File: //usr/local/share/man/man3/DBD::SQLite::Cookbook.3
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.\" ========================================================================
.\"
.IX Title "DBD::SQLite::Cookbook 3"
.TH DBD::SQLite::Cookbook 3 "2009-11-23" "perl v5.8.8" "User Contributed Perl Documentation"
.SH "NAME"
DBD::SQLite::Cookbook \- The DBD::SQLite Cookbook
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
This is the DBD::SQLite cookbook.
.PP
It is intended to provide a place to keep a variety of functions and
formals for use in callback APIs in DBD::SQLite.
.Sh "Variance"
.IX Subsection "Variance"
This is a simple aggregate function which returns a variance. It is
adapted from an example implementation in pysqlite.
.PP
.Vb 1
\& package variance;
.Ve
.PP
.Vb 1
\& sub new { bless [], shift; }
.Ve
.PP
.Vb 2
\& sub step {
\& my ( $self, $value ) = @_;
.Ve
.PP
.Vb 2
\& push @$self, $value;
\& }
.Ve
.PP
.Vb 2
\& sub finalize {
\& my $self = $_[0];
.Ve
.PP
.Vb 1
\& my $n = @$self;
.Ve
.PP
.Vb 2
\& # Variance is NULL unless there is more than one row
\& return undef unless $n || $n == 1;
.Ve
.PP
.Vb 5
\& my $mu = 0;
\& foreach my $v ( @$self ) {
\& $mu += $v;
\& }
\& $mu /= $n;
.Ve
.PP
.Vb 5
\& my $sigma = 0;
\& foreach my $v ( @$self ) {
\& $sigma += ($x - $mu)**2;
\& }
\& $sigma = $sigma / ($n - 1);
.Ve
.PP
.Vb 2
\& return $sigma;
\& }
.Ve
.PP
.Vb 3
\& # NOTE: If you use an older DBI (< 1.608),
\& # use $dbh->func(..., "create_aggregate") instead.
\& $dbh->sqlite_create_aggregate( "variance", 1, 'variance' );
.Ve
.PP
The function can then be used as:
.PP
.Vb 3
\& SELECT group_name, variance(score)
\& FROM results
\& GROUP BY group_name;
.Ve
.Sh "Variance (Memory Efficient)"
.IX Subsection "Variance (Memory Efficient)"
A more efficient variance function, optimized for memory usage at the
expense of precision:
.PP
.Vb 1
\& package variance2;
.Ve
.PP
.Vb 3
\& my $sum = 0;
\& my $count = 0;
\& my %hash;
.Ve
.PP
.Vb 1
\& sub new { bless [], shift; }
.Ve
.PP
.Vb 2
\& sub step {
\& my ( $self, $value ) = @_;
.Ve
.PP
.Vb 11
\& # by truncating and hashing, we can comsume many more data points
\& $value = int($value); # change depending on need for precision
\& # use sprintf for arbitrary fp precision
\& if (defined $hash{$value}) {
\& $hash{$value}++;
\& } else {
\& $hash{$value} = 1;
\& }
\& $sum += $value;
\& $count++;
\& }
.Ve
.PP
.Vb 2
\& sub finalize {
\& my $self = $_[0];
.Ve
.PP
.Vb 2
\& # Variance is NULL unless there is more than one row
\& return undef unless $count > 1;
.Ve
.PP
.Vb 2
\& # calculate avg
\& my $mu = $sum / $count;
.Ve
.PP
.Vb 5
\& my $sigma = 0;
\& foreach my $h (keys %hash) {
\& $sigma += (($h - $mu)**2) * $hash{$h};
\& }
\& $sigma = $sigma / ($count - 1);
.Ve
.PP
.Vb 2
\& return $sigma;
\& }
.Ve
.PP
The function can then be used as:
.PP
.Vb 3
\& SELECT group_name, variance2(score)
\& FROM results
\& GROUP BY group_name;
.Ve
.Sh "Variance (Highly Scalable)"
.IX Subsection "Variance (Highly Scalable)"
A third variable implementation, designed for arbitrarily large data sets:
.PP
.Vb 1
\& package variance;
.Ve
.PP
.Vb 3
\& my $mu = 0;
\& my $count = 0;
\& my $S = 0
.Ve
.PP
.Vb 1
\& sub new { bless [], shift; }
.Ve
.PP
.Vb 7
\& sub step {
\& my ( $self, $value ) = @_;
\& $count++;
\& $delta = $value - $mu;
\& $mu = $mu + $delta/$count
\& $S = $S + $delta*($value - $mu);
\& }
.Ve
.PP
.Vb 4
\& sub finalize {
\& my $self = $_[0];
\& return $S / ($count - 1);
\& }
.Ve
.PP
The function can then be used as:
.PP
.Vb 3
\& SELECT group_name, variance3(score)
\& FROM results
\& GROUP BY group_name;
.Ve
.SH "SUPPORT"
.IX Header "SUPPORT"
Bugs should be reported via the \s-1CPAN\s0 bug tracker at
.PP
<http://rt.cpan.org/NoAuth/ReportBug.html?Queue=DBD\-SQLite>
.SH "TO DO"
.IX Header "TO DO"
* Add more and varied cookbook recipes, until we have enough to
turn them into a seperate \s-1CPAN\s0 distribution.
.PP
* Create a series of tests scripts that validate the cookbook recipies.
.SH "AUTHOR"
.IX Header "AUTHOR"
Adam Kennedy <adamk@cpan.org>
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2009 Adam Kennedy.
.PP
This program is free software; you can redistribute
it and/or modify it under the same terms as Perl itself.
.PP
The full text of the license can be found in the
\&\s-1LICENSE\s0 file included with this module.