## table of contents

Stats(3pm) | User Contributed Perl Documentation | Stats(3pm) |

# NAME¶

PDL::Stats - a collection of statistics modules in Perl Data Language, with a quick-start guide for non-PDL people.

# DESCRIPTION¶

Loads modules named below, making the functions available in the current namespace.

Properly formatted documentations online at http://pdl-stats.sf.net

# SYNOPSIS¶

use PDL::LiteF; # loads less modules use PDL::NiceSlice; # preprocessor for easier pdl indexing syntax use PDL::Stats; # Is equivalent to the following: use PDL::Stats::Basic; use PDL::Stats::GLM; use PDL::Stats::Kmeans; use PDL::Stats::TS; # and the following if installed; use PDL::Stats::Distr; use PDL::GSL::CDF;

# QUICK-START FOR NON-PDL PEOPLE¶

Enjoy PDL::Stats without having to dive into PDL, just wet your feet a little. Three key words two concepts and an icing on the cake, you should be well on your way there.

## pdl¶

The magic word that puts PDL::Stats at your disposal. pdl creates a PDL numeric data object (a pdl, or ndarray) from perl array or array ref. All PDL::Stats methods, unless meant for regular perl array, can then be called from the data object.

my @y = 0..5; my $y = pdl @y; # a simple function my $stdv = $y->stdv; # you can skip the intermediate $y my $stdv = stdv( pdl @y ); # a more complex method, skipping intermediate $y my @x1 = qw( y y y n n n ); my @x2 = qw( 1 0 1 0 1 0 ) # do a two-way analysis of variance with y as DV and x1 x2 as IVs my %result = pdl(@y)->anova( \@x1, \@x2 ); print "$_\t$result{$_}\n" for (sort keys %result);

If you have a list of list, ie array of array refs, pdl will create a multi-dimensional data object.

my @a = ( [1,2,3,4], [0,1,2,3], [4,5,6,7] ); my $a = pdl @a; print $a . $a->info; # here's what you will get [ [1 2 3 4] [0 1 2 3] [4 5 6 7] ] PDL: Double D [4,3]

PDL::Stats puts observations in the first dimension and variables in the second dimension, ie pdl [obs, var]. In PDL::Stats the above example represents 4 observations on 3 variables.

# you can do all kinds of fancy stuff on such a 2D pdl. my %result = $a->kmeans( {NCLUS=>2} ); print "$_\t$result{$_}\n" for (sort keys %result);

Make sure the array of array refs is rectangular. If the array refs are of unequal sizes, pdl will pad it out with 0s to match the longest list.

## info¶

Tells you the data type (yes pdls are typed, but you shouldn't have to worry about it here*) and dimensionality of the pdl, as seen in the above example. I find it a big help for my sanity to keep track of the dimensionality of a pdl. As mentioned above, PDL::Stats uses 2D pdl with observation x variable dimensionality.

*pdl uses double precision by default. If you are working with things like epoch time, then you should probably use pdl(long, @epoch) to maintain the precision.

## list¶

Come back to the perl reality from the PDL wonder land. list turns a pdl data object into a regular perl list. Caveat: list produces a flat list. The dimensionality of the data object is lost.

## Signature¶

This is not a function, but a concept. You will see something like this frequently in the pod:

stdv Signature: (a(n); float+ [o]b())

The signature tells you what the function expects as input and what kind of output it produces. a(n) means it expects a 1D pdl with n elements; [o] is for output, b() means its a scalar. So stdv will take your 1D list and give back a scalar. float+ you can ignore; but if you insist, it means the output is at float or double precision. The name a or b or c is not important. What's important is the thing in the parenthesis.

corr Signature: (a(n); b(n); float+ [o]c())

Here the function corr takes two inputs, two 1D pdl with the same numbers of elements, and gives back a scalar.

t_test Signature: (a(n); b(m); float+ [o]t(); [o]d())

Here the function t_test can take two 1D pdls of unequal size (n==m is certainly fine), and give back two scalars, t-value and degrees of freedom. Yes we accommodate t-tests with unequal sample sizes.

assign Signature: (data(o,v); centroid(c,v); byte [o]cluster(o,c))

Here is one of the most complicated signatures in the package. This is a function from Kmeans. assign takes data of observation x variable dimensions, and a centroid of cluster x variable dimensions, and returns an observation x cluster membership pdl (indicated by 1s and 0s).

Got the idea? Then we can see how PDL does its magic :)

## Threading¶

Another concept. The first thing to know is that, threading is optional.

PDL threading means automatically repeating the operation on extra elements or dimensions fed to a function. For a function with a signature like this

gsl_cdf_tdist_P Signature: (double x(); double nu(); [o]out())

the signatures says that it takes two scalars as input, and returns a scalar as output. If you need to look up the p-values for a list of t's, with the same degrees of freedom 19,

my @t = ( 1.65, 1.96, 2.56 ); my $p = gsl_cdf_tdist_P( pdl(@t), 19 ); print $p . "\n" . $p->info; # here's what you will get [0.94231136 0.96758551 0.99042586] PDL: Double D [3]

The same function is repeated on each element in the list you provided. If you had different degrees of freedoms for the t's,

my @df = (199, 39, 19); my $p = gsl_cdf_tdist_P( pdl(@t), pdl(@df) ); print $p . "\n" . $p->info; # here's what you will get [0.94973979 0.97141553 0.99042586] PDL: Double D [3]

The df's are automatically matched with the t's to give you the results.

An example of threading thru extra dimension(s):

stdv Signature: (a(n); float+ [o]b())

if the input is of 2D, say you want to compute the stdv for each of the 3 variables,

my @a = ( [1,1,3,4], [0,1,2,3], [4,5,6,7] ); # pdl @a is pdl dim [4,3] my $sd = stdv( pdl @a ); print $sd . "\n" . $sd->info; # this is what you will get [ 1.2990381 1.118034 1.118034] PDL: Double D [3]

Here the function was given an input with an extra dimension of size 3, so it repeats the stdv operation on the extra dimension 3 times, and gives back a 1D pdl of size 3.

Threading works for arbitrary number of dimensions, but it's best to refrain from higher dim pdls unless you have already decided to become a PDL wiz / witch.

Not all PDL::Stats methods thread. As a rule of thumb, if a function has a signature attached to it, it threads.

## perldl¶

Essentially a perl shell with "use PDL;" at start up. Comes with the PDL installation. Very handy to try out pdl operations, or just plain perl. print is shortened to p to avoid injury from excessive typing. "my" goes out of scope at the end of (multi)line input, so mostly you will have to drop the good practice of my here.

## For more info¶

PDL::Course

# AUTHOR¶

~~~~~~~~~~~~ ~~~~~ ~~~~~~~~ ~~~~~ ~~~ `` ><(((">

Copyright (C) 2009-2015 Maggie J. Xiong <maggiexyz users.sourceforge.net>

All rights reserved. There is no warranty. You are allowed to redistribute this software / documentation as described in the file COPYING in the PDL distribution.

2021-09-01 | perl v5.32.1 |