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#1: space in variable referencing

Posted on 2008-01-12 00:03:06 by ciwei2103

Question from a newbie: how to properly use space
when reference to variables, array, hash etc.

please see the code below:
why the first line, second, and third line all output differiently.
thanks.
ciwei

========code below ======
#!/usr/bin/perl

my %ttys =();
open ( WHO, "who|") or die "can;t \n";

while ( <WHO> ){

( $user, $tty ) = split ;
push @{$ttys{ $user}} , $tty;

}

foreach $user ( sort keys %ttys ){

print "first line : $user => @{$ttys {$user}} \n";
print "second line: $user => @ {$ttys { $user}} \n";
print "third line: $user => @{$ttys{$user}} \n";

}

__OUTPUT__
first line : root =>
second line: root => @ { { root}}
third line: root => pts/1 pts/4 pts/6 pts/7 pts/8


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#2: Re: space in variable referencing

Posted on 2008-01-12 17:00:22 by Tom Phoenix

On Jan 11, 2008 3:03 PM, ciwei <ciwei2103@gmail.com> wrote:

> Question from a newbie: how to properly use space
> when reference to variables, array, hash etc.

Don't put any spaces inside a variable name, especially when interpolating it.

> print "first line : $user => @{$ttys {$user}} \n";
> print "second line: $user => @ {$ttys { $user}} \n";
> print "third line: $user => @{$ttys{$user}} \n";

Because of the spaces, the first two use $ttys (a scalar) while the
third uses %ttys (a hash). Although Perl is willing to silently ignore
some whitespace within a variable in ordinary code, quoted whitespace
is significant.

Hope this helps!

--Tom Phoenix
Stonehenge Perl Training

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#3: Re: space in variable referencing

Posted on 2008-01-12 22:32:23 by krahnj

ciwei wrote:
> Question from a newbie: how to properly use space
> when reference to variables, array, hash etc.
>
> please see the code below:
> why the first line, second, and third line all output differiently.
> thanks.
> ciwei
>
> ========code below ======
> #!/usr/bin/perl

The next two lines should be:

use warnings;
use strict;

> my %ttys =();
> open ( WHO, "who|") or die "can;t \n";

You should include the $! variable in the error message so you know why
open() failed:

open WHO, 'who |' or die "Cannot open pipe from 'who' $!";

> while ( <WHO> ){
>
> ( $user, $tty ) = split ;
> push @{$ttys{ $user}} , $tty;
>
> }

You should also close the pipe and verify that it closed correctly:

close WHO or warn $! ? "Error closing 'who' pipe: $!"
: "Exit status $? from 'who'";

> foreach $user ( sort keys %ttys ){
>
> print "first line : $user => @{$ttys {$user}} \n";
> print "second line: $user => @ {$ttys { $user}} \n";
> print "third line: $user => @{$ttys{$user}} \n";
>
> }
>
> __OUTPUT__
> first line : root =>
> second line: root => @ { { root}}
> third line: root => pts/1 pts/4 pts/6 pts/7 pts/8

If you had warnings and strict enabled you would have got this output
instead:

$ perl -e'
use warnings;
use strict;
my $user = "root";
my %ttys = ( $user => [ qw(pts/1 pts/4 pts/6 pts/7 pts/8) ] );
print "first line : $user => @{$ttys {$user}} \n";
print "second line: $user => @ {$ttys { $user}} \n";
print "third line: $user => @{$ttys{$user}} \n";
'
Global symbol "$ttys" requires explicit package name at -e line 6.
Global symbol "$ttys" requires explicit package name at -e line 7.
Execution of -e aborted due to compilation errors.



John
--
Perl isn't a toolbox, but a small machine shop where you
can special-order certain sorts of tools at low cost and
in short order. -- Larry Wall

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