in reply to Bottom-Up Data Mining with Perl

Alot of data processing is having your data in the right components which align with your view of it.. Or rather I would touch upon uses for arrays, hashes, AoAs, AoHs, HoAs, and HoHs, and how they work together.. Possibly use the Schwartzian Transform to illustrate the ability to use those kinds of data strucures to perfom complex sorting in an elegant way...

Off the top of my head, a decent example would be a hash of arrays which contains the months names, the numerical value and how many days are in them ala
%months = (
   Jan => [1,  '31'], Feb => [2,  ''  ], Mar => [3,  '31'],
   Apr => [4,  '30'], May => [5,  '31'], Jun => [6,  '30'],
   Jul => [7,  '31'], Aug => [8,  '31'], Sep => [9,  '30'],
   Oct => [10, '31'], Nov => [11, '30'], Dec => [12, '31']
);

# Get number of days in Jan
$days = $months{Jan}->[1];

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It could also be written as a hash of hashes, but I think that shows what I mean.. I also avoided adding the logic to determine if its a leap year, and appropriatly set Feb->1, as I thoroughly *hate* that aspect of our calendar system..

Anyway what I was trying to say is we now have all the info we need about the months of the year.. a third element could be the month prior to, and a fourth element could be the next month.. you could extract the months in order or reverse via 
# in numerical order
for ( sort { $a->[0] <=> $b->[0] } keys %months ) {

# or reversed
for ( sort { $b->[0] <=> $a->[0] } keys %months ) {

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The possibilities are endless.. I see alot of people who dont fully utilize the data structures to represent their data and how it relates to itself.. Maybe they attempt to have a bunch or arrays and loop through one while pulling data from another (nothing wrong with this approach), as opposed to slapping the data into a single array of arrays.

Also maybe touch upon the speed factors of using refs as opposed to passing stuff around by value, and the tradeoffs of arrays vs hashes.. If they are doing 5 extra calulations attempting to figure out which array index to get, maybe they should be using hashes, especially if its in a tight loop etc (trivial off the top of my head example)..


/* And the Creator, against his better judgement, wrote man.c */

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Re: Re: Bottom-Up Data Mining with Perl
by tachyon (Chancellor) on Mar 05, 2003 at 23:52 UTC

    Here is a much better data structure that also shows you how you can set a hash value to the return val of a sub and thus give feb a correct value. It also shows semi dynamic hash construction using map FWIW. 

    my @month_names = qw( undef, jan feb mar apr may jun jul aug sept oct 
+nov dec );
my @month_days = ( undef, 31, &feb_days, 31, 30, 31, 30, 31, 31, 30, 3
+1, 30, 31 );

my $months = {
    name   => { map{ $_, $month_names[$_]} 1..12 },
    number => { map{ $month_names[$_], $_ } 1..12 },
    days   => { map{ $_, $month_days[$_], $month_names[$_], $month_day
+s[$_] } 1..12 },
};

print "Month number 4 is ",   $months->{name}->{4}, "\n";
print "Month mar is number ", $months->{number}->{mar}, "\n";
print "Days in month feb ",   $months->{days}->{feb}, "\n";
print "Days in month 4 ",     $months->{days}->{4},   "\n";


use Data::Dumper;
print Dumper $months;

sub feb_days { check_leap_year( get_year() ) ? 29 : 28 }

sub get_year { (localtime())[5] + 1900 }

sub check_leap_year {
    my $year = shift;
    my $leap_year = 0;
    $leap_year = 1 if $year % 4 == 0;
    $leap_year = 0 if $year % 100 == 0 and $year % 400 != 0;
  return $leap_year;
}

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    cheers

    tachyon

    s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print

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[d/l]

      and make a note that if you start the script in December of a leap year it will be wrong come February. but i guess you would use a module in that case.

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        Of course but how likely is that....

        cheers

        tachyon

        s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print

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