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:

Quote:

---

Name: Mr. X

Phone: 0000000000

Continue [Y/N] ? : y

---

Second Run:

Quote:

---

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!

Quote:

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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?))

---

Quote:

---

Originally Posted by devarishi

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.

---

fgets() definitely can be used with files, but it can also be used with the standard input stream, stdin.

Quote:

---

Originally Posted by devarishi

Or could you please give an example?

---

If you need a little searching, or even just looking around, you might have spotted this recent thread where I posted a relevant link: prompting to enter a name in a loop skips the name enter part.

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.

Quote:

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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.

Quote:

---

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.

---

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'.

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!

Quote:

---

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.

Quote:

---

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');
}

---

---

You are certain that the number you entered did not exceed 9 characters (including separators and stuff)? Because that you indeed explain why.

Quote:

---

Originally Posted by devarishi

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

---

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)

---

Quote:

---

Originally Posted by devarishi

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.

---

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().

Hi,


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

Cheers!

Dev.

Quote:

---

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().

---

Quote:

---

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.

---

Unless the user typed a stray space at the end of the line, in which case this space gets consumed, not the newline, and we are back at the same problem again.

You need to consume characters all the way to the end of the line.

Code:

---

void flushRestOfLine(FILE *in)
{
    int ch;

    while((ch = fgetc(in)) != EOF && ch != '\n')
        ;
}

---

Hi all,

The easiest way is to add the line

scanf("%*c");

This will take a way the end of line in the buffer.

I'm not a Pro Programmer, but then I could suggest you to use a single line and get rid off your probrlem!!! :D

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);
        fflush(stdin); // flush the standard input stream after scanf

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

---

fflush(stdin) flushes the standard input stream... fflush() function is defined in <conio.h> and is supplied with all the compilers I've used!! :D

Quote:

---

Originally Posted by musaid

fflush(stdin) flushes the standard input stream.

---

No, it results in undefined behaviour. fflush() is only defined for output and update streams.

Quote:

---

Originally Posted by musaid

fflush() function is defined in <conio.h>

---

No, it is declared in <stdio.h>

Quote:

---

Originally Posted by musaid

and is supplied with all the compilers I've used!

---

That is probably true, since it is part of the C standard library.

Quote:

---

Originally Posted by musaid

I'm not a Pro Programmer, but then I could suggest you to use a single line and get rid off your probrlem!!! :D

fflush(stdin) flushes the standard input stream... fflush() function is defined in <conio.h> and is supplied with all the compilers I've used!! :D

---

before suggesting code you should read FAQ

about gets, flush sdtdin, void main...

Quote:

---

Originally Posted by vohkiem

The easiest way is to add the line

scanf("%*c");

This will take a way the end of line in the buffer.

---

what this line actually did.i put it and yes the problem was solved.can you explian it a bit.

also when i put this line instead of scanf

Code:

---

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

---

then also the same problem persisted.why?does getchar also leaves the any character in the buffer?
Thank You