I don't understand this grep

gwhite has asked for the wisdom of the Perl Monks concerning the following question:

I had a consultant do some a work a couple of years ago and am now trying to solve a problem and I can't figure what this is doing:

...in program...
$MSG->trace( "my message");

...in module..
sub trace    { shift->_message( TRACE, @_ )   }
sub channels_by_fh    { map { $_->[0] } @{ shift->{handles} } }
sub channels_by_file  { () }
sub channels_by_email { () }
sub channels_by_level { grep { $_[0]->level( $_[1] ) } $_[0]->channels
+ }
    
sub _message 
    {
    my $self  = shift;
    my $level = shift;
        
    foreach my $channel ( $self->channels_by_level( $level ) )
        {
        print $channel $self->tag($level) . "@_" . "\n";
        }
    
    }
[download]

Right now I am getting an error message of "Can't locate object method "channels" via package..." which is pointed at the line "sub channels_by_level", I am self taught and don't understand what the subroutine is doing, I am not familiar with what grep does or which direction this lines is working. I am hoping someone can explain this for me.

g_White

Comment on I don't understand this grep Download Code

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Re: I don't understand this grep by ccn (Vicar) on Nov 05, 2008 at 17:52 UTC
The following code `sub channels_by_level { grep { $_[0]->level( $_[1] ) } $_[0]->channels + }` [download] can be written more clear such as: `sub channels_by_level { my $self = shift; my $level = shift; return grep { $self->level( $level ) } $self->channels; }` [download] Method `$obj->channels_by_level($level)` returns those channels which has level given in an argument. The problem is that your object doesn't have channels. To understand why it doesn't more code need to be shown.	[reply] [d/l] [select]
Re^2: I don't understand this grep by gone2015 (Deacon) on Nov 05, 2008 at 18:07 UTC
The curious thing is that the test in the `grep`: `return grep { $self->level( $level ) } $self->channels;` [download] contains nothing that depends on whatever $self->channels is or returns. It could equally be written: `return $self->level($level) ? $self->channels : () ;` [download] but that doesn't answer the question of why `$self->channels` appears to be missing.	[reply] [d/l] [select]
Re^3: I don't understand this grep by ikegami (Patriarch) on Nov 05, 2008 at 18:50 UTC
I wonder if it's supposed to be `return grep { $_->level( $level ) } $self->channels;` [download]	[reply] [d/l]
Re^4: I don't understand this grep by ccn (Vicar) on Nov 05, 2008 at 19:17 UTC
Re^4: I don't understand this grep by gwhite (Friar) on Nov 05, 2008 at 19:25 UTC
Re^3: I don't understand this grep by gwadej (Chaplain) on Nov 05, 2008 at 18:11 UTC
~~Well, that won't work if `$self->channels` returns a list, which seems likely, based on the name.~~ Update: Need to engage eyeballs before fingers. Thanks massa and jwkrahn for an appropriate whack on the side of the head. G. Wade	[reply] [d/l]
Re^4: I don't understand this grep by jwkrahn (Abbot) on Nov 05, 2008 at 18:37 UTC
Re^4: I don't understand this grep by massa (Hermit) on Nov 05, 2008 at 19:00 UTC
Re^5: I don't understand this grep by gwhite (Friar) on Nov 05, 2008 at 19:21 UTC
Re^5: I don't understand this grep by gwadej (Chaplain) on Nov 05, 2008 at 19:28 UTC
Re: I don't understand this grep by toolic (Bishop) on Nov 05, 2008 at 18:06 UTC
I am not familiar with what grep does As the documentation says, grep filters a list according to a specified condition, and returns a new list, when called in a list context, as your code does. Since your sub does not have an explicit return, the sub returns what grep returns, namely a list.	[reply]