Beefy Boxes and Bandwidth Generously Provided by pair Networks
Keep It Simple, Stupid
 
PerlMonks  

Comment on

( #3333=superdoc: print w/ replies, xml ) Need Help??
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.

In reply to Re^4: aliasing arrays using typeglob under strict by BrowserUk
in thread aliasing arrays using typeglob under strict by RockyMtn

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":



  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.
  • Log In?
    Username:
    Password:

    What's my password?
    Create A New User
    Chatterbox?
    and the web crawler heard nothing...

    How do I use this? | Other CB clients
    Other Users?
    Others cooling their heels in the Monastery: (12)
    As of 2015-07-28 20:27 GMT
    Sections?
    Information?
    Find Nodes?
    Leftovers?
      Voting Booth?

      The top three priorities of my open tasks are (in descending order of likelihood to be worked on) ...









      Results (258 votes), past polls