Re: Latest Decoders package 3.0.7+

On Fri, 4 Feb 2005, Chris Peters wrote:

>
> Actually this is not as much of a bug as it might first appear, due to 
> perhaps a
> quirk of perl.
>
> David when I first read your email, your theory looked correct and it made me
> run to check the code (we have ldm running here at TWC and use the metar
> decoder.)  However, I was unable to confirm the bug with a suite of tests I
> wrote.  The tests take some canned metar examples and run them through the
> decoder, checking for known values. A metar rep with 14/00 in the T/Td group
> correctly decoded the dewpoint as 0.

Chris,

you are correct. i greped some raw metar data files and all the 0 TD's
where coded as /M00 none coded as /M0.  so the present code works correctly but
i did the fix incase of future metar coding changes.

thanks,
robb...


>
> It turns out that perl doesn't consider the string "00" to be false.  You can
> confirm this by running the following perl code on your system:
>
> #!/usr/bin/perl -w
> use strict;
>
> print "OK - 1" if ("00");
> print "OK - 2" if ("0");
> print "OK - 3" if (0);
> print "OK - 4" if (00);
>
>
> It will only print out the first line (OK -1).   For some reason, probably
> involving perl's type system and the way it converts strings to numbers, the
> string "00" does NOT evaluate to false.  Fortunately, 99% of the time I 
> believe
> a metar reports a dewpoint/temp of zero as "00" or "M00".  So due to
> coincidence, this bug probably never reared its' head.
>
> You might argue that to be safe, one should go ahead and use the defined test
> anyways, as to not do so is to rely on a coincidence that might change in a
> future version of perl.
>
> Chris
> =======================================================
> Christopher Peters
> Senior Wx Systems Developer - The Weather Channel
> address@hidden
> Phone:  770-226-2039
>
>
>
>
>                       Robb Kambic
>                       <address@hidden        To:       David Larson 
> <address@hidden>
>                       r.edu>                      cc:       address@hidden, 
> decoders
>                       Sent by:                     <address@hidden>
>                       owner-decoders@unida        Subject:  Re: Latest 
> Decoders package 3.0.7+
>                       ta.ucar.edu
>
>
>                       02/04/2005 01:59 PM
>
>
>
>
>
>
> On Fri, 4 Feb 2005, David Larson wrote:
>
> > Robb,
> >
> > There seems to be a problem with the dewpoint processing in the decoders
> > package (since the dawn of time, perhaps) ... The metar2nc script leaves
> > the TD variable as undefined, and therefore it is processed as "missing"
> > when the dewpoint is "00".
> >
> >         # get temperature and dew point
> >         if( s#^(M)?(\d{2})/(M)?(\d{2})?\s+## ) {
> >             $T = $2 ;
> >             $T *= -1 if( $1 ) ;
> >             $TD = $4 if( $4 ) ;
> >             $TD *= -1 if( $3 ) ;
> >         }
> David,
>
> you are correct, TD of value 0 fails to get set with the current code.
> your fix seems reasonable, i'll fix it and make another release.
>
> thanks,
> robb...
>
>
> >
> > The solution is to change the code to use if( defined($4) ) because
> > otherwise a zero value is not considered true and therefore the
> > assignment to TD does not occur.
> >
> > Let me know if you have a better way to handle this.
> > Dave
> >
>
> ===============================================================================
> Robb Kambic                                               Unidata Program 
> Center
> Software Engineer III                                     Univ. Corp for
> Atmospheric Research
> address@hidden                            WWW:
> http://www.unidata.ucar.edu/
> ===============================================================================
>
>
>
>
>

===============================================================================
Robb Kambic                                Unidata Program Center
Software Engineer III                      Univ. Corp for Atmospheric Research
address@hidden             WWW: http://www.unidata.ucar.edu/
===============================================================================