading in characters from the input buffer */
    }
}

Better?

**s_siouris** · 09-08-2006

I think I understand now, so thanks. I don't want to be a pain, but I've got another question. Why is:

Code:

        while ((ch = getchar()) != '\n' && ch != EOF);
        do {
                printf("Ring elements? (yes/no) ");
                ch=toupper(getchar());
                switch(ch) {
                        case('Y'):
                                for (i=11; i<=15; i++) {
                                        printf("Group %s: ", group_name[i]);
                                        scanf("%i", &groups[i]);
                                }
                                break;
                        case('N'):
                                break;
                        default: printf("Incorrect choice, try again\n");
                }
                if (ch=='Y' || ch=='N') {
                        break;
                }
        } while (1);

working, and

Code:

        do {
                printf("Ring elements? (yes/no) ");
                while ((ch = toupper(getchar())) != '\n' && ch != EOF);
                switch(ch) {
                        case('Y'):
                                for (i=11; i<=15; i++) {
                                        printf("Group %s: ", group_name[i]);
                                        scanf("%i", &groups[i]);
                                }
                                break;
                        case('N'):
                                break;
                        default: printf("Incorrect choice, try again\n");
                }
                if (ch=='Y' || ch=='N') {
                        break;
                }
        } while (1);

isn't (ie still printing twice)?

**dwks** · 09-08-2006

Neither of them should work; the first one has the input-clearer outside the loop, so unless you have input-reading code before the loop it won't do what you want; and the seond is an infinite/one-time loop because you got rid of the getchar(). [edit] Or rather, ch will always contain '\n' or EOF. Which is still infinite. [/edit]

This should work:

Code:

int c;
        while(1) {
                printf("...question... (yes/no) ");
                ch=toupper(getchar());
                while ((c = getchar()) != '\n' && c != EOF);
                switch(ch) {
                        case('Y'): 
                                operation1;
                                break;
                        case('N'):
                                operation2
                                break;
                        default: printf("Incorrect choice, try again\n");
                }
                if (ch=='Y' || ch=='N') {
                        break;
                }
        }

See, the thing is that you have to use a different variable in the input-clearer than the rest of the loop. The variable used in the input-clearer should be unique and not used elsewhere. If it is, it will always contain '\n' or EOF, and never 'Y' or whatever the user entered.

**s_siouris** · 09-08-2006

thank you very much for your time, now I understand very well what's going on.

Spiros

**jaguar23** · 09-08-2006

Hey Spiros,

Nothing wrong with your original code. It works. I just compiled it and executed with Turbo C.
Cause for your problem may be a previous input. It probably gets the enter of your previous input.

Jag

**dwks** · 09-08-2006

Yes, that's true, there may be a '\n' already sitting in the buffer; but what happens if you enter "hmm... yes" with the original code? First of all, you get lots of "incorrect choise"es, second lots of stuff is left in the buffer (which also happens with the updated code).

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