in reply to how to find hash keys in a string ?

Another method not involving regular expressions -- well, okay, ignoring the one in split -- is the following:
use strict;
my %temphash = ("tom" => "good", "dick" => "bad", "hary" => "ugly");
my $string = 'This string contains tom and dick';
my @occurrences = grep {exists $temphash{$_}} split(/\s/, $string);
print scalar @occurrences;

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If you want to know where the matches occur, you can use map instead of grep.

Is this faster than the regexp method? I'm not sure. The hash lookups are certainly quick enough...

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Re: Re: how to find hash keys in a string ?
by BrowserUk (Patriarch) on May 03, 2003 at 20:57 UTC

    Excellent++. With 100 keys looked up in a string that contains them all in random order. The big regex beats out the map option, but your invert-the-logic hash lookup wins easily (2 1/2 x quicker than the regex).

    By the time you get to 1,000 keys, the map overtakes the regex but yours is an even clearer winner (nearly 30x faster).

    By the time you get to 10,000 keys, the margin is over 250x as fast.

    Results

         s/iter   regex    map   hash
regex   119     --    -71%  -100%
map     34.7   243%     --   -99%
hash   0.443 26705%  7715%     --
10000:10000:10000

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    I think its better to use the magical split ' ', rather than /\s/, as the latter will generate nulls for consecutive whitespace and even /\s+/ will generate one null if there is any leading whitespace. 

    print join '|', split /\s/, '         the  quick    brown    fox     j
+umps          '
|||||||||the||quick||||brown||||fox|||||jumps

print join '|', split /\s+/, '         the  quick    brown    fox     
+jumps          '
|the|quick|brown|fox|jumps

print join '|', split ' ', '         the  quick    brown    fox     ju
+mps          '
the|quick|brown|fox|jumps

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    Benchmark

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    Examine what is said, not who speaks.
    "Efficiency is intelligent laziness." -David Dunham
    "When I'm working on a problem, I never think about beauty. I think only how to solve the problem. But when I have finished, if the solution is not beautiful, I know it is wrong." -Richard Buckminster Fuller
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Re: Re: how to find hash keys in a string ?
by Limbic~Region (Chancellor) on May 04, 2003 at 03:54 UTC
    Dr. Mu,
    Ok - I admit defeat. This method is certainly faster than mine. There are a couple of things I would like to point out though.

    You have chosen to work of words in the string rather than the keys of the hash. If someone were really doing this type of analysis on a large data set, it might be worthwhile to have multiple methods and choose the best one based on the data set. For instance - imagine that there are only 3 hash keys, but the string is 1,500 words long. It certainly doesn't make sense to use this method.

    You have also assumed that the hash keys will not contain any spaces. My solution allows for this, but still has the requirement of having a word border on both ends. For insance: 

    my $temphash{'good boy'} = "blah";
my $string = "He was a good boy when we went to the store";
my $otherstring = "He is worthless, not a good boyfriend at all";

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    Your method will not work in either case. My method will correctly match in $string but will correctly fail in $otherstring.
    Of course, knowing your data is what counts so that you can code for it.

    My hat is off to you for such an innovative solution.

    Cheers - L~R

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      All good points! -- except the admitting defeat part. ;-) The ever-unfolding wonder of PM is the diversity of approaches to any given problem and the discussions that they stimulate. We all win as a consequence. Frankly, I didn't really consider my method to be that innovative, hoping instead to employ (and learn about) hash slices in lieu of the grep -- but to no avail. Nonetheless, what resulted was different enough, and it worked -- subject to the very real limitations you point out. Thanks and ++ to BrowserUK for troubling to do the efficiency analysis that I was too lazy to perform!
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