Beefy Boxes and Bandwidth Generously Provided by pair Networks vroom
The stupid question is the question not asked
 
PerlMonks  

Re^4: aliasing arrays using typeglob under strict

by BrowserUk (Pope)
on Feb 08, 2013 at 18:44 UTC ( #1017876=note: print w/ replies, xml ) Need Help??


in reply to Re^3: aliasing arrays using typeglob under strict
in thread aliasing arrays using typeglob under strict

BrowserUk operates in an environment in which it is vital to squeeze every last, living computron from any processor, algorithm or function with which he deals, so I am not inclined to dispute his assertion that indirect access is "vastly less efficient".

The "vastly less efficient" was in response *only* to 7stud's suggested: my @ydata = @{shift()};.

Duplicating multiple, multi-dimensional arrays (in the case of mat mult), in order to achieve the same, non-indirected notation as you get with aliasing is "vastly less efficient".

Taking that quote, out of that context, is a strawman.

I would say that the (untested) way I have re-written mmMxM() below is, to my taste, at least as clear as the original.

For this simple example, it's not horribly more complex I grant you. But that is the tip of the iceberg.

Rather than this hard-coded sized, square-matrix multiply, let's take the more general form of MxN MatMult:

sub mmMxN { our( @M, @N ); local( *M, *N ) = @_; die "Incompatible matrix dimensions" unless @M == @{ $N[0] }; my @C = map[ (0) x @M ], 0 .. $#{ $N[ 0 ] }; for my $i ( 0 .. $#M ) { for my $j ( 0 .. $#{ $N[0] } ) { $C[ $i ][ $j ] += $M[ $i ][ $_ ] * $N[ $_ ][ $j] for 0 .. + $#N; } } return \@C; }

Now the indirect notation would take a somewhat higher cost on clarity.

Now consider that this is a method within an OO module, and the matrices in question are named instance variables in the object:

sub mmMxN { my $self = shift; die "Incompatible matrix dimensions" unless @{ $self->{M} } == @{ +$self->{N}->[0] }; $self->{R} = map[ (0) x @{ $self->{M} } ], 0 .. $#{ $self->{N}->[ +0 ] }; for my $i ( 0 .. $#{ $self->{M} } ) { for my $j ( 0 .. $#{ $self->{N}->[0] } ) { $self->{R}->[$i]->[$j] += $self->{M}->[$i]->[$_] * $self-> +{N}->[$_]->[$j] for 0 .. $#{ $self->{N} }; } } return $self->{R}; }

Yes, you can use temporaries for the array refs; and I've also exaggerated the syntax problem for effect, but even without that, this is preferable:

sub mmMxN { our ( @M, @N ); my $self = shift; local( *M, *N ) = @{ $self }{ M, N }; die "Incompatible matrix dimensions" unless @{ $M } == @{ $N[0] }; my @R = map[ (0) x @M ], 0 .. $#N[ 0 ]; for my $i ( 0 .. $#M ) { for my $j ( 0 .. $#{ $N[0] } ) { $R[$i][$j] += $M[$i][$_] * $N[$_][$j] for 0 .. $#N; } } $self->{R} = \@R; }

(IMO anyway :)

PS. See also The seven good uses for local.


With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.


Comment on Re^4: aliasing arrays using typeglob under strict
Select or Download Code

Log In?
Username:
Password:

What's my password?
Create A New User
Node Status?
node history
Node Type: note [id://1017876]
help
Chatterbox?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others wandering the Monastery: (13)
As of 2014-04-17 19:19 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?

    April first is:







    Results (454 votes), past polls