1MATLAB(1)             User Contributed Perl Documentation            MATLAB(1)
2
3
4

NAME

6       PDL::MATLAB - A guide for MATLAB users.
7

INTRODUCTION

9       If you are a MATLAB user, this page is for you. It explains the key
10       differences between MATLAB and PDL to help you get going as quickly as
11       possible.
12
13       This document is not a tutorial. For that, go to PDL::QuickStart. This
14       document complements the Quick Start guide, as it highlights the key
15       differences between MATLAB and PDL.
16

Perl

18       The key differences between MATLAB and PDL are broadcasting, and Perl.
19
20       Broadcasting means you can get a reference to just a part of your data,
21       and operate on it in a way that makes sense for your application. Those
22       operations will be reflected in the original data.
23
24       Perl is a general purpose programming language with thousands of
25       modules freely available on the web. PDL is an extension of Perl. This
26       gives PDL programs access to more features than most numerical tools
27       can dream of.  At the same time, most syntax differences between MATLAB
28       and PDL are a result of its Perl foundation.
29
30       You do not have to learn much Perl to be effective with PDL. But if you
31       wish to learn Perl, there is excellent documentation available on-line
32       (<http://perldoc.perl.org>) or through the command "perldoc perl".
33       There is also a beginner's portal (<http://perl-begin.org>).
34
35       Perl's module repository is called CPAN (<http://www.cpan.org>) and it
36       has a vast array of modules. Run "perldoc cpan" for more information.
37

TERMINOLOGY: NDARRAY

39       MATLAB typically refers to vectors, matrices, and arrays. Perl already
40       has arrays, and the terms "vector" and "matrix" typically refer to one-
41       and two-dimensional collections of data. Having no good term to
42       describe their object, PDL developers coined the term "ndarray" to give
43       a name to their data type.
44
45       An ndarray consists of a series of numbers organized as an
46       N-dimensional data set. ndarrays provide efficient storage and fast
47       computation of large N-dimensional matrices. They are highly optimized
48       for numerical work.
49
50       For more information, see "ndarrays vs Perl Arrays" later in this
51       document.
52

COMMAND WINDOW AND IDE

54       Unlike MATLAB, PDL does not come with a dedicated IDE. It does however
55       come with an interactive shell and you can use a Perl IDE to develop
56       PDL programs.
57
58   PDL interactive shell
59       To start the interactive shell, open a terminal and run "perldl" or
60       "pdl2".  As in MATLAB, the interactive shell is the best way to learn
61       the language. To exit the shell, type "exit", just like MATLAB.
62
63   Writing PDL programs
64       One popular IDE for Perl is called Padre (<http://padre.perlide.org>).
65       It is cross platform and easy to use.
66
67       Whenever you write a stand-alone PDL program (i.e. outside the "perldl"
68       or "pdl2" shell) you must start the program with "use PDL;".  This
69       command imports the PDL module into Perl. Here is a sample PDL program:
70
71         use PDL;             # Import main PDL module.
72         use PDL::NiceSlice;  # Import additional PDL module.
73         use PDL::AutoLoader; # Import additional PDL module.
74
75         $y = pdl [2,3,4];              # Statements end in semicolon.
76         $A = pdl [ [1,2,3],[4,5,6] ];  # 2-dimensional matrix.
77
78         print $A x $y->transpose;
79
80       Save this file as "myprogram.pl" and run it with:
81
82         perl myprogram.pl
83
84   New: Flexible syntax
85       In current versions of PDL (version 2.4.7 or later) there is a flexible
86       matrix syntax that can look extremely similar to MATLAB:
87
88       1) Use spaces to separate elements:
89
90         $y = pdl q[ 2 3 4 ];
91
92       2) Use a ';' to delimit rows:
93
94         $A = pdl q[ 1,2,3 ; 4,5,6 ];
95
96       Basically, as long as you put a "q" in front of the opening bracket,
97       PDL should "do what you mean". So you can write in a syntax that is
98       more comfortable for you.
99

MODULES FOR MATLAB USERS

101       There are two modules that MATLAB users will want to use:
102
103       PDL::NiceSlice
104            Gives PDL a syntax for slices (sub-matrices) that is shorter and
105            more familiar to MATLAB users.
106
107              % MATLAB
108              b(1:5)            -->  Selects the first 5 elements from b.
109
110              # PDL without NiceSlice
111              $y->slice("0:4")  -->  Selects the first 5 elements from $y.
112
113              # PDL with NiceSlice
114              $y(0:4)           -->  Selects the first 5 elements from $y.
115
116       PDL::AutoLoader
117            Provides a MATLAB-style autoloader for PDL. If an unknown function
118            "foo()" is called, PDL looks for a file called "foo.pdl". If it
119            finds one, it reads it.
120

BASIC FEATURES

122       This section explains how PDL's syntax differs from MATLAB. Most MATLAB
123       users will want to start here.
124
125   General "gotchas"
126       Indices
127            In PDL, indices start at '0' (like C and Java), not 1 (like MATLAB
128            or FORTRAN).  For example, if $y is an array with 5 elements, the
129            elements would be numbered from 0 to 4. This is different, but
130            less difficult as soon as you need to do calculations based on
131            offsets.
132
133       Displaying an object
134            MATLAB normally displays object contents automatically. In the PDL
135            shells you display objects explicitly with the "print" command or
136            the shortcut "p":
137
138            MATLAB:
139
140             >> a = 12
141             a =  12
142             >> b = 23;       % Suppress output.
143             >>
144
145            PDL Shell (perldl or pdl2):
146
147             pdl> $x = 12    # No output.
148             pdl> print $x   # Print object.
149             12
150             pdl> p $x       # "p" is a shorthand for "print" in the shell.
151             12
152             pdl>
153
154            In pdl2 there is the "do_print" command that will toggle the
155            "quiet" mode, which defaults to on. In "print" mode, expressions
156            you enter on the command line will have their values printed.
157
158   Creating ndarrays
159       Variables in PDL
160            Variables always start with the '$' sign.
161
162             MATLAB:    value  = 42
163             PerlDL:    $value = 42
164
165       Basic syntax
166            Use the "pdl" constructor to create a new ndarray.
167
168             MATLAB:    v  = [1,2,3,4]
169             PerlDL:    $v = pdl [1,2,3,4]
170
171             MATLAB:    A  =      [ 1,2,3  ;  3,4,5 ]
172             PerlDL:    $A = pdl [ [1,2,3] , [3,4,5] ]
173
174       Simple matrices
175                                  MATLAB       PDL
176                                  ------       ------
177              Matrix of ones      ones(5)      ones 5,5
178              Matrix of zeros     zeros(5)     zeros 5,5
179              Random matrix       rand(5)      random 5,5
180              Linear vector       1:5          sequence 5
181
182            Notice that in PDL the parenthesis in a function call are often
183            optional.  It is important to keep an eye out for possible
184            ambiguities. For example:
185
186              pdl> p zeros 2, 2 + 2
187
188            Should this be interpreted as "zeros(2,2) + 2" or as "zeros 2,
189            (2+2)"?  Both are valid statements:
190
191              pdl> p zeros(2,2) + 2
192              [
193               [2 2]
194               [2 2]
195              ]
196              pdl> p zeros 2, (2+2)
197              [
198               [0 0]
199               [0 0]
200               [0 0]
201               [0 0]
202              ]
203
204            Rather than trying to memorize Perl's order of precedence, it is
205            best to use parentheses to make your code unambiguous. Remember
206            you may need to come back to your code, and parentheses make your
207            own (as well as others') comprehension easier.
208
209       Linearly spaced sequences
210              MATLAB:   >> linspace(2,10,5)
211                        ans = 2 4 6 8 10
212
213              PerlDL:   pdl> p zeroes(5)->xlinvals(2,10)
214                        [2 4 6 8 10]
215
216            Explanation: Start with a 1-dimensional ndarray of 5 elements and
217            give it equally spaced values from 2 to 10.
218
219            MATLAB has a single function call for this. On the other hand,
220            PDL's method is more flexible:
221
222              pdl> p zeros(5,5)->xlinvals(2,10)
223              [
224               [ 2  4  6  8 10]
225               [ 2  4  6  8 10]
226               [ 2  4  6  8 10]
227               [ 2  4  6  8 10]
228               [ 2  4  6  8 10]
229              ]
230              pdl> p zeros(5,5)->ylinvals(2,10)
231              [
232               [ 2  2  2  2  2]
233               [ 4  4  4  4  4]
234               [ 6  6  6  6  6]
235               [ 8  8  8  8  8]
236               [10 10 10 10 10]
237              ]
238              pdl> p zeros(3,3,3)->zlinvals(2,6)
239              [
240               [
241                [2 2 2]
242                [2 2 2]
243                [2 2 2]
244               ]
245               [
246                [4 4 4]
247                [4 4 4]
248                [4 4 4]
249               ]
250               [
251                [6 6 6]
252                [6 6 6]
253                [6 6 6]
254               ]
255              ]
256
257       Slicing and indices
258            Extracting a subset from a collection of data is known as slicing.
259            PDL and MATLAB have a similar syntax for slicing, but there are
260            two important differences:
261
262            1) PDL indices start at 0, as in C and Java. MATLAB starts indices
263            at 1.
264
265            2) In MATLAB you think "rows and columns". In PDL, think "x and
266            y".
267
268              MATLAB                         PerlDL
269              ------                         ------
270              >> A                           pdl> p $A
271              A =                            [
272                   1   2   3                  [1 2 3]
273                   4   5   6                  [4 5 6]
274                   7   8   9                  [7 8 9]
275                                             ]
276              -------------------------------------------------------
277              (row = 2, col = 1)             (x = 0, y = 1)
278              >> A(2,1)                      pdl> p $A(0,1)
279              ans =                          [
280                     4                        [4]
281                                             ]
282              -------------------------------------------------------
283              (row = 2 to 3, col = 1 to 2)   (x = 0 to 1, y = 1 to 2)
284              >> A(2:3,1:2)                  pdl> p $A(0:1,1:2)
285              ans =                          [
286                     4   5                    [4 5]
287                     7   8                    [7 8]
288                                             ]
289
290            Warning
291                 When you write a stand-alone PDL program, if you want the
292                 "nice slice" syntax, you have to include the PDL::NiceSlice
293                 module. See the previous section "MODULES FOR MATLAB USERS"
294                 for more information.
295
296                   use PDL;             # Import main PDL module.
297                   use PDL::NiceSlice;  # Nice syntax for slicing.
298                   use PDL::AutoLoader; # MATLAB-like autoloader.
299
300                   $A = random 4,4;
301                   print $A(0,1);
302
303   Matrix Operations
304       Matrix multiplication
305                  MATLAB:    A * B
306                  PerlDL:    $A x $B
307
308       Element-wise multiplication
309                  MATLAB:    A .* B
310                  PerlDL:    $A * $B
311
312       Transpose
313                  MATLAB:    A'
314                  PerlDL:    $A->transpose
315
316   Functions that aggregate data
317       Some functions (like "sum", "max" and "min") aggregate data for an
318       N-dimensional data set. This is a place where MATLAB and PDL take a
319       different approach:
320
321       In MATLAB, these functions all work along one dimension.
322                   >> A = [ 1,5,4  ;  4,2,1 ]
323                   A = 1  5  4
324                       4  2  1
325                   >> max(A)
326                   ans = 4  5  4
327                   >> max(A')
328                   ans = 5  4
329
330                 If you want the maximum for the entire data set, you can use
331                 the special A(:) notation which basically turns the entire
332                 data set into a single 1-dimensional vector.
333
334                   >> max(A(:))
335                   ans =  5
336                   >> A = ones(2,2,2,2)
337                   >> max(A(:))
338                   ans = 1
339
340       PDL offers two functions for each feature.
341                   sum   vs   sumover
342                   avg   vs   average
343                   max   vs   maximum
344                   min   vs   minimum
345
346                 The long name works over a dimension, while the short name
347                 works over the entire ndarray.
348
349                   pdl> p $A = pdl [ [1,5,4] , [4,2,1] ]
350                   [
351                    [1 5 4]
352                    [4 2 1]
353                   ]
354                   pdl> p $A->maximum
355                   [5 4]
356                   pdl> p $A->transpose->maximum
357                   [4 5 4]
358                   pdl> p $A->max
359                   5
360                   pdl> p ones(2,2,2)->max
361                   1
362                   pdl> p ones(2,2,2,2)->max
363                   1
364
365       Note Notice that PDL aggregates horizontally while MATLAB aggregates
366            vertically. In other words:
367
368              MATLAB              PerlDL
369              max(A)       ==     $A->transpose->maximum
370              max(A')      ==     $A->maximum
371
372            TIP: In MATLAB you think "rows and columns". In PDL, think "x and
373            y".
374
375   Higher dimensional data sets
376       A related issue is how MATLAB and PDL understand data sets of higher
377       dimension. MATLAB was designed for 1D vectors and 2D matrices. Higher
378       dimensional objects ("N-D arrays") were added on top. In contrast, PDL
379       was designed for N-dimensional ndarrays from the start. This leads to a
380       few surprises in MATLAB that don't occur in PDL:
381
382       MATLAB sees a vector as a 2D matrix.
383              MATLAB                       PerlDL
384              ------                       ------
385              >> vector = [1,2,3,4];       pdl> $vector = pdl [1,2,3,4]
386              >> size(vector)              pdl> p $vector->dims
387              ans = 1 4                    4
388
389            MATLAB sees "[1,2,3,4]" as a 2D matrix (1x4 matrix). PDL sees it
390            as a 1D vector: A single dimension of size 4.
391
392       But MATLAB ignores the last dimension of a 4x1x1 matrix.
393              MATLAB                       PerlDL
394              ------                       ------
395              >> A = ones(4,1,1);          pdl> $A = ones 4,1,1
396              >> size(A)                   pdl> p $A->dims
397              ans = 4 1                    4 1 1
398
399       And MATLAB treats a 4x1x1 matrix differently from a 1x1x4 matrix.
400              MATLAB                       PerlDL
401              ------                       ------
402              >> A = ones(1,1,4);          pdl> $A = ones 1,1,4
403              >> size(A)                   pdl> p $A->dims
404              ans = 1 1 4                  1 1 4
405
406       MATLAB has no direct syntax for N-D arrays.
407              pdl> $A = pdl [ [[1,2,3],[4,5,6]], [[2,3,4],[5,6,7]] ]
408              pdl> p $A->dims
409              3 2 2
410
411       Feature support.
412            In MATLAB, several features such as sparse matrix support are not
413            available for N-D arrays. In PDL, just about any feature supported
414            by 1D and 2D ndarrays, is equally supported by N-dimensional
415            ndarrays.  There is usually no distinction.
416
417   Loop Structures
418       Perl has many loop structures, but we will only show the one that is
419       most familiar to MATLAB users:
420
421         MATLAB              PerlDL
422         ------              ------
423         for i = 1:10        for $i (1..10) {
424             disp(i)             print $i
425         endfor              }
426
427       Note Never use for-loops for numerical work. Perl's for-loops are
428            faster than MATLAB's, but they both pale against a "vectorized"
429            operation.  PDL has many tools that facilitate writing vectorized
430            programs.  These are beyond the scope of this guide. To learn
431            more, see: PDL::Indexing, PDL::Broadcasting, and PDL::PP.
432
433            Likewise, never use 1..10 for numerical work, even outside a for-
434            loop.  1..10 is a Perl array. Perl arrays are designed for
435            flexibility, not speed. Use ndarrays instead. To learn more, see
436            the next section.
437
438   ndarrays vs Perl Arrays
439       It is important to note the difference between an ndarray and a Perl
440       array. Perl has a general-purpose array object that can hold any type
441       of element:
442
443         @perl_array = 1..10;
444         @perl_array = ( 12, "Hello" );
445         @perl_array = ( 1, 2, 3, \@another_perl_array, sequence(5) );
446
447       Perl arrays allow you to create powerful data structures (see Data
448       structures below), but they are not designed for numerical work.  For
449       that, use ndarrays:
450
451         $pdl = pdl [ 1, 2, 3, 4 ];
452         $pdl = sequence 10_000_000;
453         $pdl = ones 600, 600;
454
455       For example:
456
457         $points =  pdl  1..10_000_000    # 4.7 seconds
458         $points = sequence 10_000_000    # milliseconds
459
460       TIP: You can use underscores in numbers ("10_000_000" reads better than
461       10000000).
462
463   Conditionals
464       Perl has many conditionals, but we will only show the one that is most
465       familiar to MATLAB users:
466
467         MATLAB                          PerlDL
468         ------                          ------
469         if value > MAX                  if ($value > $MAX) {
470             disp("Too large")               print "Too large\n";
471         elseif value < MIN              } elsif ($value < $MIN) {
472             disp("Too small")               print "Too small\n";
473         else                            } else {
474             disp("Perfect!")                print "Perfect!\n";
475         end                             }
476
477       Note Here is a "gotcha":
478
479              MATLAB:  elseif
480              PerlDL:  elsif
481
482            If your conditional gives a syntax error, check that you wrote
483            your "elsif"'s correctly.
484
485   TIMTOWDI (There Is More Than One Way To Do It)
486       One of the most interesting differences between PDL and other tools is
487       the expressiveness of the Perl language. TIMTOWDI, or "There Is More
488       Than One Way To Do It", is Perl's motto.
489
490       Perl was written by a linguist, and one of its defining properties is
491       that statements can be formulated in different ways to give the
492       language a more natural feel. For example, you are unlikely to say to a
493       friend:
494
495        "While I am not finished, I will keep working."
496
497       Human language is more flexible than that. Instead, you are more likely
498       to say:
499
500        "I will keep working until I am finished."
501
502       Owing to its linguistic roots, Perl is the only programming language
503       with this sort of flexibility. For example, Perl has traditional while-
504       loops and if-statements:
505
506         while ( ! finished() ) {
507             keep_working();
508         }
509
510         if ( ! wife_angry() ) {
511             kiss_wife();
512         }
513
514       But it also offers the alternative until and unless statements:
515
516         until ( finished() ) {
517             keep_working();
518         }
519
520         unless ( wife_angry() ) {
521             kiss_wife();
522         }
523
524       And Perl allows you to write loops and conditionals in "postfix" form:
525
526         keep_working() until finished();
527
528         kiss_wife() unless wife_angry();
529
530       In this way, Perl often allows you to write more natural, easy to
531       understand code than is possible in more restrictive programming
532       languages.
533
534   Functions
535       PDL's syntax for declaring functions differs significantly from
536       MATLAB's.
537
538         MATLAB                          PerlDL
539         ------                          ------
540         function retval = foo(x,y)      sub foo {
541             retval = x.**2 + x.*y           my ($x, $y) = @_;
542         endfunction                         return $x**2 + $x*$y;
543                                         }
544
545       Don't be intimidated by all the new syntax. Here is a quick run through
546       a function declaration in PDL:
547
548       1) "sub" stands for "subroutine".
549
550       2) "my" declares variables to be local to the function. This helps you
551       not accidentally use undeclared variables, which is enforced if you
552       "use strict". See strict for more.
553
554       3) "@_" is a special Perl array that holds all the function parameters.
555       This might seem like a strange way to do functions, but it allows you
556       to make functions that take a variable number of parameters. For
557       example, the following function takes any number of parameters and adds
558       them together:
559
560         sub mysum {
561             my ($i, $total) = (0, 0);
562             for $i (@_) {
563                 $total += $i;
564             }
565             return $total;
566         }
567
568       In more recent versions of Perl, you can "use signatures" for a
569       different syntax for declaring function parameters. See signatures for
570       more.
571
572       4) You can assign values to several variables at once using the syntax:
573
574         ($x, $y, $z) = (1, 2, 3);
575
576       So, in the previous examples:
577
578         # This declares two local variables and initializes them to 0.
579         my ($i, $total) = (0, 0);
580
581         # This takes the first two elements of @_ and puts them in $x and $y.
582         my ($x, $y) = @_;
583
584       5) The "return" statement gives the return value of the function, if
585       any.
586

ADDITIONAL FEATURES

588   ASCII File IO
589       To read data files containing whitespace separated columns of numbers
590       (as would be read using the MATLAB load command) one uses the PDL rcols
591       in PDL::IO::Misc.  For a general review of the IO functionality
592       available in PDL, see the documentation for PDL::IO, e.g., "help
593       PDL::IO" in the pdl2 shell or " pdldoc PDL::IO " from the shell command
594       line.
595
596   Data structures
597       To create complex data structures, MATLAB uses "cell arrays" and
598       "structure arrays". Perl's arrays and hashes offer similar
599       functionality but are more powerful and flexible. This section is only
600       a quick overview of what Perl has to offer. To learn more about this,
601       please go to <http://perldoc.perl.org/perldata.html> or run the command
602       "perldoc perldata".
603
604       Arrays
605            Perl arrays are similar to MATLAB's cell arrays, but more
606            flexible. For example, in MATLAB, a cell array is still
607            fundamentally a matrix. It is made of rows, and rows must have the
608            same length.
609
610              MATLAB
611              ------
612              array = {1, 12, 'hello'; rand(3, 2), ones(3), 'junk'}
613              => OK
614              array = {1, 12, 'hello'; rand(3, 2), ones(3) }
615              => ERROR
616
617            A Perl array is a general purpose, sequential data structure. It
618            can contain any data type.
619
620              PerlDL
621              ------
622              @array = ( [1, 12, 'hello'] , [ random(3,2), ones(3,3), 'junk' ] )
623              => OK
624              @array = ( [1, 12, 'hello'] , [ random(3,2), ones(3,3) ] )
625              => OK
626              @array = ( 5 , {'name' => 'Mike'} , [1, 12, 'hello'] )
627              => OK
628
629            Notice that Perl array's start with the "@" prefix instead of the
630            "$" used by ndarrays.
631
632            To learn about Perl arrays, please go to
633            <http://perldoc.perl.org/perldata.html> or run the command
634            "perldoc perldata".
635
636       Hashes
637            Perl hashes are similar to MATLAB's structure arrays:
638
639              MATLAB
640              ------
641              >> drink = struct('type', 'coke', 'size', 'large', 'myarray', {1,2,3})
642              >> drink.type = 'sprite'
643              >> drink.price = 12          % Add new field to structure array.
644
645              PerlDL
646              ------
647              pdl> %drink = ( type => 'coke' , size => 'large', myndarray => ones(3,3,3) )
648              pdl> $drink{type} = 'sprite'
649              pdl> $drink{price} = 12   # Add new field to hash.
650
651            Notice that Perl hashes start with the "%" prefix instead of the
652            "@" for arrays and "$" used by ndarrays.
653
654            To learn about Perl hashes, please go to
655            <http://perldoc.perl.org/perldata.html> or run the command
656            "perldoc perldata".
657
658   Performance
659       PDL has powerful performance features, some of which are not normally
660       available in numerical computation tools. The following pages will
661       guide you through these features:
662
663       PDL::Indexing
664            Level: Beginner
665
666            This beginner tutorial covers the standard "vectorization" feature
667            that you already know from MATLAB. Use this page to learn how to
668            avoid for-loops to make your program more efficient.
669
670       PDL::Broadcasting
671            Level: Intermediate
672
673            PDL's "vectorization" feature goes beyond what most numerical
674            software can do. In this tutorial you'll learn how to "broadcast"
675            over higher dimensions, allowing you to vectorize your program
676            further than is possible in MATLAB.
677
678       Benchmarks
679            Level: Intermediate
680
681            Perl comes with an easy to use benchmarks module to help you find
682            how long it takes to execute different parts of your code. It is a
683            great tool to help you focus your optimization efforts. You can
684            read about it online (<http://perldoc.perl.org/Benchmark.html>) or
685            through the command "perldoc Benchmark".
686
687       PDL::PP
688            Level: Advanced
689
690            PDL's Pre-Processor is one of PDL's most powerful features. You
691            write a function definition in special markup and the pre-
692            processor generates real C code which can be compiled. With PDL:PP
693            you get the full speed of native C code without having to deal
694            with the full complexity of the C language.
695
696   Plotting
697       PDL has full-featured plotting abilities. Unlike MATLAB, PDL relies
698       more on third-party libraries (pgplot and PLplot) for its 2D plotting
699       features.  Its 3D plotting and graphics uses OpenGL for performance and
700       portability.  PDL has three main plotting modules:
701
702       PDL::Graphics::PGPLOT
703            Best for: Plotting 2D functions and data sets.
704
705            This is an interface to the venerable PGPLOT library. PGPLOT has
706            been widely used in the academic and scientific communities for
707            many years. In part because of its age, PGPLOT has some
708            limitations compared to newer packages such as PLplot (e.g. no RGB
709            graphics).  But it has many features that still make it popular in
710            the scientific community.
711
712       PDL::Graphics::PLplot
713            Best for: Plotting 2D functions as well as 2D and 3D data sets.
714
715            This is an interface to the PLplot plotting library. PLplot is a
716            modern, open source library for making scientific plots.  It
717            supports plots of both 2D and 3D data sets. PLplot is best
718            supported for unix/linux/macosx platforms. It has an active
719            developers community and support for win32 platforms is improving.
720
721       PDL::Graphics::TriD
722            Best for: Plotting 3D functions.
723
724            The native PDL 3D graphics library using OpenGL as a backend for
725            3D plots and data visualization. With OpenGL, it is easy to
726            manipulate the resulting 3D objects with the mouse in real time.
727
728   Writing GUIs
729       Through Perl, PDL has access to all the major toolkits for creating a
730       cross platform graphical user interface. One popular option is wxPerl
731       (<http://wxperl.sourceforge.net>). These are the Perl bindings for
732       wxWidgets, a powerful GUI toolkit for writing cross-platform
733       applications.
734
735       wxWidgets is designed to make your application look and feel like a
736       native application in every platform. For example, the Perl IDE Padre
737       is written with wxPerl.
738
739   Simulink
740       Simulink is a graphical dynamical system modeler and simulator. It can
741       be purchased separately as an add-on to MATLAB.  PDL and Perl do not
742       have a direct equivalent to MATLAB's Simulink.  If this feature is
743       important to you, then take a look at Scilab:
744
745       <http://www.scilab.org>
746
747       Scilab is another numerical analysis software. Like PDL, it is free and
748       open source. It doesn't have PDL's unique features, but it is very
749       similar to MATLAB. Scilab comes with Xcos (previously Scicos), a
750       graphical system modeler and simulator similar to Simulink.
751

COMPARISON: REPEATED COPY OF MATRIX

753       In MATLAB, the "repmat" function works like so:
754
755         > A = reshape(0:5, 3, 2)' # similar to PDL::sequence(3, 2)
756         ans =
757            0   1   2
758            3   4   5
759         > repmat(A, 2, 3) # double rows, triple columns
760         ans =
761            0   1   2   0   1   2   0   1   2
762            3   4   5   3   4   5   3   4   5
763            0   1   2   0   1   2   0   1   2
764            3   4   5   3   4   5   3   4   5
765
766       This works (at least in Octave) at least up to 3 dimensions.
767
768       The PDL analog:
769
770         sub repmat {
771           my $f=shift;
772           my @n=@_; #number of repetitions along dimension
773           my $sl = join ',', map ":,*$_", @n; # insert right-size dummy after each real
774           my $r = $f->slice($sl); #result
775           $r = $r->clump($_, $_+1) for 0..$#n;
776           $r;
777         }
778         > p $x = sequence(3,2)
779         [
780          [0 1 2]
781          [3 4 5]
782         ]
783         > p repmat($x, 3, 2) # triple columns, double rows
784         [
785          [0 1 2 0 1 2 0 1 2]
786          [3 4 5 3 4 5 3 4 5]
787          [0 1 2 0 1 2 0 1 2]
788          [3 4 5 3 4 5 3 4 5]
789         ]
790

COMPARISON: FLOYD-WARSHALL ALGORITHM

792       In graph theory <https://en.wikipedia.org/wiki/Graph_theory>, an
793       apparently-simple but difficult problem is the "shortest path" problem,
794       of finding the shortest path between any two nodes. A famous solution
795       to this, albeit expensive (it is "O(V^3)" where "V" is the number of
796       vertices) is the Floyd-Warshall algorithm, which iterates through all
797       the possible paths.
798
799       Both the MATLAB solution and the PDL solution use vectorisation, so
800       hopefully this is a useful comparison. The MATLAB version started with
801       the code in code by Giorgos Dim
802       <https://uk.mathworks.com/matlabcentral/fileexchange/67503-floyd-
803       warshall-vectorized>, but modified as that code produces an incorrect
804       path matrix.
805
806       Sample data (reflected on both the Wikipedia page, and the Rosetta Code
807       website) for the weighted-edges matrix is, in PDL format:
808
809         my $we = pdl q[
810          [Inf Inf  -2 Inf]
811          [  4 Inf   3 Inf]
812          [Inf Inf Inf   2]
813          [Inf  -1 Inf Inf]
814         ];
815
816       and in MATLAB format:
817
818         A = [0 Inf -2 Inf; 4 0 3 Inf; Inf Inf 0 2; Inf -1 Inf 0]
819
820   PDL version
821       To solve for only distances without capturing the shortest actual
822       paths:
823
824         $we .= $we->hclip($we->mslice('X', $_) + $we->mslice($_, 'X'))
825           for 0..($we->dim(0)-1);
826
827       This loops over each possible intermediate point ("k" in the other
828       literature), setting it to $_ (a Perl idiom). It uses "hclip" in
829       PDL::Primitive for vectorised calculation of the distance between the
830       intermediate point's predecessors and successors. Those are the two
831       components of the addition expression, using "slices" alluded to above.
832       The ".=" is the PDL syntax for updating an ndarray.
833
834       To capture the shortest-path "next vertex" matrix as well:
835
836         use PDL::Lite;
837         my $d = $we->copy->inplace;
838         $d->diagonal(0, 1) .= 0;
839         my $suc = $we->copy->inplace;
840         my $adjacent_coords = PDL::whichND($we->isfinite);
841         $suc->indexND($adjacent_coords) .= $adjacent_coords->slice(0)->flat;
842         $suc->diagonal(0, 1) .= PDL::Basic::sequence($d->dim(0));
843         for (my $k = $d->dim(0)-1; $k >= 0; $k--) {
844           my $d_copy = $d->copy;
845           $d .= $d->hclip($d->mslice('X', $k) + $d->mslice($k, 'X'));
846           my $coords = PDL::whichND($d < $d_copy);
847           my $from_coords = $coords->copy->inplace;
848           $from_coords->slice(0) .= $k;
849           $suc->indexND($coords) .= $suc->indexND($from_coords);
850         }
851
852       The "diagonal" and "slice" expressions show how to update data via a
853       query syntax.
854
855   MATLAB version
856       Path-lengths only:
857
858         function D = FloydWarshall(D)
859           for k = 1:length(D)
860             D = min(D,D(:,k) + D(k,:));
861           end
862         end
863
864       The path vertices-capturing as well:
865
866         function [D,P] = FloydWarshall(D)
867           P = D;
868           n = length(D);
869           coords = find(isfinite(P));
870           P(coords) = floor((coords-1) / n)+1; % the col in 1-based
871           for v = 1:n; P(v, v) = v; end
872           for k = 1:n
873             prevD = D;
874             D = min(D,D(:,k) + D(k,:));
875             coords = find(D<prevD);
876             from_coords = n * (k-1) + mod(coords-1, n) + 1; % change col to k in 1-based
877             P(coords) = P(from_coords);
878           end
879         end
880
881       By comparison, the lack of "broadcasting" means that to update the
882       diagonal requires a for-loop, which in the sphere of vectorised
883       calculations is a bad thing. The calculations of coordinates are
884       complicated by the 1-based counting.
885
887       Copyright 2010 Daniel Carrera (dcarrera@gmail.com). You can distribute
888       and/or modify this document under the same terms as the current Perl
889       license.
890
891       See: <http://dev.perl.org/licenses/>
892

ACKNOWLEDGEMENTS

894       I'd like to thank David Mertens, Chris Marshall and Sigrid Carrera for
895       their immense help reviewing earlier drafts of this guide. Without
896       their hours of work, this document would not be remotely as useful to
897       MATLAB users as it is today.
898
899
900
901perl v5.34.0                      2022-02-28                         MATLAB(1)
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