Provided by: pdl_2.085-1ubuntu1_amd64 
NAME
PDL::Scilab - A guide for Scilab users.
INTRODUCTION
If you are a Scilab user, this page is for you. It explains the key differences between Scilab and PDL to
help you get going as quickly as possible.
This document is not a tutorial. For that, go to PDL::QuickStart. This document complements the Quick
Start guide, as it highlights the key differences between Scilab and PDL.
Perl
The key difference between Scilab and PDL is Perl.
Perl is a general purpose programming language with thousands of modules freely available on the web. PDL
is an extension of Perl. This gives PDL programs access to more features than most numerical tools can
dream of. At the same time, most syntax differences between Scilab and PDL are a result of its Perl
foundation.
You do not have to learn much Perl to be effective with PDL. But if you wish to learn Perl, there is
excellent documentation available on-line (<http://perldoc.perl.org>) or through the command "perldoc
perl". There is also a beginner's portal (<http://perl-begin.org>).
Perl's module repository is called CPAN (<http://www.cpan.org>) and it has a vast array of modules. Run
"perldoc cpan" for more information.
TERMINOLOGY: NDARRAY
Scilab typically refers to vectors, matrices, and arrays. Perl already has arrays, and the terms "vector"
and "matrix" typically refer to one- and two-dimensional collections of data. Having no good term to
describe their object, PDL developers coined the term "ndarray" to give a name to their data type.
An ndarray consists of a series of numbers organized as an N-dimensional data set. ndarrays provide
efficient storage and fast computation of large N-dimensional matrices. They are highly optimized for
numerical work.
For more information, see "ndarrays vs Perl Arrays" later in this document.
COMMAND WINDOW AND IDE
PDL does not come with a dedicated IDE. It does however come with an interactive shell and you can use a
Perl IDE to develop PDL programs.
PDL interactive shell
To start the interactive shell, open a terminal and run "perldl" or "pdl2". As in Scilab, the
interactive shell is the best way to learn the language. To exit the shell, type "exit", just like
Scilab.
Writing PDL programs
One popular IDE for Perl is called Padre (<http://padre.perlide.org>). It is cross platform and easy to
use.
Whenever you write a stand-alone PDL program (i.e. outside the "perldl" or "pdl2" shells) you must start
the program with "use PDL;". This command imports the PDL module into Perl. Here is a sample PDL
program:
use PDL; # Import main PDL module.
use PDL::NiceSlice; # Import additional PDL module.
$y = pdl [2,3,4]; # Statements end in semicolon.
$A = pdl [ [1,2,3],[4,5,6] ]; # 2-dimensional ndarray.
print $A x $y->transpose;
Save this file as "myprogram.pl" and run it with:
perl myprogram.pl
New: Flexible syntax
In very recent versions of PDL (version 2.4.7 or later) there is a flexible matrix syntax that can look
extremely similar to Scilab:
1) Use a ';' to delimit rows:
$y = pdl q[ 2,3,4 ];
$A = pdl q[ 1,2,3 ; 4,5,6 ];
2) Use spaces to separate elements:
$y = pdl q[ 2 3 4 ];
$A = pdl q[ 1 2 3 ; 4 5 6 ];
Basically, as long as you put a "q" in front of the opening bracket, PDL should "do what you mean". So
you can write in a syntax that is more comfortable for you.
A MODULE FOR SCILAB USERS
Here is a module that Scilab users will want to use:
PDL::NiceSlice
Gives PDL a syntax for slices (sub-matrices) that is shorter and more familiar to Scilab users.
// Scilab
b(1:5) --> Selects the first 5 elements from b.
# PDL without NiceSlice
$y->slice("0:4") --> Selects the first 5 elements from $y.
# PDL with NiceSlice
$y(0:4) --> Selects the first 5 elements from $y.
BASIC FEATURES
This section explains how PDL's syntax differs from Scilab. Most Scilab users will want to start here.
General "gotchas"
Indices
In PDL, indices start at '0' (like C and Java), not 1 (like Scilab). For example, if $y is an array
with 5 elements, the elements would be numbered from 0 to 4.
Displaying an object
Scilab normally displays object contents automatically. In PDL you display objects explicitly with
the "print" command or the shortcut "p":
Scilab:
--> a = 12
a = 12.
--> b = 23; // Suppress output.
-->
PerlDL:
pdl> $x = 12 # No output.
pdl> print $x # Print object.
12
pdl> p $x # "p" is a shorthand for "print" in the shell.
12
Creating ndarrays
Variables in PDL
Variables always start with the '$' sign.
Scilab: value = 42
PerlDL: $value = 42
Basic syntax
Use the "pdl" constructor to create a new ndarray.
Scilab: v = [1,2,3,4]
PerlDL: $v = pdl [1,2,3,4]
Scilab: A = [ 1,2,3 ; 3,4,5 ]
PerlDL: $A = pdl [ [1,2,3] , [3,4,5] ]
Simple matrices
Scilab PDL
------ ------
Matrix of ones ones(5,5) ones 5,5
Matrix of zeros zeros(5,5) zeros 5,5
Random matrix rand(5,5) random 5,5
Linear vector 1:5 sequence 5
Notice that in PDL the parenthesis in a function call are often optional. It is important to keep
an eye out for possible ambiguities. For example:
pdl> p zeros 2, 2 + 2
Should this be interpreted as "zeros(2,2) + 2" or as "zeros 2, (2+2)"? Both are valid statements:
pdl> p zeros(2,2) + 2
[
[2 2]
[2 2]
]
pdl> p zeros 2, (2+2)
[
[0 0]
[0 0]
[0 0]
[0 0]
]
Rather than trying to memorize Perl's order of precedence, it is best to use parentheses to make
your code unambiguous.
Linearly spaced sequences
Scilab: --> linspace(2,10,5)
ans = 2. 4. 6. 8. 10.
PerlDL: pdl> p zeroes(5)->xlinvals(2,10)
[2 4 6 8 10]
Explanation: Start with a 1-dimensional ndarray of 5 elements and give it equally spaced values from
2 to 10.
Scilab has a single function call for this. On the other hand, PDL's method is more flexible:
pdl> p zeros(5,5)->xlinvals(2,10)
[
[ 2 4 6 8 10]
[ 2 4 6 8 10]
[ 2 4 6 8 10]
[ 2 4 6 8 10]
[ 2 4 6 8 10]
]
pdl> p zeros(5,5)->ylinvals(2,10)
[
[ 2 2 2 2 2]
[ 4 4 4 4 4]
[ 6 6 6 6 6]
[ 8 8 8 8 8]
[10 10 10 10 10]
]
pdl> p zeros(3,3,3)->zlinvals(2,6)
[
[
[2 2 2]
[2 2 2]
[2 2 2]
]
[
[4 4 4]
[4 4 4]
[4 4 4]
]
[
[6 6 6]
[6 6 6]
[6 6 6]
]
]
Slicing and indices
Extracting a subset from a collection of data is known as slicing. The PDL shell and Scilab have a
similar syntax for slicing, but there are two important differences:
1) PDL indices start at 0, as in C and Java. Scilab starts indices at 1.
2) In Scilab you think "rows and columns". In PDL, think "x and y".
Scilab PerlDL
------ ------
--> A pdl> p $A
A = [
1. 2. 3. [1 2 3]
4. 5. 6. [4 5 6]
7. 8. 9. [7 8 9]
]
-------------------------------------------------------
(row = 2, col = 1) (x = 0, y = 1)
--> A(2,1) pdl> p $A(0,1)
ans = [
4. [4]
]
-------------------------------------------------------
(row = 2 to 3, col = 1 to 2) (x = 0 to 1, y = 1 to 2)
--> A(2:3,1:2) pdl> p $A(0:1,1:2)
ans = [
4. 5. [4 5]
7. 8. [7 8]
]
Warning
When you write a stand-alone PDL program you have to include the PDL::NiceSlice module. See the
previous section "MODULES FOR SCILAB USERS" for more information.
use PDL; # Import main PDL module.
use PDL::NiceSlice; # Nice syntax for slicing.
$A = random 4,4;
print $A(0,1);
Matrix Operations
Matrix multiplication
Scilab: A * B
PerlDL: $A x $B
Element-wise multiplication
Scilab: A .* B
PerlDL: $A * $B
Transpose
Scilab: A'
PerlDL: $A->transpose
Functions that aggregate data
Some functions (like "sum", "max" and "min") aggregate data for an N-dimensional data set. Scilab and PDL
both give you the option to apply these functions to the entire data set or to just one dimension.
Scilab In Scilab, these functions work along the entire data set by default, and an optional parameter
"r" or "c" makes them act over rows or columns.
--> A = [ 1,5,4 ; 4,2,1 ]
A = 1. 5. 4.
4. 2. 1.
--> max(A)
ans = 5
--> max(A, "r")
ans = 4. 5. 4.
--> max(A, "c")
ans = 5.
4.
PDL PDL offers two functions for each feature.
sum vs sumover
avg vs average
max vs maximum
min vs minimum
The long name works over a dimension, while the short name works over the entire ndarray.
pdl> p $A = pdl [ [1,5,4] , [4,2,1] ]
[
[1 5 4]
[4 2 1]
]
pdl> p $A->maximum
[5 4]
pdl> p $A->transpose->maximum
[4 5 4]
pdl> p $A->max
5
Higher dimensional data sets
A related issue is how Scilab and PDL understand data sets of higher dimension. Scilab was designed for
1D vectors and 2D matrices with higher dimensional objects added on top. In contrast, PDL was designed
for N-dimensional ndarrays from the start. This leads to a few surprises in Scilab that don't occur in
PDL:
Scilab sees a vector as a 2D matrix.
Scilab PerlDL
------ ------
--> vector = [1,2,3,4]; pdl> $vector = pdl [1,2,3,4]
--> size(vector) pdl> p $vector->dims
ans = 1 4 4
Scilab sees "[1,2,3,4]" as a 2D matrix (1x4 matrix). PDL sees it as a 1D vector: A single dimension
of size 4.
But Scilab ignores the last dimension of a 4x1x1 matrix.
Scilab PerlDL
------ ------
--> A = ones(4,1,1); pdl> $A = ones 4,1,1
--> size(A) pdl> p $A->dims
ans = 4 1 4 1 1
And Scilab treats a 4x1x1 matrix differently from a 1x1x4 matrix.
Scilab PerlDL
------ ------
--> A = ones(1,1,4); pdl> $A = ones 1,1,4
--> size(A) pdl> p $A->dims
ans = 1 1 4 1 1 4
Scilab has no direct syntax for N-D arrays.
pdl> $A = pdl [ [[1,2,3],[4,5,6]], [[2,3,4],[5,6,7]] ]
pdl> p $A->dims
3 2 2
Feature support.
In Scilab, several features are not available for N-D arrays. In PDL, just about any feature
supported by 1D and 2D ndarrays, is equally supported by N-dimensional ndarrays. There is usually no
distinction:
Scilab PerlDL
------ ------
--> A = ones(3,3,3); pdl> $A = ones(3,3,3);
--> A' pdl> transpose $A
=> ERROR => OK
Loop Structures
Perl has many loop structures, but we will only show the one that is most familiar to Scilab users:
Scilab PerlDL
------ ------
for i = 1:10 for $i (1..10) {
disp(i) print $i
end }
Note Never use for-loops for numerical work. Perl's for-loops are faster than Scilab's, but they both
pale against a "vectorized" operation. PDL has many tools that facilitate writing vectorized
programs. These are beyond the scope of this guide. To learn more, see: PDL::Indexing,
PDL::Broadcasting, and PDL::PP.
Likewise, never use 1..10 for numerical work, even outside a for-loop. 1..10 is a Perl array. Perl
arrays are designed for flexibility, not speed. Use ndarrays instead. To learn more, see the next
section.
ndarrays vs Perl Arrays
It is important to note the difference between an ndarray and a Perl array. Perl has a general-purpose
array object that can hold any type of element:
@perl_array = 1..10;
@perl_array = ( 12, "Hello" );
@perl_array = ( 1, 2, 3, \@another_perl_array, sequence(5) );
Perl arrays allow you to create powerful data structures (see Data structures below), but they are not
designed for numerical work. For that, use ndarrays:
$pdl = pdl [ 1, 2, 3, 4 ];
$pdl = sequence 10_000_000;
$pdl = ones 600, 600;
For example:
$points = pdl 1..10_000_000 # 4.7 seconds
$points = sequence 10_000_000 # milliseconds
TIP: You can use underscores in numbers ("10_000_000" reads better than 10000000).
Conditionals
Perl has many conditionals, but we will only show the one that is most familiar to Scilab users:
Scilab PerlDL
------ ------
if value > MAX if ($value > $MAX) {
disp("Too large") print "Too large\n";
elseif value < MIN } elsif ($value < $MIN) {
disp("Too small") print "Too small\n";
else } else {
disp("Perfect!") print "Perfect!\n";
end }
Note Here is a "gotcha":
Scilab: elseif
PerlDL: elsif
If your conditional gives a syntax error, check that you wrote your "elsif"'s correctly.
TIMTOWDI (There Is More Than One Way To Do It)
One of the most interesting differences between PDL and other tools is the expressiveness of the Perl
language. TIMTOWDI, or "There Is More Than One Way To Do It", is Perl's motto.
Perl was written by a linguist, and one of its defining properties is that statements can be formulated
in different ways to give the language a more natural feel. For example, you are unlikely to say to a
friend:
"While I am not finished, I will keep working."
Human language is more flexible than that. Instead, you are more likely to say:
"I will keep working until I am finished."
Owing to its linguistic roots, Perl is the only programming language with this sort of flexibility. For
example, Perl has traditional while-loops and if-statements:
while ( ! finished() ) {
keep_working();
}
if ( ! wife_angry() ) {
kiss_wife();
}
But it also offers the alternative until and unless statements:
until ( finished() ) {
keep_working();
}
unless ( wife_angry() ) {
kiss_wife();
}
And Perl allows you to write loops and conditionals in "postfix" form:
keep_working() until finished();
kiss_wife() unless wife_angry();
In this way, Perl often allows you to write more natural, easy to understand code than is possible in
more restrictive programming languages.
Functions
PDL's syntax for declaring functions differs significantly from Scilab's.
Scilab PerlDL
------ ------
function retval = foo(x,y) sub foo {
retval = x.**2 + x.*y my ($x, $y) = @_;
endfunction return $x**2 + $x*$y;
}
Don't be intimidated by all the new syntax. Here is a quick run through a function declaration in PDL:
1) "sub" stands for "subroutine".
2) "my" declares variables to be local to the function.
3) "@_" is a special Perl array that holds all the function parameters. This might seem like a strange
way to do functions, but it allows you to make functions that take a variable number of parameters. For
example, the following function takes any number of parameters and adds them together:
sub mysum {
my ($i, $total) = (0, 0);
for $i (@_) {
$total += $i;
}
return $total;
}
4) You can assign values to several variables at once using the syntax:
($x, $y, $z) = (1, 2, 3);
So, in the previous examples:
# This declares two local variables and initializes them to 0.
my ($i, $total) = (0, 0);
# This takes the first two elements of @_ and puts them in $x and $y.
my ($x, $y) = @_;
5) The "return" statement gives the return value of the function, if any.
ADDITIONAL FEATURES
Data structures
To create complex data structures, Scilab uses "lists" and "structs". Perl's arrays and hashes offer
similar functionality. This section is only a quick overview of what Perl has to offer. To learn more
about this, please go to <http://perldoc.perl.org/perldata.html> or run the command "perldoc perldata".
Arrays
Perl arrays are similar to Scilab's lists. They are both a sequential data structure that can
contain any data type.
Scilab
------
list( 1, 12, "hello", zeros(3,3) , list( 1, 2) );
PerlDL
------
@array = ( 1, 12, "hello" , zeros(3,3), [ 1, 2 ] )
Notice that Perl array's start with the "@" prefix instead of the "$" used by ndarrays.
To learn about Perl arrays, please go to <http://perldoc.perl.org/perldata.html> or run the command
"perldoc perldata".
Hashes
Perl hashes are similar to Scilab's structure arrays:
Scilab
------
--> drink = struct('type', 'coke', 'size', 'large', 'myarray', ones(3,3,3))
--> drink.type = 'sprite'
--> drink.price = 12 // Add new field to structure array.
PerlDL
------
pdl> %drink = ( type => 'coke' , size => 'large', myndarray => ones(3,3,3) )
pdl> $drink{type} = 'sprite'
pdl> $drink{price} = 12 # Add new field to hash.
Notice that Perl hashes start with the "%" prefix instead of the "@" for arrays and "$" used by
ndarrays.
To learn about Perl hashes, please go to <http://perldoc.perl.org/perldata.html> or run the command
"perldoc perldata".
Performance
PDL has powerful performance features, some of which are not normally available in numerical computation
tools. The following pages will guide you through these features:
PDL::Indexing
Level: Beginner
This beginner tutorial covers the standard "vectorization" feature that you already know from
Scilab. Use this page to learn how to avoid for-loops to make your program more efficient.
PDL::Broadcasting
Level: Intermediate
PDL's "vectorization" feature goes beyond what most numerical software can do. In this tutorial
you'll learn how to "broadcast" over higher dimensions, allowing you to vectorize your program
further than is possible in Scilab.
Benchmarks
Level: Intermediate
Perl comes with an easy to use benchmarks module to help you find how long it takes to execute
different parts of your code. It is a great tool to help you focus your optimization efforts. You
can read about it online (<http://perldoc.perl.org/Benchmark.html>) or through the command "perldoc
Benchmark".
PDL::PP
Level: Advanced
PDL's Pre-Processor is one of PDL's most powerful features. You write a function definition in
special markup and the pre-processor generates real C code which can be compiled. With PDL:PP you
get the full speed of native C code without having to deal with the full complexity of the C
language.
Plotting
PDL has full-featured plotting abilities. Unlike Scilab, PDL relies more on third-party libraries (pgplot
and PLplot) for its 2D plotting features. Its 3D plotting and graphics uses OpenGL for performance and
portability. PDL has three main plotting modules:
PDL::Graphics::PGPLOT
Best for: Plotting 2D functions and data sets.
This is an interface to the venerable PGPLOT library. PGPLOT has been widely used in the academic
and scientific communities for many years. In part because of its age, PGPLOT has some limitations
compared to newer packages such as PLplot (e.g. no RGB graphics). But it has many features that
still make it popular in the scientific community.
PDL::Graphics::PLplot
Best for: Plotting 2D functions as well as 2D and 3D data sets.
This is an interface to the PLplot plotting library. PLplot is a modern, open source library for
making scientific plots. It supports plots of both 2D and 3D data sets. PLplot is best supported
for unix/linux/macosx platforms. It has an active developers community and support for win32
platforms is improving.
PDL::Graphics::TriD
Best for: Plotting 3D functions.
The native PDL 3D graphics library using OpenGL as a backend for 3D plots and data visualization.
With OpenGL, it is easy to manipulate the resulting 3D objects with the mouse in real time.
Writing GUIs
Through Perl, PDL has access to all the major toolkits for creating a cross platform graphical user
interface. One popular option is wxPerl (<http://wxperl.sourceforge.net>). These are the Perl bindings
for wxWidgets, a powerful GUI toolkit for writing cross-platform applications.
wxWidgets is designed to make your application look and feel like a native application in every platform.
For example, the Perl IDE Padre is written with wxPerl.
Xcos / Scicos
Xcos (formerly Scicos) is a graphical dynamical system modeler and simulator. It is part of the standard
Scilab distribution. PDL and Perl do not have a direct equivalent to Scilab's Xcos. If this feature is
important to you, you should probably keep a copy of Scilab around for that.
COPYRIGHT
Copyright 2010 Daniel Carrera (dcarrera@gmail.com). You can distribute and/or modify this document under
the same terms as the current Perl license.
See: http://dev.perl.org/licenses/
perl v5.38.2 2024-04-10 SCILAB(1p)