in reply to Common Perl Idioms

Regarding in_list I personally would prefer either: 

sub in_list {my $target=shift; return 0+grep { $_ eq $target } @_}

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if you really care about the count (and want to use grep) or if you want fast then: 

sub in_list {my $target=shift; $_ eq $target and return 1 for @_; retu
+rn}

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using the for modifier avoids creating and managing an extra scope (like grep,map and for statements) and avoiding the slice I think is more efficient. Afaik perl will have to create the sliced list first before iterating so you are really just copying the whole array which could get expensive for a large list.

Anyway heres an interesting/useful one : 

sub find_first_index {
  my $target=shift;
  push @_,$target;
  my $i=0;
  $i++ while $_[$i] ne $target;
  return $i==$#_ ? undef : $i
}

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Which is a nice trick to reduce the cost of the tight loop. Dunno if more modern perls have been optimised enough that these tricks are unnecessary though. Its been a while since I benchmarked this stuff last.


---
demerphq

    First they ignore you, then they laugh at you, then they fight you, then you win.
    -- Gandhi

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Re^2: Common Perl Idioms
by ysth (Canon) on Jul 23, 2004 at 19:59 UTC
    I would have said !!grep, not 0+grep. 0+ will return 0 or 1. !! will return canonical true or false.
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      Well I'm not so sure how important that is, especially as it loses useful and already obtained information (ie the count). Also for the canonical return isn't 0 < grep better? It would also return the canonical true/false but is only a single operator...


      ---
      demerphq

        First they ignore you, then they laugh at you, then they fight you, then you win.
        -- Gandhi
[reply]
[d/l]
        Ah, but !! is an idiom, and more readily recognizable to my eyes. With 0< it takes me a second to figure out what the code is trying to compare. In any case, 0 < has to load a constant then do a compare, so it's still two ops.
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Re^2: Common Perl Idioms
by ihb (Deacon) on Jul 24, 2004 at 00:45 UTC

    An alternative to your &find_first_index that I prefer (for smaller lists) is 

        use List::Util 'first';

    sub find_first_index {
        my $target = shift;
        first { $_[$_] eq $target } 0 .. $#_;
    }

    # Simple enough to write inline:

    my $idx = first { $ary[$_] eq $target } 0 .. $#ary;

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    I don't know about any efficiency issues though.

    Update: Fixed a off-by-one bug in the sub and added parenthesis.

    ihb

    Read argumentation in its context!

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[d/l]
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      I don't know about any efficiency issues though.

      Doesn't look good:

      Read more... (2 kB)
                 Rate ihb_zip ihb_hit dmq_zip dmq_hit
ihb_zip  8461/s      --     -5%    -44%    -50%
ihb_hit  8869/s      5%      --    -42%    -47%
dmq_zip 15177/s     79%     71%      --    -10%
dmq_hit 16782/s     98%     89%     11%      --

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      And thats using the XS List::Util implementation from 5.8.2. Theres a lot of overhead in calling the closure, and I think that the range operator isn't lazy in this usage, so it has to manufacture a list of indexes to use.


      ---
      demerphq

        First they ignore you, then they laugh at you, then they fight you, then you win.
        -- Gandhi
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[d/l]
[select]

        sub firstIdx { 
    $_[ $_ ] eq $_[ 0 ] and return --$_ for 1 .. @_; 
    return; 
}

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        Examine what is said, not who speaks.
        "Efficiency is intelligent laziness." -David Dunham
        "Think for yourself!" - Abigail
        "Memory, processor, disk in that order on the hardware side. Algorithm, algoritm, algorithm on the code side." - tachyon
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[d/l]

In Section Meditations