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open failure code

by RolandGunslinger (Curate)
on Nov 26, 2003 at 23:11 UTC ( #310407=perlquestion: print w/replies, xml ) Need Help??

RolandGunslinger has asked for the wisdom of the Perl Monks concerning the following question:

This is starting to bother me. I wrote up a perl script and what I had wanted to do was this 
$down="false";
open (FH, $log) || $down = "true";

[download]
I obviously got an error about trying to mod a scalar value I found a way to get what I wanted by calling subroutines instead; 
use vars qw/$down/;

$down="false";
open (FH, $log) || &woops($host);
if ($down eq "false") {
....send tail of log file...
} else {
....send "server is down"
}

sub woops {
  ...do some stuff....
 $down = "true";
}

[download]
I know this may look like a really stupid bit of perl code, I only code perl for my own enjoyment and I've only been doing it since September. What would you folks recommend? Thanks much.

Replies are listed 'Best First'.
Re: open failure code
by tachyon (Chancellor) on Nov 26, 2003 at 23:17 UTC

    Precedence and operator binding is the issue. But...why not just..... 

    if (open FH, $log) {
....send tail of log file...
} else {
....send "server is down. Perl say $!"
}

    [download]

    Note the error code (string reason) is set in $! if open, print, close, mkdir, unlink.....fail. Internal Perl functions return 1 if they succeed, 0 or undef if they fail. If you want to do the  || $scalar = 'blah' you can do this: 

     
blah() || do{ $scalar = 'blah'; more_stuff() }

    [download]

    cheers

    tachyon

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Re: open failure code
by cchampion (Curate) on Nov 26, 2003 at 23:25 UTC

    In addition to the useful advice you've got so far, I would like to point out that programming with flag variables is not always a good idea.

    See this discussion on flag variables for more material on this subject.

    HTH

[reply]
      Excellent advice. Using flag variables is a bad habit I've developed over the last 8 years. Thanks very much for your helpful response, and thanks everybody else. I feel like such an idiot, but at least I'm smarter than I was before.
[reply]
        Nope, an idiot would have asked "My Perl code doesn't work. Why?" (without anything else, such as code, examples, assumptions, et cetera).
[reply]
Re: open failure code
by Revelation (Deacon) on Nov 26, 2003 at 23:17 UTC
    It's all about operator precedence. For more information about precedence theory checkout perlop.
    Regarding your specific problem:
    The line  open ($fh,$log) || $down = 'moo'
    is evaluated as (open ($fh,$log) || $down) = 'moo';
    Obviously, open isn't an lvalue function; you can't assign it a value. 
    $log = "moooojlfdkafj.txt";
$down="false";
open (FH, $log) || ( $down = "true" );
print $down;

    [download]
    or 
    $log = "moooojlfdkafj.txt";
$down="false";
open (FH, $log) or $down = "true";
print $down;

    [download]
    work.

    Gyan Kapur
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Re: open failure code
by mpeppler (Vicar) on Nov 26, 2003 at 23:18 UTC
    You could do 
    open(FH, $log) || do { $down = 'true'; };

    [download]
    However that's really bad form, IMHO.

    I might do something like this: 

    if(open(FH, $log)) {
    send tail of log file
    close(FH);
} else {
    ... send server down...
}

    [download]
    ... and there are various other ways of coding this as well.

    Michael

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Re: open failure code
by Anonymous Monk on Nov 26, 2003 at 23:38 UTC
    What all these wise monks forgot to tell you is how they arrived at the answer :)
    perl -MO=Deparse,-p
$down="false";
open (FH, $log) || $down = "true";
__END__
Can't modify logical or (||) in scalar assignment at - line 2, near ""
+true";"
- had compilation errors.
($down = 'false');
((open(FH, $log) || $down) = 'true');

    [download]
    B::Deparse
[reply]
[d/l]

      You could also use this type of contruct 

      open(FH,"<".$log) ? print "opened\n" : print "unable to open $!\n";

      [download]
      it's not any better at all just making you aware of another option.

      Grygonos
[reply]
[d/l]
        Though this works, the ternary operator, like the grep and map operators, should not really be use in void context. 
        if (open(FH,"<".$log) {
    print "opened\n";
}
else {
print "unable to open $!\n";
}

        [download]
        is preferred.


        Who is Kayser Söze?
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[d/l]

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