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Extracting an array from a module

by jonnyfolk (Vicar)
on Oct 13, 2007 at 07:21 UTC ( #644609=perlquestion: print w/ replies, xml ) Need Help??
jonnyfolk has asked for the wisdom of the Perl Monks concerning the following question:

I have written a module which contains some oft used subroutines.

One routine is simply extracting data from a file into an array. I have written the code in a couple of ways, both seem to do the job ok:

# from wslib.pm sub gencli { open FH, '<', $gencli or die "Can't open $gencli $!"; flock (FH, 1) or die "Can't lock $gencli for reading: $!"; my @gencli = <FH>; close FH; return \@gencli; } #from script, subroutine call my $gencli_scalar = gencli(); my @gencli = @{ $gencli_scalar };

The other way

#from wslib.pm sub gencli { open FH, '<', $gencli or die "Can't open $gencli $!"; flock (FH, 1) or die "Can't lock $gencli for reading: $!"; @_ = <FH>; close FH; } #from script &gencli; my @gencli = @_; @_=();

In the former example can I get the scalar out of the subroutine without creating a sacrificial $gencli? Is the second way 'wrong'?

thank you

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Re: Extracting an array from a module
by shmem (Canon) on Oct 13, 2007 at 09:05 UTC
    Retrieving @_ outside a sub indeed looks wrong to me. I wouldn't do it. I use @_ only inside subroutines. If you use a lexical filehandle, you can drop the close
    use Fcntl ':flock'; # import LOCK_* constants sub gencli { open my $fh, '<', $gencli or die "Can't open $gencli $!"; flock ($fh, LOCK_EX) or die "Can't lock $gencli for reading: $!"; wantarray ? <$fh> : [<$fh>]; } my $gencli_ref = gencli(); # both of these my @gencli_ary = gencli(); # work
    since it is done implicitly (and the lock removed) at subroutine return.

    --shmem

    _($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                                  /\_¯/(q    /
    ----------------------------  \__(m.====·.(_("always off the crowd"))."·
    ");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}
      I quite agree with the 'looks wrong' of the second example. I shall follow your advice re LOCK_* constants. Thank you.
Re: Extracting an array from a module
by GrandFather (Cardinal) on Oct 13, 2007 at 23:32 UTC

    What is it that actually bothers you with the first sub? The   return \@gencli; is about the cheapest way there is of returning an array from a sub - which one way or another is what you are doing.

    I would be much more worried about the potential confusion in the mind of a reader of the code caused by overloading gencli three ways - as a sub name, a scalar containing a file name, and as an array.

    Note too that using & to call a sub is generally frowned on except in a few rare special situations where the calling sub's argument list (@_) is to be passed into the called sub or you wish to ignore a sub's prototype.

    The two lines:

    my $gencli_scalar = gencli(); my @gencli = @{ $gencli_scalar };

    might be better rewritten:

    my @gencli = @{gencli()};

    (although I'd use a different identifier for the array). However, generally there is probably no need to actually copy the array at all, but simply dereference the array reference returned by the sub as required.


    Perl is environmentally friendly - it saves trees
      Au contraire, I was pretty happy with the first sub, it was the way I was extracting the information that I was uncertain about (rightly). Your explanations are spot on and you have shown me the 'better way' that I was looking for. Thank you.

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