=head1 NAME

Statistics::Basic::LeastSquareFit - find the least square fit for two lists

=head1 SYNOPSIS

A machine to calculate the Least Square Fit of given vectors x and y.

The module returns the alpha and beta filling this formula:

    $y = $beta * $x + $alpha

for a given set of I<x> and I<y> co-ordinate pairs.

Say you have the set of Cartesian coordinates:

    my @points = ( [1,1], [2,2], [3,3], [4,4] );

The simplest way to find the LSF is as follows:

    my $lsf = lsf()->set_size(int @points);
       $lsf->insert(@$_) for @points;

Or this way:

    my $xv  = vector( map {$_->[0]} @points );
    my $yv  = vector( map {$_->[1]} @points );
    my $lsf = lsf($xv, $yv);

And then either query the values or print them like so:

    print "The LSF for $xv and $yv: $lsf\n";
    my ($yint, $slope) =
    my ($alpha, $beta) = $lsf->query;

L<LSF|Statistics::Basic::LeastSquareFit> is meant for finding a line of best
fit.  C<$beta> is the slope of the line and C<$alpha> is the y-offset.  Suppose
you want to draw the line.  Use these to calculate the C<x> for a given C<y> or
vice versa:

    my $y = $lsf->y_given_x( 7 );
    my $x = $lsf->x_given_y( 7 );

(Note that C<x_given_y()> can sometimes produce a divide-by-zero error since it
has to divide by the C<$beta>.)

Create a 20 point "moving" L<LSF|Statistics::Basic::LeastSquareFit> like so:

    use Statistics::Basic qw(:all nofill);

    my $sth = $dbh->prepare("select x,y from points where something");
    my $len = 20;
    my $lsf = lsf()->set_size($len);

    $sth->execute or die $dbh->errstr;
    $sth->bind_columns( my ($x, $y) ) or die $dbh->errstr;

    my $count = $len;
    while( $sth->fetch ) {
        $lsf->insert( $x, $y );
        if( defined( my ($yint, $slope) = $lsf->query ) {
            print "LSF: y= $slope*x + $yint\n";
        }

        # This would also work:
        # print "$lsf\n" if $lsf->query_filled;
    }

=head1 METHODS

This list of methods skips the methods inherited from
L<Statistics::Basic::_TwoVectorBase> (things like
L<insert()|Statistics::Basic::_TwoVectorBase/insert()>, and
L<ginsert()|Statistics::Basic::_TwoVectorBase/ginsert()>).

=over 4

=item B<new()>

Create a new L<Statistics::Basic::LeastSquareFit> object.  This function takes
two arguments -- which can either be arrayrefs or L<Statistics::Basic::Vector>
objects.  This function is called when the
L<leastsquarefirt()|Statistics::Basic/leastsquarefit() LSF() lsf()>
shortcut-function is called.

=item B<query()>

L<LSF|Statistics::Basic::LeastSquareFit> is meant for finding a line of best
fit.  C<$beta> is the slope of the line and C<$alpha> is the y-offset.

    my ($alpha, $beta) = $lsf->query;

=item B<y_given_x()>

Automatically calculate the y-value on the line for a given x-value.

    my $y = $lsf->y_given_x( 7 );

=item B<x_given_y()>

Automatically calculate the x-value on the line for a given y-value.

    my $x = $lsf->x_given_y( 7 );

C<x_given_y()> can sometimes produce a divide-by-zero error since it
has to divide by the C<$beta>.  This might be helpful:

    if( defined( my $x = eval { $lsf->x_given_y(7) } ) ) {
        warn "there is no x value for 7";

    } else {
        print "x (given y=7): $x\n";
    }

=item B<query_vector1()>

Return the L<Statistics::Basic::Vector> for the first vector used in the
computation of alpha and beta.

=item B<query_vector2()>

Return the L<Statistics::Basic::Vector> object for the second vector used in the
computation of alpha and beta.

=item B<query_mean1()>

Returns the L<Statistics::Basic::Mean> object for the first vector used in the
computation of alpha and beta.

=item B<query_variance1()>

Returns the L<Statistics::Basic::Variance> object for the first vector used in
the computation of alpha and beta.

=item B<query_covariance()>

Returns the L<Statistics::Basic::Covariance> object used in the computation of
alpha and beta.

=back

=head1 OVERLOADS

This object is overloaded.  It tries to return an appropriate string for the
calculation, but raises an error in numeric context.

In boolean context, this object is always true (even when empty).

=head1 AUTHOR

Paul Miller C<< <jettero@cpan.org> >>

=head1 COPYRIGHT

Copyright 2009 Paul Miller -- Licensed under the LGPL

=head1 SEE ALSO

perl(1), L<Statistics::Basic>, L<Statistics::Basic::_TwoVectorBase>, L<Statistics::Basic::Vector>

=cut