1TIPS(1) User Contributed Perl Documentation TIPS(1)
2
6 PDL::Tips - Small tidbits of useful arcana. Programming tidbits and
7 such.
8
10 use PDL;
11 # Whatever happens here.
13
15 This page documents useful idioms, helpful hints and tips for using
16 Perl Data Language v2.0.
17 Help
19 Use "help help" within perldl or the "pdldoc" program from the command
21 line for access to the PerlDL documentation. HTML versions of the
22 pages should also be present, in the HtmlDocs/PDL directory of the PDL
23 distribution. To find this directory, try the following
24 perldl> foreach ( map{"$_/PDL/HtmlDocs"}@INC ) { p "$_\n" if -d $_ }
26 Indexing idioms
28 The following code normalizes a bunch of vectors in $a. This works
30 regardless of the dimensionality of $a.
31 $a /= $a->sumover->dummy(0);
33 What is actually happening?
35 If you want to see what the code is actually doing, try the command
37 PDL::Core::set_debugging(1);
39 somewhere. This spews out a huge amount of debug info for PDL into STD‐
41 OUT. It is planned to eventually make this redirectable and the mes‐
42 sages selectable more accurately.
43 Many of the messages come from "Basic/Core/pdlapi.c" and you can look
45 at the source to see what is going on.
46 If you have any extra time to work on these mechanisms, infrom the pdl-
48 porters mailing list.
49 Memory savings
51 If you are running recursively something that selects certain indices
53 of a large piddle, like
54 while(1) {
56 $inds = where($a>0);
57 $a = $a->index($inds);
58 $b = $b->index($inds);
59 func($b,$a);
60 }
61 If you are not writing to $b, it saves a lot of memory to change this
63 to
64 $b = $b->index($inds)->sever;
66 The new method "sever" is a causes the write-back relation to be for‐
68 gotten. It is like copy except it changes the original piddle and
69 returns it).
70 Of course, the probably best way to do the above is
72 $inds = xvals ($a->long);
74 while(1) {
75 $inds0 = where($a>0);
76 $inds1 = $inds->index($inds)->sever;
77 $a = $a0->index($inds1);
78 $b = $b->index($inds1)->sever;
79 func($b,$a);
80 }
81 which doesn't save all the temporary instances of $a in memory. See
83 "mandel.pl" in the Demos subdirectory of the PerlDL distribution for an
84 example.
85 PP speed
87 If you really want to write speedy PP code, the first thing you need to
89 do is to make sure that your C compiler is allowed to do the necessary
90 optimizations.
91 What this means is that you have to allow as many variables as possible
93 to go into registers:
94 loop(a) %{
96 $a() += $COMP(foo_member) * $b()
97 %}
98 expands to
100 for(i=0; i<10000; i++) {
102 a[i] += __privtrans->foo_member * b[i];
103 }
104 is about the worst you can do, since your C compiler is not allowed to
106 assume that "a" doesn't clobber "foo_member" which completely inhibits
107 vectorization. Instead, do
108 float foo = $COMP(foo_member);
110 loop(a) %{
111 $a() += foo * $b();
112 %}
113 This is not a restriction caused by PP but by ANSI C semantics. Of
115 course, we could copy the struct into local varibles and back but that
116 could cause very strange things sometimes.
117 There are many other issues on organizing loops.
119 We are currently planning to make PP able to do fixed-width things as
121 well as physical piddles (where looping over the first dimensions would
122 be cheaper as there are less distinct increments, which might make a
123 difference on machines with a small number of registers).
124
126 Copyright (C) Tuomas J. Lukka 1997. All rights reserved. Duplication
127 in the same form and printing a copy for yourself allowed.
128perl v5.8.8 2003-05-21 TIPS(1)