Re: [Pdl-devel] Fwd: Re: [Pdl-general] Weird behaviour of stats with bad values

[Marek G. <mar...@gm...>]

OK, thank you for your explanations. I'm not sure if all of this makes
sense. If you try

print $s > 5;

in the above example, you will get 'BAD'. 5 is just a scalar and not a bad
value. And $s is a one-dimensional piddle with a value different from the
bad value. Neither of the values in this conditional expression is bad. Why
does it return a string 'BAD'?

Marek

On Tue, 16 Jun 2015 at 14:24 Chris Marshall <dev...@gm...> wrote:

>  This result seems to violate the principle of least surprise.
> On the LHS we have a PDL with badvalue(0) and on the RHS we
> have a perl scalar with value 0 (which happens to be the bad
> value for the PDL).
>
> It seems surprising that $pdl > 0 is actually $pdl > pdl(0)->badvalue(0)
>
> Thoughts?
> Chris
>
>
> -------- Forwarded Message --------  Subject: Re: [Pdl-general] Weird
> behaviour of stats with bad values  Date: Tue, 16 Jun 2015 09:18:29 -0400  From:
> Chris Marshall <dev...@gm...> <dev...@gm...>  To: Derek
> Lamb <de...@bo...> <de...@bo...>, Marek Gierliński
> <mar...@gm...> <mar...@gm...>  CC:
> pdl...@li...
>
>
>  I think the problem is that if $s is a badvalue piddle with the
> badvalue=0
>  then $s>0 is $s>BAD which is BAD.  If you use a value for badvalue that is
>  not in the comparison, you should be good.
>
>  --Chris
>
>  On 6/16/2015 09:11, Derek Lamb wrote:
>
> Thanks for the simple example.  That helps a lot.
>
>  The actual problem is that piddles with the badflags set always evaluate
> to BAD when element-wise logical operations are performed on them.
>
>   "print $s<0" prints BAD instead of 0 or 1.  "print $x>0" also prints
> [BAD BAD BAD BAD].
>
>  $m and $s have their badflags set (as they should, since they were made
> from partially-bad data).  You can check if they can be safely cleared, and
> clear them if so, with $s->check_badflag, etc.  Since $m and $s are
> 1-element piddles, you can also just set them directly to their scalar
> values, for example
>
>  map{$_=$_->sclr} ($m,$s)=stats($x); #if you know they are only going to
> be 1 element piddles
> OR
> map{$_=$_->((0))} ($m,$s)=stats($x); #if you just want the first value
>
>  The fact that $m and $s are 1-element piddles is likely what is causing
> your problem.  My guess is that this behavior has changed (perhaps
> intentionally, perhaps not) between your old version and more recent
> versions, and if you were getting $m and $s as simple perl scalars before
> you would not have seen this problem.  I can verify that stats() has
> returned 1-piddles and not perl scalars at least as far back as PDL-2.4.11.
>
>  cheers,
> Derek
>
>   On Jun 16, 2015, at 6:29 AM, Marek Gierliński <
> mar...@gm...> wrote:
>
>  Oops, sorry, I missed a line when copying the text. Here is the code
> again.
>
>  use PDL;
> use PDL::NiceSlice;
>
>
>  my $x = pdl(1, 2, 3, 0);
> $x->badflag(1);
> $x->badvalue(0);
>
>  print "x = $x\n";
>
>  my ($m, $s) = stats($x);
>
>  print "m = $m, s = $s\n";
> print "s greater than zero\n" if $s > 0;
> print "s less than zero\n" if $s < 0;
> print "s equals zero\n" if $s == 0;
>
>
>  On Tue, 16 Jun 2015 at 12:20 Chris Marshall <dev...@gm...>
> wrote:
>
>>  Hi Marek-
>>
>>  Do you have a complete example showing the problem?
>>  You don't show where $m and $s are being set below.
>>
>>  Thanks,
>>  Chris
>>
>>
>>  On 6/16/2015 07:04, Marek Gierliński wrote:
>>
>> I have recently encountered a weird problem with stats (or statsover)
>> using bad values. The resulting numbers have a strange property, they
>> always return true in comparisons. This code:
>>
>>   use PDL;
>> use PDL::NiceSlice;
>>
>>  my $x = pdl(1, 2, 3, 0);
>> $x->badflag(1); $x->badvalue(0);
>>
>>   print "m = $m, s = $s\n";
>> print "s greater than zero\n" if $s > 0;
>> print "s less than zero\n" if $s < 0;
>> print "s equals zero\n" if $s == 0;
>>
>>  returns the following output:
>>
>>  x = [1 2 3 BAD]
>> m = 2, s = 1
>> s greater than zero
>> s less than zero
>> s equals zero
>>
>>  which doesn't make any sense. Can anyone explain this? I always assumed
>> that 'stats' returned ordinary Perl scalars, but this must be something
>> different.
>>
>>  I'm using PDL version 2.007. We had an older version installed for a
>> long time and I think this problem appeared when we upgraded to 2.007.
>>
>>  Marek
>>
>>
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