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Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead?

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On 03/30/2014 10:04 PM, rivkaumiller@gmail.com wrote:
> How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead?
> 
> For example, compact while loops are written like this in k&r
> 
> while ((c=getchar())!=NULL){
> 
>   // do certain actions
> 
> }
> 
> and I want to convert a stanza like
> 
> fscanf ( FpSource, "%d" , &input );
> while (input != EOF){
> 
>   // do certain actions
> 
>   fscanf ( FpSource, "%d" , &input );
> }
> 
> into the above form with getchar() so that there is a single instance of fscanf.
> 
> More than one alternative would be appreciated.

The following code has the exact same behavior as your loop, except that
it contains only one call to fscanf():

while(fscanf ( FpSource, "%d" , &input ), input != EOF)
{
   // do certain actions
}

However, that isn't a very good idea.
"The fscanf function returns the value of the macro EOF if an input
failure occurs before the first conversion (if any) has completed.
Otherwise, the function returns the number of input items assigned,
which can be fewer than provided for, or even zero, in the event of an
early matching failure." (7.21.6.2p16)

Your code pays no attention to the value returned by fscanf(), which
means it does nothing to deal with the possibility that anything might
go wrong when you call it. This could lead, among other possibilities,
to the value of 'input' being unequal to EOF, and never changing,
because fscanf() has reached the end of the file or has an input
failure, so you've written an infinite loop.

What your code does is check input != EOF, which will be false only
under two circumstances:
a) input has a value of EOF at the start of the loop, and the first call
to fscanf() fails, leaving that value unchanged.
b) fscanf() completes successfully, loading input with a value equal to
EOF that was read from FpSource.

In principle, this could be a legitimate way of marking the end of your
input data. However, it suggests to me the possibility that you're
confused about how fscanf() works, and thought that input == EOF would
be the way to determine whether or not the call to fscanf() succeeded.

int status;
while((status = fscanf(FpSource, "%d", &input)) != EOF)
{
   if(status != 1)
   {
      // Handle early matching failure
   }
   else
   {
      // do certain actions
   }
}
if(ferror(FpSource))
{
    // Handle I/O error
}
else
{
    // Handle end of file
}

How you handle each of those situations is up to you. End of File, in
particular, is a perfectly normal thing, except when it occurs at a time
when it shouldn't be occurring, such as after the tenth number in a file
that was supposed to contain 100 numbers.

If you choose to simplify the code by removing all of the handling
sections, you can drop the corresponding else blocks, and even the
second if(). However, do NOT remove the if(status != 1) test. If status
is not equal to 1, then the value of 'input' is indeterminate. That
means it can be either an unspecified integer, or a trap
representations. Any attempt to use a trap representation has undefined
behavior, but very few implementations support 'int' types with trap
representations. Much more important is the fact that the value of
'input' is unspecified. That doesn't mean that it's random - it could be
highly predictable if you know enough about that particular
implementation of the C standard library. However, it does mean that
there's no guarantees about which value it has. There's nothing useful
your program can portably do with that value, so it should not "do
certain actions" using that value.
-- 
James Kuyper

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How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? rivkaumiller@gmail.com - 2014-03-30 19:04 -0700
  Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? Richard Damon <Richard@Damon-Family.org> - 2014-03-30 22:31 -0400
    Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? rivkaumiller@gmail.com - 2014-03-30 19:43 -0700
      Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? Richard Damon <Richard@Damon-Family.org> - 2014-03-30 23:09 -0400
  Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? rivkaumiller@gmail.com - 2014-03-30 19:35 -0700
  Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? rivkaumiller@gmail.com - 2014-03-30 19:47 -0700
    Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? "BartC" <bc@freeuk.com> - 2014-03-31 11:53 +0100
      Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-03-31 17:50 +0000
        Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? Kaz Kylheku <kaz@kylheku.com> - 2014-03-31 18:55 +0000
          Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? "BartC" <bc@freeuk.com> - 2014-03-31 22:51 +0100
            Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? Kaz Kylheku <kaz@kylheku.com> - 2014-03-31 22:46 +0000
              Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? "BartC" <bc@freeuk.com> - 2014-04-01 00:03 +0100
              Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-03-31 20:33 -0400
                Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? "BartC" <bc@freeuk.com> - 2014-04-01 09:35 +0100
                Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? Keith Thompson <kst-u@mib.org> - 2014-04-01 07:35 -0700
                Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? "BartC" <bc@freeuk.com> - 2014-04-01 17:19 +0100
                Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? Keith Thompson <kst-u@mib.org> - 2014-04-01 10:14 -0700
                Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? "BartC" <bc@freeuk.com> - 2014-04-01 18:59 +0100
                Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? Öö Tiib <ootiib@hot.ee> - 2014-04-04 00:29 -0700
                Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? "BartC" <bc@freeuk.com> - 2014-04-04 12:35 +0100
                Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? Seungbeom Kim <musiphil@bawi.org> - 2014-04-08 11:48 -0700
                Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? Öö Tiib <ootiib@hot.ee> - 2014-04-08 15:52 -0700
                Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-02 18:38 +0000
                Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? Keith Thompson <kst-u@mib.org> - 2014-04-02 11:48 -0700
                Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-02 23:18 +0000
                Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? Kaz Kylheku <kaz@kylheku.com> - 2014-04-03 00:05 +0000
                Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? James Kuyper <jameskuyper@verizon.net> - 2014-04-02 21:12 -0400
                Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? Keith Thompson <kst-u@mib.org> - 2014-04-02 19:16 -0700
                Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-03 03:16 +0000
                Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? James Kuyper <jameskuyper@verizon.net> - 2014-04-02 15:07 -0400
                Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-04-01 23:00 +0100
                Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? Seungbeom Kim <musiphil@bawi.org> - 2014-04-08 11:56 -0700
            Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2014-04-01 02:14 +0300
            Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-01 04:52 +0000
              Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? "BartC" <bc@freeuk.com> - 2014-04-01 09:56 +0100
  Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-03-30 22:49 -0400
  Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? James Kuyper <jameskuyper@verizon.net> - 2014-03-30 22:58 -0400
    Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? rivkaumiller@gmail.com - 2014-03-31 09:10 -0700
      Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? James Kuyper <jameskuyper@verizon.net> - 2014-03-31 12:25 -0400
      Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? Keith Thompson <kst-u@mib.org> - 2014-03-31 10:56 -0700
  Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? James Kuyper <jameskuyper@verizon.net> - 2014-03-30 23:09 -0400
  Re: How to modify ((c=getchar())!=NULL) to use fscanf or scanf instead? Keith Thompson <kst-u@mib.org> - 2014-03-31 08:55 -0700

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