Better than scanf Post: 302407775

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Top Forums Programming Better than scanf Post 302407775 by Corona688 on Friday 26th of March 2010 11:22:49 AM

03-26-2010

Registered User

scanf may be easier(or at least more obvious) but will cause strange problems -- as you've already seen. And there's others too. Try this:

Code:

#include <stdio.h>

int main(void)
{
        int fail=1, n;
        while(fail)
        {
                fail=0;

                printf("\nPlease enter a number: ");
                if(scanf("%d", &n) != 1)
                        fail=1;
        }

        printf("You input %d\n", n);
        return(0);
}

When you enter a number, it works fine. When you enter qwertyuiop, it goes into an infinite loop. This is because scanf only reads one single character, finds that it's not a number, gives up, and puts it back so the data you get the next loop is the same you had last time. You have to fflush(stdin); to get rid of the backed-up garbage.

If you're inputting whole lines, scanf is the wrong function to use.

Often I use fgets and scanf together to avoid that buffer-problem, like this:

Code:

char buf[512];
int n;
// Read in an entire line
fgets(buf,512,stdin);
// scan the string for the data you want
sscanf(buf, "%d", &n);

Note the extra s, that's intentional. sscanf works just like scanf except it scans strings. This way, if scanf has an error, the data won't remain stuck in the buffer until flushed since it's already been read.

Last edited by Corona688; 03-26-2010 at 12:40 PM..

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LEARN ABOUT OPENSOLARIS

fgets

gets(3C)						   Standard C Library Functions 						  gets(3C)

NAME

       gets, fgets - get a string from a stream

SYNOPSIS

       #include <stdio.h>

       char *gets(char *s);

       char *fgets(char *s, int n, FILE *stream);

DESCRIPTION

       The gets() function reads bytes from the standard input stream (see Intro(3)), stdin, into the array pointed to by s, until a newline char-
       acter is read or an end-of-file condition is encountered. The newline character is discarded and the string is terminated with a null byte.

       If the length of an input line exceeds the size of s, indeterminate behavior may result. For this reason, it is strongly  recommended  that
       gets() be avoided in favor of fgets().

       The  fgets()  function reads bytes from the stream into the array pointed to by s, until n-1 bytes are read, or a newline character is read
       and transferred to s, or an end-of-file condition is encountered. The string is then terminated with a null byte.

       The fgets() and gets() functions may mark the st_atime field of the file associated with stream for update.  The  st_atime  field  will	be
       marked  for  update  by	the  first  successful	execution  of fgetc(3C), fgets(), fread(3C), fscanf(3C), getc(3C), getchar(3C), gets(), or
       scanf(3C) using stream that returns data not supplied by a prior call to ungetc(3C) or ungetwc(3C).

RETURN VALUES

       If end-of-file is encountered and no bytes have been read, no bytes are transferred to s and a null pointer is returned. For  standard-con-
       forming	(see  standards(5))  applications,  if	the  end-of-file indicator for the stream is set, no bytes are transferred to s and a null
       pointer is returned whether or not the stream is at end-of-file. If a read error occurs, such as trying to use these functions  on  a  file
       that  has  not been opened for reading, a null pointer is returned and the error indicator for the stream is set. If end-of-file is encoun-
       tered, the EOF indicator for the stream is set.	Otherwise s is returned.

ERRORS

       Refer to fgetc(3C).

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Standard			   |
       +-----------------------------+-----------------------------+
       |MT-Level		     |MT-Safe			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       lseek(2), read(2), ferror(3C), fgetc(3C), fgetwc(3C), fopen(3C), fread(3C), getchar(3C),  scanf(3C),  stdio(3C),  ungetc(3C),  ungetwc(3C),
       attributes(5), standards(5)

SunOS 5.11							    15 Oct 2003 							  gets(3C)