in reply to Re: Matching First Character of Strings Efficiently
in thread Matching First Character of Strings Efficiently

In my precedent message, I proposed a solution that was not considered maybe because it was in a test context. As a newbie in Perl, I would like to know if my solution is good or wrong. 
  my $fc = substr($str_a,0,1);
  for my $str_b ( @list ) {
    next if ord($fc) == ord((split(//, $str_b))[0]);
    expensive_function ( $str_a, $str_b );
  }

[download]
Tnx to all. :^)
----------
kral
(sorry for my english)

Replies are listed 'Best First'.
Re: Re: Re: Matching First Character of Strings Efficiently
by Limbic~Region (Chancellor) on Mar 16, 2004 at 15:02 UTC
    kral,
    Actually, I just missed it when I was doing the benchmark. I would give you an award for having the most un-necessary steps. Let me explain why.
    • You use substr to get 1st char even though ord returns numeric value of 1st char
    • You use split to get a list of individual characters and throw them away using a slice
    • You then use ord again for the test
    There is however, nothing wrong with it.

    Cheers - L~R

[reply]
      You use substr to get 1st char even though ord returns numeric value of 1st char
      You use split to get a list of individual characters and throw them away using a slice
      I don't want to seem obstinate (I'm here for learning), but I made some tests: 
        #!/usr/bin/perl -w

  use strict;
  use Benchmark qw(:all);

  my $str_a = "dlajsdlkajslkdjasldjasljdaskjd";

  cmpthese(
    -5,
    {
      'Test1' => sub { ord( $str_a ) },
      'Test2' => sub { ord( ( split ( //, $str_a ) )[0] ) },
      'Test3' => sub { ord( substr $str_a, 0, 1 ) },
    }
  );

__DATA__

Results:
           Rate  Test2  Test3  Test1
Test2    6321/s     --   -99%  -100%
Test3  516114/s  8065%     --   -72%
Test1 1827646/s 28815%   254%     --

      [download]
      I convene that isn't much readable, but it seems like the ord function works better with only one character as argument.
      Update: I have misunderstood the result, sorry!
      ----------
      kral
      (sorry for my english)
[reply]
[d/l]
        kral,
        I think you are misreading something. The first test by far surpassed the other two. In the first test you are using a long string, not a single character.

        Additionally, it isn't that ord works better with only one character as an argument - it is that the other two are doing a lot more work. Using substr or split to get the first character is extra operations that are completely un-necessary. The ord function (perldoc -f ord) would work the same if it was a single character or a long string. It is for this reason that the second two tests take more time. They are doing un-necessary work.

        I hope this clears things up - L~R
[reply]

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