feof fails

feof fails

[Christian Stalp]

Hello together,
I have a simple problem. I opened a file with fopen and read from the file 
with:
  while ( !feof( infile ) )
 {
   fgets( buffer, BUFFER_SIZE, infile );
   new_number = atoi ( buffer );
.....

The file is filled withe a stack of numbers, 7 charakters and each is 
terminated by an '\n'. 

My problem now is, that the last number is been read twice. What is the 
problem?

Gruss Christian


-- 
Christian Stalp

Institut für Medizinische Biometrie, Epidemiologie und Informatik (IMBEI)
Obere Zahlbacher Straße 69
55131 Mainz
Tel.: 06131/ 17-6852

E-Mail: stalp@imbei.uni-mainz.de
Internet: www.imbei.de
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Re: feof fails

[Pankaj]

hi,
Check whether u have put a \n at the end of last character..
u might have missed it.

Pankaj
On Wednesday 15 Sep 2004 3:09 pm, Christian Stalp wrote:
> Hello together,
> I have a simple problem. I opened a file with fopen and read from the file
> with:
>   while ( !feof( infile ) )
>  {
>    fgets( buffer, BUFFER_SIZE, infile );
>    new_number = atoi ( buffer );
> ......
>
> The file is filled withe a stack of numbers, 7 charakters and each is
> terminated by an '\n'.
>
> My problem now is, that the last number is been read twice. What is the
> problem?
>
> Gruss Christian

Re: feof fails

[Charlie Gordon]

"Christian Stalp" <stalp@imbei.uni-mainz.de> wrote in message
news:B0003441729@hermes.imsd.uni-mainz.de...
Hello together,
I have a simple problem. I opened a file with fopen and read from the file
with:
  while ( !feof( infile ) )
 {
   fgets( buffer, BUFFER_SIZE, infile );
   new_number = atoi ( buffer );
.....

The end of file condition is detected when trying to read from the file and
failing.
Thus feof(infile)  doesn't return non zero until after fgets fails at least
once.
You should test fgets return value to detect end of file.
The reason the last number is read twice is that your particular
implementation of fgets doesn't modify buffer upon end of file, it just
returns NULL, which you don't check before parsing the contents of the
buffer.

try this:

while (fgets(buffer, BUFFER_SIZE, infile)) {
    new_number = atoi(buffer);
    ...
}

simpler and better !

Chqrlie.

PS: wether the file is terminated with a newline or not is irrelevant to
this question.

RE: feof fails

[Huber, George K RDECOM CERDEC STCD SRI]

Christian wrote:

>I have a simple problem. I opened a file with fopen and read from the file 
>with:
>  while ( !feof( infile ) )
> {
>   fgets( buffer, BUFFER_SIZE, infile );
>   new_number = atoi ( buffer );
>.....

>The file is filled withe a stack of numbers, 7 charakters and each is 
>terminated by an '\n'. 

>My problem now is, that the last number is been read twice. What is the 
>problem?

The problem is not with feof, but rather how input works (this issue plagued
me for a number of years).  Lets assume that the last line of your file is
(where
^ represents the new-line `character', and the arrow is the current file
pointer):

    1234567^
   ^
   |

Now, when fgets process this line, it reads up to and including the the new
line
character, moving the file pointer thus:

    1234567^
            ^
            |

At this point, we have not hit the EOF `character' as it is the next
`character' to
be read in.  So we will go through the loop one more time.  This time the
call to 
fgets will fail due to hitting the end-of-file as the first character (and
thus not 
placing anything in the output buffer).  The contents the buffer are left
unmodified 
and will be processed by the remainder of the loop body.  Now the next time
through 
the loop, feof will return true and the loop will be exited - thus the
problem of 
reading the last line twice.  You can see this behaviour in the following
code 
snippet:

while(!feof(fp))
{
    //memset((void*)buffer, '\0', BUFFER_SIZE);
    fgets(buffer, BUFFER_SIZE, fp);
    buffer[strlen(buffer)-1] = '\0';              // eat new line
    printf("***%s***",buffer);
}

Now if the input file contains the lines (where ^ represents a new line):

123456^
234567^
345678^

The output is:

***123456***
***234567***
***345678***
***34567***

(the last line is truncated due to the logic to eat the new line, the
contencts 
 of buffer is shortened by one twice).

If the memset line is uncomment, the output is:

***123456***
***234567***
***345678***
******

We have not gained much, at least the last line will not be reprocessed
since we 
clear the buffer prior to each read.  But it does not really solve your
problem 
(yet).  The hint, is to look at the return value of fgets which is either
the 
buffer or NULL on error of if EOF is encountered.  With this, we can change
the 
logic of the above loop to:

while(!feof(fp))
{
    memset((void*)buffer, '\0', BUFFER_SIZE);
    if(NULL == fgets(buffer, BUFFER_SIZE, fp))
    {
	if(ferror(fp))
      {
	   fprintf(stderr, "fgets failed.");
      }

      break;
    }
    else
    {
       int nLen = strlen(buffer) - 1;
       if(buffer[nLen] == '\n') buffer[nLen] = '\0';
       printf("***%s***",buffer);
    }
}

Using the above code, the output is:

***123456***
***234567***
***345678***

Also, with the change in logic of handling new-lines, we can now handle
files of 
the sort:

123456^
234567^
345678

i.e. - files that do not end with a new line.  

Hope this helps,

George