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From: rhorizon74 on 31 Jan 2006 11:24
In the code below the print val statement actually creates an key 'ABC'
in the hash if it does not exist ( i wanted to print the value of the
0th element in an array , the reference to which is stored in a hash
with key 'ABC').

Why should a print create a new key, shouldnt it just print undefined,
and not create any new key.

Thank you
Code:

use strict;
use Data::Dumper;
&main();

sub main
{
my %a;
my $aref = \%a;

print "val: @{$aref->{ABC}}[0]\n";
print Dumper($aref)."\n";
}


Output:
val:
$VAR1 = {
'ABC' => []
};

From: A. Sinan Unur on 31 Jan 2006 11:29
"rhorizon74" <rhorizon74(a)yahoo.com> wrote in
news:1138724646.585398.120570(a)g43g2000cwa.googlegroups.com:

> In the code below the print val statement actually creates an key
> 'ABC' in the hash if it does not exist ( i wanted to print the value
> of the 0th element in an array , the reference to which is stored in a
> hash with key 'ABC').

That's called autovivification (sp?) You can read Uri Guttman's article:

http://www.sysarch.com/Perl/tutorials/autoviv.html

If you want to avoid it, you need to use exists, and print only when the
said element does exist.

if( exists $hash{$key} ) {
print "$hash{$key}\n";
}

> Why should a print create a new key, shouldnt it just print undefined,
> and not create any new key.

Why do you think so?

> use strict;

use warnings;

missing.

> use Data::Dumper;
> &main();

Do you know what '&' does here? If not, then don't use it.

See

perldoc perlsub


Sinan
--
A. Sinan Unur <1usa(a)llenroc.ude.invalid>
(reverse each component and remove .invalid for email address)

comp.lang.perl.misc guidelines on the WWW:
http://mail.augustmail.com/~tadmc/clpmisc/clpmisc_guidelines.html
From: it_says_BALLS_on_your forehead on 31 Jan 2006 11:30

rhorizon74 wrote:
> In the code below the print val statement actually creates an key 'ABC'
> in the hash if it does not exist ( i wanted to print the value of the
> 0th element in an array , the reference to which is stored in a hash
> with key 'ABC').
>
> Why should a print create a new key, shouldnt it just print undefined,
> and not create any new key.
>
> Thank you
> Code:
>
> use strict;
> use Data::Dumper;
> &main();
>
> sub main
> {
> my %a;
> my $aref = \%a;
>
> print "val: @{$aref->{ABC}}[0]\n";
> print Dumper($aref)."\n";
> }
>
>
> Output:
> val:
> $VAR1 = {
> 'ABC' => []
> };

lookup autovivification.

From: rhorizon74 on 31 Jan 2006 11:38
Thanks for the help.

That explains what is happening (i had used the exists approach to work
around).
But it does seem bogus that for a simple print statement i need to
check each and every level. I havent seen too many languages do
something like this (i may be wrong).
But a print statement shouldnt really be creating any data.


Thanks for the help though.

From: A. Sinan Unur on 31 Jan 2006 11:46
"rhorizon74" <rhorizon74(a)yahoo.com> wrote in news:1138725497.627908.102130
@g44g2000cwa.googlegroups.com:

> But it does seem bogus that for a simple print statement i need to
> check each and every level. I havent seen too many languages do
> something like this

Please feel free to use one of the languages that behave in the way you
think they should.

Sinan
--
A. Sinan Unur <1usa(a)llenroc.ude.invalid>
(reverse each component and remove .invalid for email address)

comp.lang.perl.misc guidelines on the WWW:
http://mail.augustmail.com/~tadmc/clpmisc/clpmisc_guidelines.html

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