Path: utzoo!utgpu!attcan!uunet!husc6!uwvax!oddjob!ncar!ames!oliveb!sun!thetone!swilson
From: swilson%thetone@Sun.COM (Scott Wilson)
Newsgroups: comp.sys.mac.programmer
Subject: Re: LSC 3.0 scanf deletes
Keywords: delete, scanf
Message-ID: <62725@sun.uucp>
Date: 4 Aug 88 00:58:32 GMT
References: <2559@pt.cs.cmu.edu> <587@helios.ee.lbl.gov> <5794@haddock.ISC.COM>
Sender: news@sun.uucp
Reply-To: swilson@sun.UUCP (Scott Wilson)
Organization: Sun Microsystems, Mountain View
Lines: 64

scanf is not a useless function!  While I believe that to approach 100%
bullet-proof i/o you have to do everything yourself one character at a
time, scanf is still useful.  The problem is that most people don't
know how to use scanf.  Consider this example:

#include <stdio.h>

main()
{
    int i;

    do {
        printf("Enter a number between 1 and 100:  ");
        scanf("%d", &i);
    } while (i < 1 || i > 100);
    printf("You typed: %d\n", i);
}

This is a common problem when using scanf.  The user types a letter,
scanf pushes it back and the program goes into an infinite loop
(assuming the garbage value of an uninitialzed i is out of range).  The
solution:  don't use scanf?  Not necessarily.  Scanf returns a value
to help out in this situation.  From the Sun0S 4.0 scanf man page:

     scanf() returns  the  number  of  successfully  matched  and
     assigned  input  items; this number can be zero in the event
     of an early conflict between an input character and the con-
     trol  string.   The  constant  EOF  is  returned upon end of
     input. Note: this is different from 0, which means  that  no
     conversion  was  done;  if  conversion  was intended, it was
     frustrated by an inappropriate character in the input.
				  
Now I know that I have seen some scanfs that return something different
like the number of characters read.  ANSI should clear this up (using the
above I think).  My LSC manual is at home so I don't know what that
scanf returns.

So we change the example to this:

#include <stdio.h>

main()
{
    int i;

    do {
        printf("Enter a number between 1 and 100:  ");
        if (scanf("%d", &i) != 1) {
            while ((i = getchar()) != '\n')  /* skip bad stuff */
                ;
            i = 0;     /* or something else out of range */
        }
    } while (i < 1 || i > 100);
    printf("You typed: %d\n", i);
}

Note that the program does not go into an infinite loop because the
offending character that caused scanf not to match a decimal integer
has been removed from the input stream.

--
Scott Wilson		arpa: swilson@sun.com		"Why do dogs lick
Sun Microsystems	uucp: ...!sun!swilson		their balls?  Because
Mt. View, CA						they can!"