1BADVALUES(1)          User Contributed Perl Documentation         BADVALUES(1)
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NAME

6       PDL::BadValues - Discussion of bad value support in PDL
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DESCRIPTION

9   What are bad values and why should I bother with them?
10       Sometimes it's useful to be able to specify a certain value is 'bad' or
11       'missing'; for example CCDs used in astronomy produce 2D images which
12       are not perfect since certain areas contain invalid data due to
13       imperfections in the detector.  Whilst PDL's powerful index routines
14       and all the complicated business with dataflow, slices, etc etc mean
15       that these regions can be ignored in processing, it's awkward to do. It
16       would be much easier to be able to say "$c = $x + $y" and leave all the
17       hassle to the computer.
18
19       If you're not interested in this, then you may (rightly) be concerned
20       with how this affects the speed of PDL, since the overhead of checking
21       for a bad value at each operation can be large.  Because of this, the
22       code has been written to be as fast as possible - particularly when
23       operating on ndarrays which do not contain bad values.  In fact, you
24       should notice essentially no speed difference when working with
25       ndarrays which do not contain bad values.
26
27       You may also ask 'well, my computer supports IEEE NaN, so I already
28       have this'.  They are different things; a bad value signifies "leave
29       this out of processing", whereas NaN is the result of a mathematically-
30       invalid operation.
31
32       Many routines, such as "y=sin(x)", will propagate NaN's without the
33       user having to code differently, but routines such as "qsort", or
34       finding the median of an array, need to be re-coded to handle bad
35       values.  For floating-point datatypes, "NaN" and "Inf" can be used to
36       flag bad values, but by default special values are used (Default bad
37       values).
38
39       There is one default bad value for each datatype, but as of PDL 2.040,
40       you can have different bad values for separate ndarrays of the same
41       type.
42
43       You can use "NaN" as the bad value for any floating-point type,
44       including complex.
45
46   A quick overview
47        pdl> $x = sequence(4,3);
48        pdl> p $x
49        [
50         [ 0  1  2  3]
51         [ 4  5  6  7]
52         [ 8  9 10 11]
53        ]
54        pdl> $x = $x->setbadif( $x % 3 == 2 )
55        pdl> p $x
56        [
57         [  0   1 BAD   3]
58         [  4 BAD   6   7]
59         [BAD   9  10 BAD]
60        ]
61        pdl> $x *= 3
62        pdl> p $x
63        [
64         [  0   3 BAD   9]
65         [ 12 BAD  18  21]
66         [BAD  27  30 BAD]
67        ]
68        pdl> p $x->sum
69        120
70
71       "demo bad" within perldl or pdl2 gives a demonstration of some of the
72       things possible with bad values.  These are also available on PDL's
73       web-site, at http://pdl.perl.org/demos/.  See PDL::Bad for useful
74       routines for working with bad values and t/bad.t to see them in action.
75
76       To find out if a routine supports bad values, use the "badinfo" command
77       in perldl or pdl2 or the "-b" option to pdldoc.
78
79       Each ndarray contains a flag - accessible via "$pdl->badflag" - to say
80       whether there's any bad data present:
81
82       •   If false/0, which means there's no bad data here, the code supplied
83           by the "Code" option to pp_def() is executed.
84
85       •   If true/1, then this says there MAY be bad data in the ndarray, so
86           use the code in the "BadCode" option (assuming that the pp_def()
87           for this routine has been updated to have a BadCode key).  You get
88           all the advantages of broadcasting, as with the "Code" option, but
89           it will run slower since you are going to have to handle the
90           presence of bad values.
91
92       If you create an ndarray, it will have its bad-value flag set to 0. To
93       change this, use "$pdl->badflag($new_bad_status)", where
94       $new_bad_status can be 0 or 1.  When a routine creates an ndarray, its
95       bad-value flag will depend on the input ndarrays: unless over-ridden
96       (see the "CopyBadStatusCode" option to "pp_def"), the bad-value flag
97       will be set true if any of the input ndarrays contain bad values.  To
98       check that an ndarray really contains bad data, use the "check_badflag"
99       method.
100
101       NOTE: propagation of the badflag
102
103       If you change the badflag of an ndarray, this change is propagated to
104       all the children of an ndarray, so
105
106          pdl> $x = zeroes(20,30);
107          pdl> $y = $x->slice('0:10,0:10');
108          pdl> $c = $y->slice(',(2)');
109          pdl> print ">>c: ", $c->badflag, "\n";
110          >>c: 0
111          pdl> $x->badflag(1);
112          pdl> print ">>c: ", $c->badflag, "\n";
113          >>c: 1
114
115       This is also propagated to the parents of an ndarray, so
116
117          pdl> print ">>a: ", $x->badflag, "\n";
118          >>a: 1
119          pdl> $c->badflag(0);
120          pdl> print ">>a: ", $x->badflag, "\n";
121          >>a: 0
122
123       There's also the issue of what happens if you change the badvalue of an
124       ndarray - should these propagate to children/parents (yes) or whether
125       you should only be able to change the badvalue at the 'top' level -
126       i.e. those ndarrays which do not have parents.
127
128       The orig_badvalue() method returns the compile-time value for a given
129       datatype. It works on ndarrays, PDL::Type objects, and numbers - eg
130
131         $pdl->orig_badvalue(), byte->orig_badvalue(), and orig_badvalue(4).
132
133       To get the current bad value, use the badvalue() method - it has the
134       same syntax as orig_badvalue().
135
136       To change the current bad value, supply the new number to badvalue - eg
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138         $pdl->badvalue(2.3), byte->badvalue(2), badvalue(5,-3e34).
139
140       Note: the value is silently converted to the correct C type, and
141       returned - i.e. "byte->badvalue(-26)" returns 230 on my Linux machine.
142
143       Note that changes to the bad value are NOT propagated to previously-
144       created ndarrays - they will still have the bad flag set, but suddenly
145       the elements that were bad will become 'good', but containing the old
146       bad value.  See discussion below.
147
148   Bad values and boolean operators
149       For those boolean operators in PDL::Ops, evaluation on a bad value
150       returns the bad value. This:
151
152        $mask = $img > $thresh;
153
154       correctly propagates bad values. This will omit any bad values, but
155       return a bad value if there are no good ones:
156
157        $bool = any( $img > $thresh );
158
159       As of 2.077, a bad value used as a boolean will throw an exception.
160
161       When using one of the 'projection' functions in PDL::Ufunc - such as
162       orover - bad values are skipped over (see the documentation of these
163       functions for the current handling of the case when all elements are
164       bad).
165

IMPLEMENTATION DETAILS

167       A new flag has been added to the state of an ndarray - "PDL_BADVAL". If
168       unset, then the ndarray does not contain bad values, and so all the
169       support code can be ignored. If set, it does not guarantee that bad
170       values are present, just that they should be checked for.
171
172       The "pdl_trans" structure has been extended to include an integer
173       value, "bvalflag", which acts as a switch to tell the code whether to
174       handle bad values or not. This value is set if any of the input
175       ndarrays have their "PDL_BADVAL" flag set (although this code can be
176       replaced by setting "FindBadStateCode" in pp_def).
177
178   Default bad values
179       The default bad values are now stored in a structure within the Core
180       PDL structure - "PDL.bvals" (eg Basic/Core/pdlcore.h.PL); see also
181       "typedef badvals" in Basic/Core/pdl.h.PL and the BOOT code of
182       Basic/Core/Core.xs.PL where the values are initialised to (hopefully)
183       sensible values.  See "badvalue" in PDL::Bad and "orig_badvalue" in
184       PDL::Bad for read/write routines to the values.
185
186       The default/original bad values are set to the C type's maximum
187       (unsigned integers) or the minimum (floating-point and signed
188       integers).
189
190   How do I change a routine to handle bad values?
191       See "BadCode" in PDL::PP and "HandleBad" in PDL::PP.
192
193       If you have a routine that you want to be able to use as in-place, look
194       at the routines in bad.pd (or ops.pd) which use the "in-place" option
195       to see how the bad flag is propagated to children using the
196       "xxxBadStatusCode" options.  I decided not to automate this as rules
197       would be a little complex, since not every in-place op will need to
198       propagate the badflag (eg unary functions).
199
200       This all means that you can change
201
202          Code => '$a() = $b() + $c();'
203
204       to
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206          BadCode => 'if ( $ISBAD(b()) || $ISBAD(c()) ) {
207                        $SETBAD(a());
208                      } else {
209                        $a() = $b() + $c();
210                      }'
211
212       leaving Code as it is. PP::PDLCode will then create code something like
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214          if ( __trans->bvalflag ) {
215               broadcastloop over BadCode
216          } else {
217               broadcastloop over Code
218          }
219

WHAT ABOUT DOCUMENTATION?

221       One of the strengths of PDL is its on-line documentation. The aim is to
222       use this system to provide information on how/if a routine supports bad
223       values: in many cases pp_def() contains all the information anyway, so
224       the function-writer doesn't need to do anything at all! For the cases
225       when this is not sufficient, there's the "BadDoc" option. For code
226       written at the Perl level - i.e. in a .pm file - use the "=for bad" pod
227       directive.
228
229       This information will be available via man/pod2man/html documentation.
230       It's also accessible from the "perldl" or "pdl2" shells - using the
231       "badinfo" command - and the "pdldoc" shell command - using the "-b"
232       option.
233

AUTHOR

235       Copyright (C) Doug Burke (djburke@cpan.org), 2000, 2006.
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237       The per-ndarray bad value support is by Heiko Klein (2006).
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241perl v5.38.0                      2023-07-21                      BADVALUES(1)
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