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scalar my vs list my

by merlyn (Sage)
on Apr 27, 2001 at 01:11 UTC ( #75915=note: print w/replies, xml ) Need Help??

in reply to Re: my $var; vs. use constant VAR = '';
in thread my $var = ''; vs. use constant VAR => '';

my ($filename) = "/path/to/file"; my ($filemode) = "immolate"; my ($opmode) = "seek"; my ($filetype) = "image/gif";
As a matter of style (and occasionally necessity), I leave off the parens on those leftsides if the right side needs to be evaluated in a scalar context. Otherwise, I might wonder which one you meant of:
my ($filemode) = ("immolate"); # list context
my $filemode = "immolate"; # scalar context
And yes, it really does matter when you get into subroutine arg grabbing, like
my $count = @_; # get the length
or should that have been
my ($count) = @_; # get the first arg
So, make your left side and your right side agree, and I don't have to scratch my head wondering where you are headed.

-- Randal L. Schwartz, Perl hacker

Replies are listed 'Best First'.
Re: scalar my vs list my
by tadman (Prior) on Apr 27, 2001 at 01:47 UTC
    Letting go of brackets is hard if you come from the C school of bondage and discipline programming, where forgetting something minor, like brackets on a function call, is punishable by death ("Segmentation fault.").

    When declaring arguments, I have found that the code looks strange, inconsistent, and a little off-kilter when mixing and matching declaration styles, where 'off-kilter' is a euphemism for "looks broken":
    sub Foo { # Arguments: ARRAY <- ARRAY my ($ich, $bin) = @_; # Local variables: SCALAR,SCALAR,... my $x, $y, $z; # Local constants: ARRAY <- ARRAY my ($ein, $berliner) = qw [ jelly donut ]; # Single constant: SCALAR <- SCALAR my $go = 5; }
    Any statement like 'my $count = @_' can make me feel a little bit dizzy, if only because it seems like anything could happen there (i.e. concatenation with spaces, concatenation without, first element assignment, last element assignment, count of items, index of last item, etc.). Instead, I would rather explicitly specify what is intended as 'my ($count) = scalar (@_);', though rumor would have it that 'scalar' is deprecated.

    Anyway, maybe I'm just behind the times. I use brackets on int() and join(). I'll admit it! Maybe it's curable. As long as I'm careful to make sure that my assignments are array-safe (as I don't want to go to jail) then things work okay, in their own wacky way, and everything is bracketized and compartmentalized neatly.

    So, I can only suppose that you would prefer the following mutant variation, suggested for fun:
    my $filename = "/path/to/file", $filemode = "immolate", $opmode = "seek", $filetype = "image/gif";
      Hmm. Nope, you ended up with a few broken ones:
      # Local variables: SCALAR,SCALAR,... my $x, $y, $z; ... my $filename = "/path/to/file", $filemode = "immolate", $opmode = "seek", $filetype = "image/gif";
      Neither of those declare locally anything but the first var. You'd have to do something like:
      my $x, my $y, my $z;
      Just think of my as a really high precedence prefix operator that must appear before an lvalue, and you'll have the right mental model. But to figure the scalar-vs-arrayness of the rest of the expression, throw away all the my's first.

      My preference is one variable per my declaration, unless it's a bunch of scalars and an optional terminating array being fed from a list (like a subroutine argument grab).

      -- Randal L. Schwartz, Perl hacker

        At the risk of straying further, its ok to do this isn't it:
        my ($filename, $filemode, $opmode, $filetype) = ("/path/file", "immolate", "seek", "image/gif");
        Ignore me: I really should get into the habbit of reading the last paragraph of peoples' posts.

        $ perldoc perldoc

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