Re^2: problem unless I use ".="

Actually, you get a syntax error there, but only because you're missing the if keyword :-)

But you're correct; you don't get a warning. And that's because you're not trying to use the value of $baseball{yankees}, you're only modifying it (ie. appending to it).

Note that you do get a warning if $baseball{mets} is undefined, as in:


use strict;
use warnings;

my %baseball = (
    orioles   =>   "baltimore",
    twins     =>   "minnesota"
);

if ($baseball{yankees} .= $baseball{mets})  {
    foreach $_ (keys %baseball) {
        print "$_ => $baseball{$_}\n";
    }
}

__END__
Use of uninitialized value in concatenation (.) or string at test.pl l
+ine 10.
[download]

s''(q.S:$/9=(T1';s;(..)(..);$..=substr+crypt($1,$2),2,3;eg;print$..$/

Comment on Re^2: problem unless I use ".=" Select or Download Code

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Re^3: problem unless I use ".=" by cgmd (Beadle) on May 13, 2007 at 15:55 UTC
Then what is being stated is that the line of code: `if ( $baseball{yankees} = $baseball{yankees} . $baseball{mets}` [download] is not identical to: `if ($baseball{yankees} .= $baseball{mets})` [download] Does this refute the original premise that: $a=$a <operator> $b; is same as $a <operator> = $b; (Taken from "Beginning Perl" by Lee & Cozens)?	[reply] [d/l] [select]
Re^4: problem unless I use ".=" by liverpole (Monsignor) on May 13, 2007 at 16:32 UTC
That's a good question, but you'll find it in the on-line documentation. From perlop (under Assignment Operators): `Assignment operators work as in C. That is, $a += 2; is equivalent to $a = $a + 2; although without duplicating any side effects that dereferencing the lvalue might trigger` [download] It's that last part, about avoiding dereferencing side effects, that comes into play here. When you do: `$baseball{yankees} = $baseball{yankees} . $baseball{mets};` [download] you are incurring a side effect of dereferencing `$baseball{yankees}`, namely testing for definedness, which you won't incur when you use the assignment operator: `$baseball{yankees} .= $baseball{mets}` [download] s''(q.S:$/9=(T1';s;(..)(..);$..=substr+crypt($1,$2),2,3;eg;print$..$/	[reply] [d/l] [select]
Re^5: problem unless I use ".=" by demerphq (Chancellor) on May 14, 2007 at 08:50 UTC
Its not clear to me what the "avoiding dereferencing side effects" part of the documentation means exactly, however I feel pretty confident in saying that it isnt relevent here. When you do `$hash{key}.=$str;` [download] there is no dereferencing going on. With `$hash->{key}.=$str;` [download] there is, however it happens before the .= operator is involved, so it doesnt explain the special casing behaviour of .= and other mutator operators. It turns out that the documentation of this behaviour is not to be found in perlop, but rather in perlsyn (in the section Declarations): `Operators such as ++, --, +=, -=, and .=, that operate on undefined le +ft values such as: my $a; $a++; are also always exempt from such warnings.` [download] Hope this clears things up for you. --- $world=~s/war/peace/g	[reply] [d/l] [select]