in reply to finite automata

This will get you started: 

#!/usr/bin/perl -w
use strict;

my $string = 'just another perl hacker';

# generate a lookup hash containing the words in our language
# as the keys, we set the values to 1 with the ++ syntax
# so as to define the keys which are all we use
my %lang;
do{ chomp; $lang{$_}++ }for <DATA>; 

# split the test string on whitespace to give us an
# array that will contain all the 'words' where a 
# word is a character sequence
my @bits = split /\s/, $string;

# iterate over our word array seeing if they are 
# defined in our langugue specification
for (@bits) {
    die "Word '$_' not in language!\n" unless defined $lang{$_};
}

# if we have not died then all the words are OK
print "Success, \$string only contains words in language!\n";

__DATA__
just
another
finite
automaton
perl
hack

[download]

doc

print(s<>ecode?scalar reverse :p)

Replies are listed 'Best First'.
Re: Re: finite automata
by davorg (Chancellor) on Oct 02, 2001 at 16:10 UTC

    I may, of course, be misunderstanding the problem, but it sounds to me like you're not given a complete listing of the language dictionary - simply a set of rules that valid words must obey. In that case pjf's regex-based solution is far more efficient (assuming, of course, that you can represent each of the rules as a regex).

    --
    <http://www.dave.org.uk>

    "The first rule of Perl club is you don't talk about Perl club."

[reply]

      As you note it does rather depend on what the problem is. Depending on the situation and the complexity of the language a pre-generated hash lookup table will be potentially much faster than a regex solution.

      Consider a simple alphabet that may only contain words in the form: aa ab ac ad .... az. Including the overhead of generating the hash lookup table the hash method is much faster than a comparable regex method as well as being far more flexible. 

      use Benchmark;

$string = 'aa ab ac ad ae af ' x 10000 . ' ff';
@string = split /\s/, $string;

$regex = <<'CODE';
&regex;
sub regex {
    do{return 0 unless /^a[a-z]$/} for @string;
    return 1;
}
CODE

$hash = <<'CODE';
$hash{$_}++ for aa..az;
&hash;
sub hash {
    do{return 0 unless defined $hash{$_}} for @string;
    return 1;
}
CODE

timethese ( 100, { 'regex' => $regex, 'hash' => $hash } );

__END__

Benchmark: timing 100 iterations of hash, regex...
      hash: 21 wallclock secs (20.37 usr +  0.00 sys = 20.37 CPU) @  4
+.91/s (n=100)
     regex: 45 wallclock secs (45.10 usr +  0.00 sys = 45.10 CPU) @  2
+.22/s (n=100)

      [download]

      doc

      print(s??cod??scalar reverse :p)

[reply]
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