Thread: Frustrated with the instablility of scanf() and gets() functions

Well, when I run this program it works fine in the first run of it but from the second run it begins to annoy me (or the user):

Code:

#include <stdio.h>
#include <conio.h>
#include <ctype.h>
#include <process.h>

#define pause getch()
#define clear clrscr()

void main(void) {

	char UserName[25];
	char UserPhone[10];
	char UserChoice = ' ';

	clear;

	printf("\nName: ");
	gets(UserName);

	printf("\nPhone: ");
	gets(UserPhone);

	printf("\nContinue [Y/N] ? : ");
	scanf("%c", &UserChoice);

	if(toupper(UserChoice) == 'Y')
		main();
	
	printf("\nPress any key to exit...");
	pause;
	exit(0);
}

First Run:

Name: Mr. X

Phone: 0000000000

Continue [Y/N] ? : y

Second Run:

Name:
Phone: 0000000000

Continue [Y/N] ? : n

Press any key to exit...

Now, you see what is the thing that annoys? In the second run the entry field for the Name is skipped and we are at the field Phone.


Now before you suggest me anything, kindly, consider this revised version of the above program in which I have used flushall() function to avoid the annoyance:

Code:

#include <stdio.h>
#include <conio.h>
#include <ctype.h>
#include <process.h>

#define pause getch()
#define clear clrscr()

void main(void) {

	char UserName[25];
	char UserPhone[10];
	char UserChoice = ' ';

	clear;

	printf("\nName: ");
	flushall();
	gets(UserName);

	printf("\nPhone: ");
	flushall();
	gets(UserPhone);

	printf("\nContinue [Y/N] ? : ");
	flushall();
	scanf("%c", &UserChoice);

	if(toupper(UserChoice) == 'Y')
		main();
	
	printf("\nPress any key to exit...");
	pause;
	exit(0);
}

Well, I have used the flushall() function just before every gets() and scanf() because otherwise the annoyance would not be guaranted to be gone.

Now, the question is:

Do I have to user flushall() everytime I use a scanf() or gets() or any user input function?

Doesn't scanf() leave the newline character in the buffer? In which case I would have thought you would have to flush each time.

scanf leaves a newline behind, yes; hence you would only need to flush after the scanf.

(Of course the use of gets is an abomination, and flushall is presumably compiler-specific; you should fgets for the former and fgets for the latter (why use scanf when it causes problems?))

Instead of flushall, which is undefined for an input stream, use getchar(), right after the scanf() line, and all will be well with your newline problem.

Hi,

I just tried fgets instead of gets and as it is used for file handling it doesn't serve the purpose I have set in the given program.

Or could you please give an example?

Thanks!

Originally Posted by tabstop

scanf leaves a newline behind, yes; hence you would only need to flush after the scanf.

(Of course the use of gets is an abomination, and flushall is presumably compiler-specific; you should fgets for the former and fgets for the latter (why use scanf when it causes problems?))

Well, I have tried getchar() also and it does the same thing as flushall() does but with the requirement of pressing one more key after pressing Enter when a value has been entered. Besides, if the key pressed is other Enter then you have got to pull your head's hair again as you do in the original program given above.

Originally Posted by Adak

Instead of flushall, which is undefined for an input stream, use getchar(), right after the scanf() line, and all will be well with your newline problem.

Well, I have used flushall() after gets() and scanf() this time and it serves the purpose efficiently. However, I would like to eliminate the need of using flushall() at all.

Hi LaserLight,

Why didn't you choose to make your name Light of Knowledge? Haha!! Well, thanks for your support. Now I am leaving for the day as my shift has ended. So, I will try it later on if I can't do it right away.

Thanks a lot to all of you guys!

Originally Posted by laserlight

As tabstop mentioned, you only need to "flush" the input buffer once per iteration (and yes, iteration, not recursion: you do not need to call main recursively). I still am not clear where does flushall() come from (<conio.h>?), but I would do is write the function myself using the standard library.

Thus, I would propose this instead:

Code:

#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <conio.h>

char *getLine(char *str, size_t size, FILE *stream);
void clearScreen();
void ignoreUnreadInput();

int main(void) {
    int UserChoice;

    do {
        char UserName[25];
        char UserPhone[10];

        printf("\nName: ");
        getLine(UserName, sizeof(UserName), stdin);

        printf("\nPhone: ");
        getLine(UserPhone, sizeof(UserPhone), stdin);

        printf("\nContinue [Y/N] ? : ");
        UserChoice = getchar();

        ignoreUnreadInput();
        clearScreen();
    } while (toupper(UserChoice) == 'Y');

    printf("\nPress enter to exit...");
    getchar();
    return 0;
}

char *getLine(char *str, size_t size, FILE *stream)
{
    if ((str = fgets(str, size, stream)))
    {
        char *new_line = strchr(str, '\n');
        if (new_line)
        {
            *new_line = '\0';
        }
    }
    return str;
}

void clearScreen()
{
    clrscr();
}

void ignoreUnreadInput()
{
    int ch;
    while ((ch = getchar()) != EOF && ch != '\n');
}

I implemented a getLine() function to replace gets(). It uses fgets(), but additionally removes the newline that might be read into the string by fgets().

Instead of scanf() I used getchar() as suggested by tabstop.

Instead of just using one getchar() call as suggested by Adak, I implemented ignoreUnreadInput() similiar to how it was done in the tutorial I linked to, since a user could enter multiple characters instead of just 'y' or 'n'.

Okay, I compiled the program and the following warning message flagged:

Code:

if ((str = fgets(str, size, stream)))

Warning: TRYIT.CPP 37: Possibly incorrect assignment

However, I ran the program and here is the output:

First Run:

Name: Mr. X Man

Phone: 123456789

Continue [Y/N] ? : y

Screen gets cleared, as it should, and the following message is displayed:

Press enter to exit...


Second Run:

Name: Mr. X

Phone: 12345678

Continue [Y/N] ? : y

(Screen gets cleared):

[B]Name: Mr. X

....

then runs fine.

But... I changed the input a little- Phone number! The program went in an unexpected way when I supplied a nine digit number to it. But it worked fine when there were only eight digits.

But, thanks! There is something in your codes for me to learn from.

Thumps up to you! Have a nice time, friend!


Dev.

Originally Posted by laserlight

Thus, I would propose this instead:

Code:

#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <conio.h>

char *getLine(char *str, size_t size, FILE *stream);
void clearScreen();
void ignoreUnreadInput();

int main(void) {
    int UserChoice;

    do {
        char UserName[25];
        char UserPhone[10];

        printf("\nName: ");
        getLine(UserName, sizeof(UserName), stdin);

        printf("\nPhone: ");
        getLine(UserPhone, sizeof(UserPhone), stdin);

        printf("\nContinue [Y/N] ? : ");
        UserChoice = getchar();

        ignoreUnreadInput();
        clearScreen();
    } while (toupper(UserChoice) == 'Y');

    printf("\nPress enter to exit...");
    getchar();
    return 0;
}

char *getLine(char *str, size_t size, FILE *stream)
{
    if ((str = fgets(str, size, stream)))
    {
        char *new_line = strchr(str, '\n');
        if (new_line)
        {
            *new_line = '\0';
        }
    }
    return str;
}

void clearScreen()
{
    clrscr();
}

void ignoreUnreadInput()
{
    int ch;
    while ((ch = getchar()) != EOF && ch != '\n');
}

Hi,


I think I fixed it. Thanks a lot to you all!

Cheers!

Dev.

Originally Posted by laserlight

That warning is a false positive, since the assignment is correct. If you really want to avoid it, then change it to:

Code:

str = fgets(str, size, stream);
if (str)

Ah yes, I did not consider that input can be left in the buffer if the input provided is too long. You could just use ignoreUnreadInput() after each call to getLine().