1FAQ(1)                User Contributed Perl Documentation               FAQ(1)
2
3
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NAME

6       PDL::FAQ - Frequently asked questions about PDL
7

VERSION

9       Current FAQ version:  1.008
10

DESCRIPTION

12       This is version 1.008 of the PDL FAQ, a collection of  frequently asked
13       questions about PDL - the Perl Data Language.
14

ABOUT THIS DOCUMENT

16   Q: 1.1    Where to find this document
17       You can find the latest version of this document at
18       <http://pdl.perl.org/?docs=FAQ&title=Frequently%20Asked%20Questions> .
19
20   Q: 1.2    How to contribute to this document
21       This is a considerably reworked version of the PDL FAQ. As such many
22       errors might have crept in and many updates might not have made it in.
23       You are explicitly encouraged to let us know about questions which you
24       think should be answered in this document but currently aren't.
25
26       Similarly, if you think parts of this document are unclear, please tell
27       the FAQ maintainer about it. Where a specific answer is taken in full
28       from someones posting the authorship should be indicated, let the FAQ
29       maintainer know if it isn't. For more general information explicit
30       acknowledgment is not made in the text, but rather there is an
31       incomplete list of contributors at the end of this document. Please
32       contact the FAQ maintainer if you feel hard done by.
33
34       Send your comments, additions, suggestions or corrections to the PDL
35       mailing list at pdl-general@lists.sourceforge.net.  See Q: 3.2 below
36       for instructions on how to join the mailing lists.
37

GENERAL QUESTIONS

39   Q: 2.1    What is PDL ?
40       PDL stands for Perl Data  Language . To say it with the words of Karl
41       Glazebrook, initiator of the PDL project:
42
43           The PDL concept is to give standard perl5 the ability
44           to COMPACTLY store and SPEEDILY manipulate the large
45           N-dimensional data sets which are the bread and butter
46           of scientific computing. e.g. $a=$b+$c can add two
47           2048x2048 images in only a fraction of a second.
48
49       It provides tons of useful functionality for scientific and numeric
50       analysis.
51
52       For readers familiar with other scientific data evaluation packages it
53       may be helpful to add that PDL is in many respects similar to IDL,
54       MATLAB and similar packages. However, it tries to improve on a number
55       of issues which were perceived (by the authors of PDL) as shortcomings
56       of those existing packages.
57
58   Q: 2.2    Who supports PDL? Who develops it?
59       PDL is supported by its users. General informal support for PDL is
60       provided through the PDL mailing list
61       (pdl-general@lists.sourceforge.net , see below).
62
63       As a Perl extension (see Q: 2.5 below) it is devoted to the idea of
64       free and open development put forth by the Perl community. PDL was and
65       is being actively developed by a loosely knit group of people around
66       the world who coordinate their activities through the PDL development
67       mailing list (pdl-devel@lists.sourceforge.net , see Q: 3.2 below). If
68       you would like to join in the ongoing efforts to improve PDL please
69       join this list.
70
71   Q: 2.3    Why yet another Data Language ?
72       There are actually several reasons and everyone should decide for
73       himself which are the most important ones:
74
75       ·   PDL is "free software". The authors of PDL think that this concept
76           has several advantages: everyone has access to the sources ->
77           better debugging, easily adaptable to your own needs, extensible
78           for your purposes, etc... In comparison with commercial packages
79           such as MATLAB and IDL this is of considerable importance for
80           workers who want to do some work at home and cannot afford the
81           considerable cost to buy commercial packages for personal use.
82
83       ·   PDL is based on a powerful and well designed scripting language:
84           Perl. In contrast to other scientific/numeric data analysis
85           languages it has been designed using the features of a proven
86           language instead of having grown into existence from scratch.
87           Defining the control structures while features were added during
88           development leads to languages that often appear clumsy and badly
89           planned for most existing packages with similar scope as PDL.
90
91       ·   Using Perl as the basis a PDL programmer has all the powerful
92           features of Perl at his hand, right from the start. This includes
93           regular expressions, associative arrays (hashes), well designed
94           interfaces to the operating system, network, etc. Experience has
95           shown that even in mainly numerically oriented programming it is
96           often extremely handy if you have easy access to powerful semi-
97           numerical or completely non-numerical functionality as well. For
98           example, you might want to offer the results of a complicated
99           computation as a server process to other processes on the network,
100           perhaps directly accepting input from other processes on the
101           network. Using Perl and existing Perl extension packages things
102           like this are no problem at all (and it all will fit into your "PDL
103           script").
104
105       ·   Extremely easy extensibility and interoperability as PDL is a Perl
106           extension; development support for Perl extensions is an integral
107           part of Perl and there are already numerous extensions to standard
108           Perl freely available on the network.
109
110       ·   Integral language features of Perl (regular expressions, hashes,
111           object modules) immensely facilitated development and
112           implementation of key concepts of PDL. One of the most striking
113           examples for this point is probably PDL::PP (see Q: 6.16 below), a
114           code generator/parser/pre-processor that generates PDL functions
115           from concise descriptions.
116
117       ·   None of the existing data languages follow the Perl language rules,
118           which the authors firmly believe in:
119
120           ·   TIMTOWTDI: There is more than one way to do it.  Minimalist
121               languages are interesting for computer scientists, but for
122               users, a little bit of redundancy makes things wildly easier to
123               cope with and allows individual programming styles - just as
124               people speak in different ways. For many people this will
125               undoubtedly be a reason to avoid PDL ;)
126
127           ·   Simple things are simple, complicated things possible: Things
128               that are often done should be easy to do in the language,
129               whereas seldom done things shouldn't be too cumbersome.
130
131           All existing languages violate at least one of these rules.
132
133       ·   As a project for the future PDL should be able to use super
134           computer features, e.g. vector capabilities/parallel processing,
135           GPGPU acceleration. This will probably be achieved by having
136           PDL::PP (see Q: 6.16 below) generate appropriate code on such
137           architectures to exploit these features.
138
139       ·   [ fill in your personal 111 favourite reasons here...]
140
141   Q: 2.4    What is PDL good for ?
142       Just in case you do not yet know what the main features of PDL are and
143       what one could do with them, here is a (necessarily selective) list of
144       key features:
145
146       PDL is well suited for matrix computations, general handling of
147       multidimensional data, image processing, general scientific
148       computation, numerical applications. It supports I/O for many popular
149       image and data formats, 1D (line plots), 2D (images) and 3D (volume
150       visualization, surface plots via OpenGL - for instance implemented
151       using Mesa or video card OpenGL drivers), graphics display capabilities
152       and implements many numerical and semi-numerical algorithms.
153
154       Through the powerful pre-processor it is also easy to interface Perl to
155       your favorite C routines, more of that further below.
156
157   Q: 2.5    What is the connection between PDL and Perl ?
158       PDL is a Perl5 extension package. As such it needs an existing Perl5
159       installation (see below) to run. Furthermore, much of PDL is written in
160       Perl (+ some core functionality that is written in C). PDL programs are
161       (syntactically) just Perl scripts that happen to use some of the
162       functionality implemented by the package "PDL".
163
164   Q: 2.6    What do I need to run PDL on my machine ?
165       Since PDL is just a Perl5 package you need first of all an installation
166       of Perl5 on your machine. As of this writing PDL requires version
167       5.10.x of perl, or higher.  More information on where and how to get a
168       Perl installation can be found at the Perl home page
169       <http://www.perl.org> and at many CPAN sites (if you do not know what
170       CPAN is, check the answer to the next question).
171
172       To build PDL you also need a working C compiler, support for Xsubs, and
173       the package Extutils::MakeMaker. If you don't have a compiler there
174       might be a binary distribution available, see "Binary distributions"
175       below.
176
177       If you can (or cannot) get PDL working on a new (previously
178       unsupported) platform we would like to hear about it. Please, report
179       your success/failure to the PDL mailing list at
180       pdl-general@lists.sourceforge.net . We will do our best to assist you
181       in porting PDL to a new system.
182
183   Q: 2.7    Where do I get it?
184       PDL is available as source distribution in the Comprehensive Perl
185       Archive Network (or CPAN) and from the GitHub project page at
186       <https://github.com/PDLPorters/pdl>.  The CPAN archives contains not
187       only the PDL distribution but also just about everything else that is
188       Perl-related.  CPAN is mirrored by dozens of sites all over the world.
189       The main site is <http://www.cpan.org>, and local CPAN sites (mirrors)
190       can be found there. Within CPAN you find the latest released version of
191       PDL in the directory /modules/by-module/PDL/. PDL's homepage is at
192       <http://pdl.perl.org> and the latest version can also be downloaded
193       from there.
194
195   Q: 2.8    What do I have to pay to get PDL?
196       We are delighted to be able to give you the nicest possible answer on a
197       question like this: PDL is *free software* and all sources are publicly
198       available. But still, there are some copyrights to comply with. So
199       please, try to be as nice as we (the PDL authors) are and try to comply
200       with them.
201
202       Oh, before you think it is *completely* free: you have to invest some
203       time to pull the distribution from the net, compile and install it and
204       (maybe) read the manuals.
205

GETTING HELP/MORE INFORMATION

207   Q: 3.1    Where can I get information on PDL?
208       The complete PDL documentation is available with the PDL distribution.
209       Use the command "perldoc PDL" to start learning about PDL.
210
211       The easiest way by far, however, to get familiar with PDL is to use the
212       PDL on-line help facility from within the PDL shell, "pdl2"  Just type
213       "pdl2" at your system prompt. Once you are inside the "pdl2" shell type
214       "help" .  Using the "help" and "apropos" commands inside the shell you
215       should be able to find the way round the documentation.
216
217       Even better, you can immediately try your newly acquired knowledge
218       about PDL by issuing PDL/Perl commands directly at the command line. To
219       illustrate this process, here is the record of a typical "pdl2" session
220       of a PDL beginner (lengthy output is only symbolically reproduced in
221       braces ( <... ...> ) ):
222
223           unix> pdl2
224           pdl> help
225           < ... help output ... >
226           pdl> help PDL::QuickStart
227           < ... perldoc page ... >
228           pdl> $a = pdl (1,5,7.3,1.0)
229           pdl> $b = sequence float, 4, 4
230           pdl> help inner
231           < ... help on the 'inner' function ... >
232           pdl> $c = inner $a, $b
233           pdl> p $c
234           [22.6 79.8 137 194.2]
235
236       For further sources of information that are accessible through the
237       Internet see next question.
238
239   Q: 3.2    Are there other PDL information sources on the Internet?
240       First of all, for all purely Perl-related questions there are tons of
241       sources on the net. Good points to start are <http://www.perl.com> and
242       <http://www.perl.org> .
243
244       The PDL home site can be accessed by pointing your web browser to
245       <http://pdl.perl.org> . It has tons of goodies for anyone interested in
246       PDL:
247
248       ·   PDL distributions
249
250       ·   On-line documentation
251
252       ·   Pointers to an HTML archive of the PDL mailing lists
253
254       ·   A list of platforms on which PDL has been successfully tested.
255
256       ·   News about recently added features, ported libraries, etc.
257
258       ·   Name of the current pumpkin holders for the different PDL modules
259           (if you want to know what that means you better had a look at the
260           web pages).
261
262       If you are interested in PDL in general you can join the pdl-general
263       mailing list. This is a forum to discuss programming issues in PDL,
264       report bugs, seek assistance with PDL related problems, etc.
265
266       If you are interested in all the technical details of the ongoing PDL
267       development you can join the pdl-devel mailing list.
268
269       Subscription and current archive links to both mailing lists can be
270       found at <http://pdl.perl.org/?page=mailing-lists>.
271
272       Cross-posting between these lists should be avoided unless there is a
273       very good reason for doing that.
274
275       The PDL project, begun in the late 1990s, has undergone considerable
276       evolution since that time, and the support for it has as well. Thus
277       mailing-list archives are in several places.  Originally pdl-general
278       was called 'perldl', and pdl-devel was called 'pdl-porters'.
279
280       |Time Period | URL                                                   |
281       |------------|-------------------------------------------------------|
282       |1996 - 2004 | http://www.xray.mpe.mpg.de/mailing-lists/perldl/      |
283       |1997 - 2004 | http://www.xray.mpe.mpg.de/mailing-lists/pdl-porters/ |
284       |2005 - 2015 | http://perldl.jach.hawaii.narkive.com/                |
285       |2005 - 2015 | http://pdl-porters.jach.hawaii.narkive.com/           |
286       |2015 -      | https://sourceforge.net/p/pdl/mailman/pdl-general/    |
287       |2015 -      | https://sourceforge.net/p/pdl/mailman/pdl-devel/      |
288       |--------------------------------------------------------------------|
289
290   Q: 3.3    What is the current version of PDL ?
291       As of this writing (FAQ version 1.008 of 21 May 2017) the latest stable
292       version is 2.018.  The latest stable version should always be available
293       from a CPAN mirror site near you (see Question 2.7 for info on where to
294       get PDL).
295
296       The most current (possibly unstable) version of PDL can be obtained
297       from the Git repository, see Question 4.10 and periodic CPAN developers
298       releases of the Git code will be made for testing purposes and more
299       general availability.
300
301   Q: 3.4  How can PDL-2.2 be older than PDL-2.007?
302       Over its development, PDL has used both a single floating point version
303       number (from the versions 1.x through 2.005) at which point it switched
304       to a dotted triple version for 2.1.1 onward---EXCEPT for version 2.2
305       which came out which should have been 2.2.0.  To simplify and unify
306       things, PDL has reverted to a single float version representation with
307       PDL-2.006.  This can cause dependency problems for modules that set a
308       minimum PDL version of 2.2.  The work around it, note that all extant
309       PDL releases have version numbers greater than 2.2.1 so that using 0 as
310       the minimum version will work.
311
312   Q: 3.5    I want to contribute to the further development of PDL. How can I
313       help?
314       Two ways that you could help almost immediately are (1) participate in
315       CPAN Testers for PDL and related modules, and (2) proofreading and
316       clarifying the PDL documentation so that it is most useable for PDL
317       users, especially new users.
318
319       To participate in CPAN Testers and contribute test reports, the page
320       <http://wiki.cpantesters.org/wiki/QuickStart> has instructions for
321       starting for either "CPAN" or "CPANPLUS" users.
322
323       If you have a certain project in mind you should check if somebody else
324       is already working on it or if you could benefit from existing modules.
325       Do so by posting your planned project to the PDL developers mailing
326       list at pdl-devel@lists.sourceforge.net . See the subscription
327       instructions in Question 3.2.  We are always looking for people to
328       write code and/or documentation ;).
329
330   Q: 3.6    I think I have found a bug in the current version of PDL. What
331       shall I do?
332       First, make sure that the bug/problem you came across has not already
333       been dealt with somewhere else in this FAQ.  Secondly, you can check
334       the searchable archive of the PDL mailing lists to find whether this
335       bug has already been discussed.  If you still haven't found any
336       explanations you can post a bug report to
337       pdl-general@lists.sourceforge.net , or through the Bugs link on
338       <http://pdl.perl.org> .  See the BUGS file in the PDL distribution for
339       what information to include.  If you are unsure, discussions via the
340       perldl mailing list can be most helpful.
341

INSTALLATION

343   Q: 4.1    I have problems installing PDL. What shall I do?
344       First make sure you have read the file INSTALL in the distribution.
345       This contains a list of common problems which are unnecessary to repeat
346       here.
347
348       Next, check the file perldl.conf to see if by editing the configuration
349       options in that file you will be able to successfully build PDL. Some
350       of the modules need additional software installed, please refer to the
351       file DEPENDENCIES for further details. Make sure to edit the location
352       of these packages in perldl.conf if you have them in non-standard
353       locations.
354
355       N.B. Unix shell specific: If you would like to save an edited
356       perldl.conf for future builds just copy it as ~/.perldl.conf into your
357       home directory where it will be picked up automatically during the PDL
358       build process.
359
360       Also, check for another, pre-existing version of PDL on the build
361       system.  Multiple PDL installs in the same PATH or @INC can cause
362       puzzling test or build failures.
363
364       If you still can't make it work properly please submit a bug report
365       including detailed information on the problems you encountered to the
366       perldl mailing list ( pdl-general@lists.sourceforge.net , see also
367       above). Response is often rapid.
368
369   Q: 4.2    Are there configuration files for PDL I have to edit?
370       Most users should not have to edit any configuration files manually.
371       However, in some cases you might have to supply some information about
372       awkwardly placed include files/libraries or you might want to
373       explicitly disable building some of the optional PDL modules.  Check
374       the files INSTALL and perldl.conf for details.
375
376       If you had to manually edit perldl.conf and are happy with the results
377       you can keep the file handy for future reference. Place it in
378       ~/.perldl.conf where it will be picked up automatically or use "perl
379       Makefile.PL  PDLCONF=your_file_name" next time you build PDL.
380
381   Q: 4.3    Do I need other software for successful operation?
382       For the basic PDL functionality you don't need any additional software.
383       However, some of the optional PDL modules included in the distribution
384       (notably most graphics and some I/O modules) require certain other
385       libraries/programs to be installed. Check the file DEPENDENCIES in the
386       distribution for details and directions on how to get these.
387
388   Q: 4.4    How can I install PDL in a non-standard location?
389       To install PDL in a non-standard location, use the INSTALL_BASE option
390       in the "perl Makefile.PL" configure step.  For example, "perl
391       Makefile.PL INSTALL_BASE=/mydir/perl5" will configure PDL to install
392       into the tree rooted at "/mydir/perl5".  For more details see "How do I
393       keep my own module/library directory?" in perlfaq8 and subsequent
394       sections.  Another alternative is to use local::lib to do the heavy
395       listing for the needed configuration.
396
397   Q: 4.5    How can I force a completely clean installation?
398       To guarantee a completely clean installation of PDL, you will need to
399       first delete the current installation files and folders.  These will be
400       all directories named "PDL" in the Perl @INC path, files named
401       "*Pdlpp*" in any "Inline" directories, and the programs "pdl, pdldoc,
402       pdl2, perldl, and pptemplate".  Then just build and install as usual.
403       This is much easier to keep track of if you always install "PDL" into a
404       non-standard location.  See Q: 4.4 above.
405

BINARY DISTRIBUTIONS

407   Q: 4.5    What binary distributions are available?
408       Information about binary distributions of PDL can be found on
409       <http://pdl.perl.org> .  At present there are binary distributions of
410       PDL for Linux (RedHat and Debian), FreeBSD, Mac OS X and Windows,
411       though they might not be the most recent version.
412
413       If someone is interested in providing binary distributions for other
414       architectures, that would be very welcome. Let us know on the
415       pdl-devel@lists.sourceforge.net mailing list. Also check your Linux
416       distribution's package manager as many now include PDL.  PPMs for win32
417       versions (both 32bit and 64bit) are also available.
418
419   Q: 4.6    Does PDL run on Linux? (And what about packages?)
420       Yes, PDL does run on Linux and indeed much of the development has been
421       done under Linux. On <http://pdl.perl.org> you can find links to
422       packages for some of the major distributions. Also check your
423       distribution's package manager (yum, apt, urpmi, ...)  as PDL is now
424       found by many of these.
425
426   Q: 4.7    Does PDL run under Windows?
427       PDL builds fine on Win32 using MinGW or Microsoft compilers.  See the
428       win32/INSTALL file in the PDL source distribution for details.  Other
429       compilers have not been tested--input is welcome.  There is also a
430       distribution of PDL through ActiveState's ppm, though it might not
431       always be the latest version.  PDL-2.018 builds out of the box on
432       Strawberry Perl and ActiveState Perl and there are distributions of
433       Strawberry Perl with bundled PDL (see
434       <http://strawberryperl.com/releases.html>).
435

CVS, GIT, AND ON-GOING DEVELOPMENT

437   Q: 4.8    Can I get PDL via CVS?
438       No.  PDL development was conducted with a CVS repository from December
439       1999 to April 2009.  In April 2009 the project switched to the Git
440       version control system (see <http://git-scm.com>).
441
442   Q: 4.9    How do I get PDL via Git?
443       Assume you have Git installed on your system and want to download the
444       project source code into the directory "PDL". To get read-only access
445       to the repository, you type at the command line
446
447          git clone git://github.com/PDLPorters/pdl
448
449       If you wish to submit changes to PDL, you should "fork" the repository
450       from <https://github.com/PDLPorters/pdl>, then clone your fork in the
451       normal fashion.
452
453       To become an official PDL developer, you will need to be added to the
454       GitHub "PDLPorters" organisation.
455
456       For official PDL developers, to get read/write access to the repository
457       type at the command line
458
459          git clone git://github.com/PDLPorters/pdl
460
461       They can still use their own fork; at least one active developer uses
462       that model rather than branches on the main repository.
463
464   Q: 4.10   I had a problem with the Git version, how do I check if someone
465       has submitted a patch?
466       The best way is to check <https://github.com/PDLPorters/pdl/pulls> to
467       see if somebody has submitted a pull request related to your problem.
468
469       In addition, if you are not subscribing to the mailing list, check the
470       archive of the "pdl-devel" and "pdl-general" mailing lists.  See
471       Question 3.2 for details.
472
473   Q: 4.11   I have gotten developer access to Git, how do I upload my
474       changes?
475       The first thing you should do is to read the Git documentation and
476       learn the basics about Git. There are many sources available online.
477       It is very important that you use Git "best practice", with branches,
478       but fortunately this is very easy! Here are the basics.
479
480       Make sure your copy is up to date with the main repo:
481
482          git checkout master
483          git pull --rebase # rebase in case you wrongly changed your own master
484
485       Make a branch:
486
487          git checkout -b mybranch-name
488
489       Commit your changes locally:
490
491          git add <file1> <file2> ...
492          git commit
493
494       or combine these two with:
495
496          git commit -a
497
498       Test the PDL before you push it to the main repository.  If the code is
499       broken for you, then it is most likely broken for others.  Luckily, the
500       rest of this process will test that automatically to help you catch
501       such errors.
502
503       Then update the shared repository with your changes:
504
505          git push -u origin mybranch-name
506
507       This will still leave your changes on a branch, but this is good. Now
508       go to the GitHub page, <https://github.com/PDLPorters/pdl>. It will ask
509       you whether you want to make a "pull request" - you do. Follow the
510       prompts. This will then initiate the automated "continuous integration"
511       tests, on Linux and Windows, with various versions of Perl, with
512       various compilers. You will also want to get at least one other
513       developer to review your changes.
514
515       Once this review process is successfully completed, you can merge your
516       changes to the master branch!
517

PDL JARGON

519   Q: 5.1    What is threading (is PDL a newsreader) ?
520       Unfortunately, in the context of PDL the term threading can have two
521       different (but related) meanings:
522
523       ·   When mentioned in the INSTALL directions and possibly during the
524           build process we have the usual computer science meaning of multi-
525           threading in mind (useful mainly on multiprocessor machines or
526           clusters)
527
528       ·   PDL threading of operations on piddles (as mentioned in the
529           indexing docs) is the iteration of a basic operation over
530           appropriate sub-slices of piddles, e.g. the inner product "inner
531           $a, $b" of a (3) pdl $a and a (3,5,4) pdl $b results in a (5,4)
532           piddle where each value is the result of an inner product of the
533           (3) pdl with a (3) sub-slice of the (3,5,4) piddle.  For details
534           check PDL::Indexing
535
536       PDL threading leads naturally to potentially parallel code which can
537       make use of multi threading on multiprocessor machines/networks; there
538       you have the connection between the two types of use of the term.
539
540   Q: 5.2    What is a piddle?
541       Well, PDL scalar variables (which are instances of a particular class
542       of Perl objects, i.e. blessed thingies (see "perldoc perlobj" )) are in
543       common PDL parlance often called piddles (for example, check the
544       mailing list archives).  Err, clear?  If not, simply use the term
545       piddle when you refer to a PDL variable (an instance of a PDL object as
546       you might remember) regardless of what actual data the PDL variable
547       contains.
548

TECHNICAL QUESTIONS

550   Q: 6.1    What is perldl?   What is pdl2?
551       Sometimes "perldl" ("pdl2") is used as a synonym for PDL. Strictly
552       speaking, however, the name "perldl" ("pdl2") is reserved for the
553       little shell that comes with the PDL distribution and is supposed to be
554       used for the interactive prototyping of PDL scripts. For details check
555       perldl or pdl2.
556
557   Q: 6.2    How do I get on-line help for PDL?
558       Just type "help" (shortcut = "?") at the "pdl2" shell prompt and
559       proceed from there. Another useful command is the "apropos" (shortcut =
560       "??") command.  Also try the "demo" command in the "perldl" or "pdl2"
561       shell if you are new to PDL.
562

MANIPULATION OF PIDDLES

564   Q: 6.3    I want to access the third element of a pdl but $a[2] doesn't
565       work ?!
566       See answer to the next question why the normal Perl array syntax
567       doesn't work for piddles.
568
569   Q: 6.4    The docs say piddles are some kind of array. But why doesn't the
570       Perl array syntax work with piddles then ?
571       OK, you are right in a way. The docs say that piddles can be thought of
572       arrays.  More specifically, it says ( PDL::QuickStart ):
573
574           I find when using the Perl Data Language it is most useful
575           to think of standard Perl @x variables as "lists" of generic
576           "things" and PDL variables like $x as "arrays" which can be
577           contained in lists or hashes.
578
579       So, while piddles can be thought of as some kind of multi-dimensional
580       array they are  not arrays in the Perl sense. Rather, from the point of
581       view of Perl they are some special class (which is currently
582       implemented as an opaque pointer to some stuff in memory) and therefore
583       need special functions (or 'methods' if you are using the OO version)
584       to access individual elements or a range of elements. The
585       functions/methods to check are "at" / "set" (see the section 'Sections'
586       in PDL::QuickStart ) or the powerful "slice" function and friends (see
587       PDL::Slices and PDL::Indexing and especially PDL::NiceSlice ).
588
589       Finally, to confuse you completely, you can have Perl arrays of
590       piddles, e.g. $spec[3] can refer to a pdl representing ,e.g, a
591       spectrum, where $spec[3] is the fourth element of the Perl list (or
592       array ;) @spec .  This may be confusing but is very useful !
593
594   Q: 6.5    How do I concatenate piddles?
595       Most people will try to form new piddles from old piddles using some
596       variation over the theme: "$a =  pdl([$b, 0, 2])" , but this does not
597       work. The way to concatenate piddles is to use the function "cat" (see
598       also "append" and "glue"). Similarly you can split piddles using the
599       command "dog" .
600
601   Q: 6.6    Sometimes I am getting these strange results when using inplace
602       operations?
603       This question is related to the "inplace" function. From the
604       documentation (see PDL::QuickStart):
605
606           Most functions, e.g. log(), return a result which is a
607           transformation of their argument. This makes for good
608           programming practice. However many operations can be done
609           "in-place" and this may be required when large arrays are in
610           use and memory is at a premium. For these circumstances the
611           operator inplace() is provided which prevents the extra copy
612           and allows the argument to be modified. e.g.:
613
614           $x = log($array);          # $array unaffected
615           log( inplace($bigarray) ); # $bigarray changed in situ
616
617       And also from the doc !!:
618
619           Obviously when used with some functions which can not be
620           applied in situ (e.g. convolve()) unexpected effects may
621           occur!
622
623   Q: 6.7    What is this strange usage of the string concatenation operator
624       ".="  in PDL scripts?
625       See next question on assignment in PDL.
626
627   Q: 6.8    Why are there two different kinds of assignment in PDL ?
628       This is caused by the fact that currently the assignment operator "="
629       allows only restricted overloading. For some purposes of PDL it turned
630       out to be necessary to have more control over the overloading of an
631       assignment operator. Therefore, PDL peruses the operator ".=" for
632       certain types of assignments.
633
634   Q: 6.9    How do I set a set of values in a piddle?
635       In Perl 5.6.7 and higher this assignment can be made using lvalue
636       subroutines:
637
638           pdl> $a = sequence(5); p $a
639           [0 1 2 3 4]
640           pdl> $a->slice('1:2') .= pdl([5,6])
641           pdl> p $a
642           [0 5 6 3 4]
643
644       see PDL::Lvalue for more info.  PDL also supports a more matrix-like
645       slice syntax via the PDL::NiceSlice module:
646
647           pdl> $a(1:2) .= pdl([5,6])
648           pdl> p $a
649           [0 5 6 3 4]
650
651       With versions of Perl prior to 5.6.7 or when running under the perl
652       debugger this has to be done using a temporary variable:
653
654           pdl> $a = sequence(5); p $a
655           [0 1 2 3 4]
656           pdl> $tmp = $a->slice('1:2'); p $tmp;
657           [1 2]
658           pdl> $tmp .= pdl([5, 6]);    # Note .= !!
659           pdl> p $a
660           [0 5 6 3 4]
661
662       This can also be made into one expression, which is often seen in PDL
663       code:
664
665           pdl> ($tmp = $a->slice('1:2')) .= pdl([5,6])
666           pdl> p $a
667           [0 5 6 3 4]
668
669   Q: 6.10   Can I use a piddle in a conditional expression?
670       Yes you can, but not in the way you probably tried first. It is not
671       possible to use a piddle directly in a conditional expression since
672       this is usually poorly defined. Instead PDL has two very useful
673       functions: "any" and "all" . Use these to test if any or all elements
674       in a piddle fulfills some criterion:
675
676           pdl> $a=pdl ( 1, -2, 3);
677           pdl> print '$a has at least one element < 0' if (any $a < 0);
678           $a has at least one element < 0
679
680           pdl> print '$a is not positive definite' unless (all $a > 0);
681           $a is not positive definite
682
683   Q: 6.11   Logical operators and piddles -  '||' and '&&' don't work!
684       It is a common problem that you try to make a mask array or something
685       similar using a construct such as
686
687           $mask = which($piddle > 1 && $piddle < 2);   # incorrect
688
689       This  does not work! What you are looking for is the  bitwise logical
690       operators '|' and '&' which work on an element-by-element basis. So it
691       is really very simple: Do not use logical operators on multi-element
692       piddles since that really doesn't make sense, instead write the example
693       as:
694
695           $mask = which($piddle > 1 & $piddle < 2);
696
697       which works correctly.
698

ADVANCED TOPICS

700   Q: 6.12   What is a null pdl ?
701       "null" is a special token for 'empty piddle'. A null pdl can be used to
702       flag to a PDL function that it should create an appropriately sized and
703       typed piddle. Null piddles can be used in places where a PDL function
704       expects an output or temporary argument. Output and temporary arguments
705       are flagged in the signature of a PDL function with the "[o]" and "[t]"
706       qualifiers (see next question if you don't know what the signature of a
707       PDL function is).  For example, you can invoke the "sumover" function
708       as follows:
709
710           sumover $a, $b=null;
711
712       which is equivalent to
713
714           $b = sumover $a;
715
716       If this seems still a bit murky check PDL::Indexing and PDL::PP for
717       details about calling conventions, the signature and threading (see
718       also below).
719
720   Q: 6.13   What is the signature of a PDL function ?
721       The signature of a function is an important concept in PDL.  Many (but
722       not all) PDL function have a signature which specifies the arguments
723       and their (minimal) dimensionality. As an example, look at the
724       signature of the "maximum" function:
725
726           'a(n); [o] b;'
727
728       this says that "maximum" takes two arguments, the first of which is (at
729       least) one-dimensional while the second one is zero-dimensional and an
730       output argument (flagged by the "[o]" qualifier). If the function is
731       called with piddles of higher dimension the function will be repeatedly
732       called with slices of these piddles of appropriate dimension(this is
733       called threading in PDL).
734
735       For details and further explanations consult PDL::Indexing and PDL::PP
736       .
737
738   Q: 6.14   How can I subclass (inherit from) piddles?
739       The short answer is: read PDL::Objects (e.g. type "help PDL::Objects"
740       in the perldl or pdl2 shell).
741
742       The longer answer (extracted from PDL::Objects ): Since a PDL object is
743       an opaque reference to a C struct, it is not possible to extend the PDL
744       class by e.g. extra data via sub-classing (as you could do with a hash
745       based Perl object).  To circumvent this problem PDL has built-in
746       support to extend the PDL class via the has-a relation for blessed
747       hashes. You can get the HAS-A to behave like IS-A simply in that you
748       assign the PDL object to the attribute named "PDL" and redefine the
749       method initialize(). For example:
750
751           package FOO;
752
753           @FOO::ISA = qw(PDL);
754           sub initialize {
755              my $class = shift;
756              my $self = {
757                 creation_time => time(),  # necessary extension :-)
758                 PDL => PDL->null,         # used to store PDL object
759              };
760              bless $self, $class;
761           }
762
763       For another example check the script t/subclass.t in the PDL
764       distribution.
765
766   Q: 6.15   What on earth is this dataflow stuff ?
767       Dataflow is an experimental project that you don't need to concern
768       yourself with (it should not interfere with your usual programming).
769       However, if you want to know, have a look at PDL::Dataflow . There are
770       applications which will benefit from this feature (and it is already at
771       work behind the scenes).
772
773   Q: 6.16   What is PDL::PP?
774       Simple answer: PDL::PP is both a glue between external libraries and
775       PDL and a concise language for writing PDL functions.
776
777       Slightly longer answer: PDL::PP is used to compile very concise
778       definitions into XSUB routines implemented in C that can easily be
779       called from PDL and which automatically support threading, dataflow and
780       other things without you having to worry about it.
781
782       For further details check PDL::PP and the section below on Extensions
783       of PDL.
784
785   Q: 6.17   What happens when I have several references to the same PDL
786       object in different variables (cloning, etc?) ?
787       Piddles behave like Perl references in many respects. So when you say
788
789           $a = pdl [0,1,2,3];
790           $b = $a;
791
792       then both $b and $a point to the same object, e.g. then saying
793
794           $b++;
795
796       will *not* create a copy of the original piddle but just increment in
797       place, of which you can convince yourself by saying
798
799           print $a;
800           [1 2 3 4]
801
802       This should not be mistaken for dataflow which connects several
803       *different* objects so that data changes are propagated between the so
804       linked piddles (though, under certain circumstances, dataflown piddles
805       can share physically the same data).
806
807       It is important to keep the "reference nature" of piddles in mind when
808       passing piddles into subroutines. If you modify the input piddles you
809       modify the original argument, not a copy of it. This is different from
810       some other array processing languages but makes for very efficient
811       passing of piddles between subroutines. If you do not want to modify
812       the original argument but rather a copy of it just create a copy
813       explicitly (this example also demonstrates how to properly check for an
814       explicit request to process inplace, assuming your routine can work
815       inplace):
816
817           sub myfunc {
818              my $pdl = shift;
819              if ($pdl->is_inplace) {
820                 $pdl->set_inplace(0)
821              } else {
822                 # modify a copy by default
823                 $pdl = $pdl->copy
824              }
825              $pdl->set(0,0);
826              return $pdl;
827           }
828

MISCELLANEOUS

830   Q: 6.18   What I/O formats are supported by PDL ?
831       The current versions of PDL already support quite a number of different
832       I/O formats.  However, it is not always obvious which module implements
833       which formats.  To help you find the right module for the format you
834       require, here is a short list of the current list of I/O formats and a
835       hint in which module to find the implementation:
836
837       ·   A home brew fast raw (binary) I/O format for PDL is implemented by
838           the FastRaw module
839
840       ·   The FlexRaw module implements generic methods for the input and
841           output of `raw' data arrays.  In particular, it is designed to read
842           output from FORTRAN 77 UNFORMATTED files and the low-level C
843           "write" function, even if the files are compressed or gzipped.
844
845           It is possible that the FastRaw functionality will be included in
846           the FlexRaw module at some time in the future.
847
848       ·   FITS I/O is implemented by the "wfits"/"rfits" functions in
849           PDL::IO::FITS .
850
851       ·   ASCII file I/O in various formats can be achieved by using the
852           "rcols" and "rgrep" functions, also in PDL::IO::Misc .
853
854       ·   PDL::IO::Pic implements an interface to the NetPBM/PBM+ filters to
855           read/write several popular image formats; also supported is output
856           of image sequences as MPEG movies, animated GIFs and a wide variety
857           of other video formats.
858
859       ·   On CPAN you can find the PDL::NetCDF module that works with PDL
860           2.007.
861
862       For further details consult the more detailed list in the PDL::IO
863       documentation or the documentation for the individual modules.
864
865   Q: 6.19   How can I stack a set of 2D arrays (images) into a 3D piddle?
866       Assuming all arrays are of the same size and in some format recognized
867       by "rpic" (see PDL::IO::Pic ) you could say:
868
869           use PDL::IO::Pic;
870           @names = qw/name1.tif .... nameN.tif/;  # some file names
871           $dummy = PDL->rpic($names[0]);
872           $cube = PDL->zeroes($dummy->type,$dummy->dims,$#names+1); # make 3D piddle
873           for (0..$#names) {
874               # this is the slice assignment
875               ($tmp = $cube->slice(":,:,($_)")) .= PDL->rpic($names[$_]);
876           }
877
878       or
879
880           $cube(:,:,($_)) .= PDL->rpic($names[$_]);
881
882       for the slice assignment using the new PDL::NiceSlice syntax and Lvalue
883       assignments.
884
885       The for loop reads the actual images into a temporary 2D piddle whose
886       values are then assigned (using the overloaded ".=" operator) to the
887       appropriate slices of the 3D piddle $cube .
888
889   Q: 6.20   Where are test files for the graphics modules?
890       This answer applies mainly to PDL::Graphics::TriD (PDL's device
891       independent 3D graphics model) which is the trickiest one in this
892       respect. You find some test scripts in Demos/TriD in the distribution.
893       There are also 3dtest.pl and line3d.pl in the PDL/Example/TriD
894       directory.  After you have built PDL you can do:
895
896           perl -Mblib Example/TriD/3dtest.pl
897
898           perl -Mblib Example/TriD/line3d.pl
899
900       to try the two TriD test programs.  They only exercise one TriD
901       function each but their simplicity makes it easy to debug if needed
902       with the Perl debugger, see perldbug.
903
904       The programs in the Demo directory can be run most easily from the
905       "perldl" or "pdl2" interactive shell:
906
907           perl -Mblib perldl  or  perl -Mblib Perldl2/pdl2
908
909       followed by "demo 3d" or "demo 3d2" at the prompt.  "demo" by itself
910       will give you a list of the available PDL demos.
911
912       You can run the test scripts in the Demos/TriD directory manually by
913       changing to that directory and running
914
915           perl -Mblib <testfile>
916
917       where "testfile" ; should match the pattern "test[3-9].p" and watch the
918       results. Some of the tests should bring up a window where you can
919       control (twiddle) the 3D objects with the mouse. Try using mouse button
920       1 for turning the objects in 3D space, mouse button 3 to zoom in and
921       out, and 'q' to advance to the next stage of the test.
922
923   Q: 6.21   What is TriD or PDL::TriD or PDL::Graphics::TriD?
924       Questions like this should be a thing of the past with the PDL on-line
925       help system in place. Just try (after installation):
926
927           un*x> pdl2
928           pdl> apropos trid
929
930       Check the output for promising hits and then try to look up some of
931       them, e.g.
932
933           pdl> help PDL::Graphics::TriD
934
935       Note that case matters with "help" but not with "apropos" .
936
937   Q: 6.22   PGPLOT does not write out PNG files.
938       There are a few sources of trouble with PGPLOT and PNG files. First,
939       when compiling the pgplot libraries, make sure you uncomment the PNG
940       entries in the drivers.list file. Then when running 'make' you probably
941       got an error like
942
943         C<make: *** No rule to make target `png.h', needed by `pndriv.o'.  Stop.>
944
945       To fix this, find the line in the 'makefile' that starts with
946       'pndriv.o:' (it's near the bottom). Change, for example, ./png.h to
947       /usr/include/png.h, if that is where your header files are (you do have
948       the libpng and libz devel packages, don't you?).  Do this for all four
949       entries on that line, then go back and run "make".
950
951       Second, if you already have the PGPLOT Perl module and PDL installed,
952       you probably tried to write out a PNG file and got fatal error message
953       like:
954
955         C<undefined symbol: png_create_write_struct>
956
957       This is because the PGPLOT Perl module does not automatically link
958       against the png and z libraries. So when you are installing the PGPLOT
959       Perl module (version 2.19) from CPAN, don't do "install PGPLOT", but
960       just do "get PGPLOT". Then exit from CPAN and manually install PGPLOT,
961       calling the makefile thusly:
962
963         C<perl Makefile.PL EXLIB=png,z EXDIR=/usr/lib>
964
965       assuming that there exist files such as /usr/lib/libpng.so.*,
966       /usr/lib/libz.so.*. Then do the standard "make;make test;make install;"
967       sequence. Now you can write png files from PDL!
968

EXTENSIONS OF PDL

970   Q: 7.1    I am looking for a package to do XXX in PDL. Where shall I look
971       for it?
972       The first stop is again "perldl" or "pdl2" and the on-line help or the
973       PDL documentation. There is already a lot of functionality in PDL which
974       you might not be aware of.  The easiest way to look for functionality
975       is to use the "apropos" command:
976
977           pdl> apropos 'integral'
978           ceil            Round to integral values in floating-point format
979           floor           Round to integral values in floating-point format
980           intover         Project via integral to N-1 dimensions
981           rint            Round to integral values in floating-point format
982
983       Since the apropos command is no sophisticated search engine make sure
984       that you search on a couple of related topics and use short phrases.
985
986       However there is a good chance that what you need is not part of the
987       PDL distribution. You are then well advised to check out
988       <http://pdl.perl.org> where there is a list of packages using PDL. If
989       that does not solve your problem, ask on the mailing-list, if nothing
990       else you might get assistance which will let you interface your package
991       with PDL yourself, see also the next question.
992
993   Q: 7.2    Can I access my C/FORTRAN library routines in  PDL?
994       Yes, you can, in fact it is very simple for many simple applications.
995       What you want is the PDL pre-processor PP (PDL::PP ). This will allow
996       you to make a simple interface to your C routine.
997
998       The two functions you need to learn (at least first) are "pp_def" which
999       defines the calling interface to the function, specifying input and
1000       output parameters, and contains the code that links to the external
1001       library. The other command is "pp_end" which finishes the PP
1002       definitions.  For details see the PDL::PP man-page, but we also have a
1003       worked example here.
1004
1005           double eight_sum(int n)
1006           {
1007                int i;
1008                double sum, x;
1009
1010                sum = 0.0; x=0.0;
1011                for (i=1; i<=n; i++) {
1012                  x++;
1013                  sum += x/((4.0*x*x-1.0)*(4.0*x*x-1.0));
1014                }
1015                return 1.0/sum;
1016           }
1017
1018       We will here show you an example of how you interface C code with PDL.
1019       This is the first example and will show you how to approximate the
1020       number 8...
1021
1022       The C code is shown above and is a simple function returning a double,
1023       and expecting an integer - the number of terms in the sum - as input.
1024       This function could be defined in a library or, as we do here, as an
1025       inline function.
1026
1027       We will postpone the writing of the Makefile till later. First we will
1028       construct the ".pd" file. This is the file containing PDL::PP code. We
1029       call this "eight.pd" .
1030
1031           #
1032           # pp_def defines a PDL function.
1033           #
1034           pp_addhdr (
1035           '
1036           double eight_sum(int n)
1037           {
1038             int i;
1039             double sum, x;
1040
1041             sum = 0.0; x=0.0;
1042             for (i=1; i<=n; i++) {
1043              x++;
1044              sum += x/((4.0*x*x-1.0)*(4.0*x*x-1.0));
1045             }
1046            return 1.0/sum;
1047
1048           }
1049           ');
1050
1051           pp_def (
1052                   'eight',
1053                Pars => 'int a(); double [o]b();',
1054                   Code => '$b()=eight_sum($a());'
1055                  );
1056
1057           # Always make sure that you finish your PP declarations with
1058           # pp_done
1059
1060           pp_done();
1061
1062       A peculiarity with our example is that we have included the entire code
1063       with "pp_addhdr" instead of linking it in. This is only for the
1064       purposes of example, in a typical application you will use "pp_addhdr"
1065       to include header files. Note that the argument to "pp_addhdr" is
1066       enclosed in quotes.
1067
1068       What is most important in this example is however the "pp_def" command.
1069       The first argument to this is the name of the new function eight  ,
1070       then comes a hash which the real meat:
1071
1072       ·   This gives the input parameters (here  "a") and the output
1073           parameters (here  "b"). The latter are indicated by the  "[o]"
1074           specifier. Both arguments can have a type specification as shown
1075           here.
1076
1077           Many variations and further flexibility in the interface can be
1078           specified. See "perldoc PDL::PP" for details.
1079
1080       ·   This switch contains the code that should be executed. As you can
1081           see this is a rather peculiar mix of C and Perl, but essentially it
1082           is just as you would write it in C, but the variables that are
1083           passed from PDL are treated differently and have to be referred to
1084           with a preceding '$'.
1085
1086           There are also simple macros to pass pointers to data and to obtain
1087           the values of other Perl quantities, see the manual page for
1088           further details.
1089
1090       Finally note the call to "pp_done()" at the end of the file. This is
1091       necessary in all PP files.
1092
1093       OK. So now we have a file with code that we dearly would like to use in
1094       Perl via PDL. To do this we need to compile the function, and to do
1095       that we need a Makefile.
1096
1097           use PDL::Core::Dev;
1098           use ExtUtils::MakeMaker;
1099           PDL::Core::Dev->import();
1100
1101           $package = ["eight.pd",Eight,PDL::Eight];
1102           %hash = pdlpp_stdargs($package);
1103
1104           WriteMakefile( %hash );
1105
1106           sub MY::postamble {pdlpp_postamble($package)};
1107
1108       The code above should go in a file called Makefile.PL, which should
1109       subsequently be called in  the standard Perl way: "perl Makefile.PL" .
1110       This should give you a Makefile and running "make" should compile the
1111       module for you and "make install" will install it for you.
1112
1113   Q: 7.3    How can I interface package XXX in PDL?
1114       This question is closely related to the previous one, and as we said
1115       there, the PDL::PP pre-processor is the standard way of interfacing
1116       external packages with PDL. The most usual way to use PDL::PP is to
1117       write a short interface routine, see the PDL::PP perldoc page and the
1118       answer to the previous question for examples.
1119
1120       However it is also possible to interface a package to PDL by re-writing
1121       your function in PDL::PP directly. This can be convenient in certain
1122       situations, in particular if you have a routine that expects a function
1123       as input and you would like to pass the function a Perl function for
1124       convenience.
1125
1126       The PDL::PP perldoc page is the main source of information for writing
1127       PDL::PP extensions, but it is very useful to look for files in the
1128       distribution of PDL as many of the core functions are written in
1129       PDL::PP. Look for files that end in ".pd" which is the generally
1130       accepted suffix for PDL::PP files. But we also have a simple example
1131       here.
1132
1133       The following example will show you how to write a simple function that
1134       automatically allows threading. To make this concise the example is of
1135       an almost trivial function, but the intention is to show the basics of
1136       writing a PDL::PP interface.
1137
1138       We will write a simple function that calculates the minimum, maximum
1139       and average of a piddle. On my machine the resulting function is 8
1140       times faster than the built-in function "stats" (of course the latter
1141       also calculates the median).
1142
1143       Let's jump straight in. Here is the code (from a file called
1144       "quickstats.pd" )
1145
1146           #
1147           pp_def('quickstats',
1148                Pars => 'a(n); [o]avg(); [o]max(); [o]min()',
1149                Code => '$GENERIC(a) curmax, curmin;
1150                         $GENERIC(a) tmp=0;
1151                            loop(n) %{
1152                              tmp += $a();
1153                              if (!n || $a() > curmax) { curmax = $a();}
1154                              if (!n || $a() < curmin) { curmin = $a();}
1155                            %}
1156                            $avg() = tmp/$SIZE(n);
1157                         $max() = curmax;
1158                         $min() = curmin;
1159                           '
1160                );
1161
1162           pp_done();
1163
1164       The above might look like a confusing mixture of C and Perl, but behind
1165       the peculiar syntax lies a very powerful language. Let us take it line
1166       by line.
1167
1168       The first line declares that we are starting the definition of a PDL:PP
1169       function called "quickstats" .
1170
1171       The second line is very important as it specifies the input and output
1172       parameters of the function.  a(n) tells us that there is one input
1173       parameter that we will refer to as "a" which is expected to be a vector
1174       of length n (likewise matrices, both square and rectangular would be
1175       written as "a(n,n)" and "a(n,m)" respectively). To indicate that
1176       something is an output parameter we put "[o]" in front of their names,
1177       so referring back to the code we see that avg, max and min are three
1178       output parameters, all of which are scalar (since they have no
1179       dimensional size indicated.
1180
1181       The third line starts the code definition which is essentially pure C
1182       but with a couple of convenient functions.  $GENERIC is a function that
1183       returns the C type of its argument - here the input parameter a. Thus
1184       the first two lines of the code section are variable declarations.
1185
1186       The loop(n) construct is a convenience function that loops over the
1187       dimension called n in the parameter section. Inside this loop we
1188       calculate the cumulative sum of the input vector and keep track of the
1189       maximum and minimum values. Finally we assign the resulting values to
1190       the output parameters.
1191
1192       Finally we finish our function declaration with "pp_done()" .
1193
1194       To compile our new function we need to create a Makefile, which we will
1195       just list since its creation is discussed in an earlier question.
1196
1197           use PDL::Core::Dev;
1198           use ExtUtils::MakeMaker;
1199           PDL::Core::Dev->import();
1200
1201           $package = ["quickstats.pd",Quickstats,PDL::Quickstats];
1202           %hash = pdlpp_stdargs($package);
1203
1204           WriteMakefile( %hash );
1205
1206           sub MY::postamble {pdlpp_postamble($package)};
1207
1208       An example Makefile.PL
1209
1210       Our new statistic function should now compile using the tried and
1211       tested Perl way: "perl Makefile.PL; make" .
1212
1213       You should experiment with this function, changing the calculations and
1214       input and output parameters. In conjunction with the PDL::PP perldoc
1215       page this should allow you to quickly write more advanced routines
1216       directly in PDL::PP.
1217

BUGS

1219       If you find any inaccuracies in this document (or dis-functional URLs)
1220       please report to the perldl mailing list
1221       pdl-general@lists.sourceforge.net.
1222

ACKNOWLEDGMENTS

1224       Achim Bohnet (ach@mpe.mpg.de ) for suggesting CoolHTML as a
1225       prettypodder (although we have switched to XML now) and various other
1226       improvements. Suggestions for some questions were taken from Perl FAQ
1227       and adapted for PDL.
1228

CONTRIBUTORS

1230       Many people have contributed or given feedback on the current version
1231       of the FAQ, here is an incomplete list of individuals whose
1232       contributions or posts to the mailing-list have improved this FAQ at
1233       some point in time alphabetically listed by first name: Christian
1234       Soeller, Chris Marshall, Doug Burke, Doug Hunt, Frank Schmauder, Jarle
1235       Brinchmann, John Cerney, Karl Glazebrook, Kurt Starsinic, Thomas
1236       Yengst, Tuomas J. Lukka.
1237
1239       This document emerged from a joint effort of several PDL developers
1240       (Karl Glazebrook, Tuomas J. Lukka, Christian Soeller) to compile a list
1241       of the most frequently asked questions about PDL with answers.
1242       Permission is granted for verbatim copying (and formatting) of this
1243       material as part of PDL.
1244
1245       Permission is explicitly not granted for distribution in book or any
1246       corresponding form. Ask on the PDL mailing list
1247       pdl-general@lists.sourceforge.net if some of the issues covered in here
1248       are unclear.
1249
1250
1251
1252perl v5.28.1                      2018-05-05                            FAQ(1)
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