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Re^6: Is there any difference between prototype % and @ ?

by LanX (Canon)
on Feb 22, 2013 at 22:00 UTC ( #1020240=note: print w/ replies, xml ) Need Help??


in reply to Re^5: Is there any difference between prototype % and @ ?
in thread Is there any difference between prototype % and @ ?

> Then why create a list from the hash in the first place?

It's meant to be compatible with the standard interface!

You might remember that map, sort and grep can be chained and they always accept and return lists...

Cheers Rolf


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Re^7: Is there any difference between prototype % and @ ?
by BrowserUk (Pope) on Feb 23, 2013 at 07:03 UTC
    You might remember that map, sort and grep can be chained and they always accept and return lists...

    Hm. Passing a listified hash (mixed keys and values in random order) to any of those constructs is a completely pointless exercise.

    That is after all, why you are having to define an hgrep yourself.

    Better methinks to pass hgrep and hmap a reference to the hash, call the callback with pairs, and have them return a list.

    At least then they serve a purpose:

    #! perl -slw use strict; use Data::Dump qw[ pp ]; sub hgrep (&\%) { my( $code, $href ) = @_; map{ local @_; $code->( @_ = each %$href ) ? @_ : () } 1 .. keys %$href; } sub hmap (&\%) { my( $code, $href ) = @_; map{ local @_; $code->( @_ = each %$href ) } 1 .. keys %$href; } my %h = 'a'..'z'; print join ' ', hgrep{ $_[0] =~ /[a-m]/ } %h; print join ' ', hmap{ "$_[0] => $_[1]" } %h; my %orig = ( cat => 22, dog => 23, category => 66, catalyst => 77, cataclysm => 88, dogma => 89, dogstar => 92, ); my %h2 = hgrep{ $_[0] =~ /^cat/ } %orig; pp\%h2; __END__ C:\test>hfp e f a b m n c d k l g h i j w => x e => f a => b m => n s => t y => z u => v c => d k => l q => r +g => h i => j o => p { cat => 22, cataclysm => 88, catalyst => 77, category => 66 }

    hsort is left as an exercise.


    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.
      > > It's meant to be compatible with the standard interface!

      This can't be chained at least not easily w/o more transformations in the middle.

      your input is a hashref your output a list ...

      and your code is quite obfuscated...

      ... though ++ for creative use of each within map.

      Cheers Rolf

        This can't be chained at least not easily w/o more transformations in the middle.

        Show me an example of one of these mythical chains over a listified hash? (One that makes some sense!)

        and your code is quite obfuscated...

        Hm. You find that "obfuscated"? I always rated you as one of the more adept perlers.

        Besides, isn't a big part of the purpose of subroutines to hide the "difficult stuff", in order to simplify the code that calls it?

        though ++ for creative use of each within map.

        Thanks. The same method works with hsort to make sorting hashes a breeze:

        sub hsort (&\%) { my( $code, $href ) = @_; sort( $code map[ each %$href ], 1 .. keys %$href ); } my %orig = ( cat => 22, dog => 23, category => 66, catalyst => 77, cataclysm => 88, dogma => 89, dogstar => 92, ); ## Oh look! A chain :) print 'sorted by key: ', map "$_->[0]=>$_->[1] ", hsort{ $a->[0] cmp $ +b->[0] } %orig; print 'sorted by val: ', map "$_->[0]=>$_->[1] ", hsort{ $a->[1] <=> $ +b->[1] } %orig; C:\test>hFP.pl sorted by key: cat=>22 cataclysm=>88 catalyst=>77 category=>66 dog=>23 + dogma=>89 dogstar=>92 sorted by val: cat=>22 dog=>23 category=>66 catalyst=>77 cataclysm=>88 + dogma=>89 dogstar=>92

        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.

      BrowserUk: Your implementation of  hmap and  hgrep is a little tricky, but I certainly wouldn't describe it as 'quite obfuscated'! However...

      sub hmap (&\%) { my( $code, $href ) = @_; map{ local @_; $code->( @_ = each %$href ) } 1 .. keys %$href; }

      In the code above, I don't see the point of local-izing  @_ and then assigning  each to it. Passing the return of  each %$href 'directly' to the coderef seems to work just as well.

      >perl -wMstrict -MData::Dump -le "sub hmap (&\%) { my( $code, $href ) = @_; map{ $code->(each %$href) } 1 .. keys %$href; } ;; my %h = 'a'..'z'; ;; print join ' ', hmap { qq{$_[0] => $_[1]} } %h; ;; my %orig = ( cat => 22, dog => 23, category => 66, catalyst => 77, cataclysm => 88, dogma => 89, dogstar => 92, ); my %h2 = hmap { $_[0], $_[1] + $_[1] } %orig; dd \%h2; " w => x e => f a => b m => n s => t y => z u => v c => d k => l q => r +g => h i => j o => p { cat => 44, cataclysm => 176, catalyst => 154, category => 132, dog => 46, dogma => 178, dogstar => 184, }

        You're right! It's a left-over from starting with a copy of hgrep where I needed to keep a copy of the pair in order to return it conditionally.

        Thanks for pointing out the simplification.


        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.

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