Timothy J Massey wrote at about 12:05:02 -0400 on Thursday, May 31, 2012:
> Pascal Mosimann <pascal.mosimann AT alternatique DOT ch> wrote on 05/31/2012
> 11:12:54 AM:
>
> > > Why does BackupPC run the parity command if I've told it not to by
> > passing
> > > it a 0? And how do I return to the 3.1 behavior? According to the
> > > documentation, setting it to 0 should disable it, not cause it to run
>
> > with
> > > a parameter of 0... And if for some reason we would *want* the
> > parity to
> > > run with a parity of 0, could we have a parameter that disables it?
> > Maybe
> > > -1? How do you calculate *negative* parity! :)
> >
> > Hi Tim,
> >
> > Same situation here: I don't want to run par2. Because the archive is
> > done on USB drives and it takes too long.
> >
> > It looks between version 1.16 and 1.17 of BackupPC_archiveHost (see
> > http://backuppc.cvs.sourceforge.net/viewvc/backuppc/BackupPC/bin/
> > BackupPC_archiveHost?r1=1.16&r2=1.17),
> > the condition to run par2 has changed on line 157
> > From
> > ...
> > if ( $parfile != 0 ) {
> > ...
> >
> > to
> > ...
> > if ( length($parfile) ) {
> > ...
> >
> > I've modified the condition back to "if ( $parfile != 0 )" and now it
> > skips the par2 execution.
>
> Thank you for the followup! I actually changed it to:
>
> if ( length($parfile) && $parfile != 0 ) {
>
> (and sent a patch to the list that was seemingly ignored).
>
> My perl-fu is not the greatest; the reason why I changed it to the above
> is to be able to handle two conditions: parfile set to 0, *and* the
> parfile parameter unset (which is the default, by the way) or set to an
> empty string. Perl considers unset variables equal to an empty string,
> which also is equal to 0 in a comparison, but I don't like depending on
> such assumptions. I'd rather the code be explicit about what it's trying
> to accomplish. Maybe that makes me inelegant.
Well, Perl "purposely" does such casting to simplify the type of test
you are attempting to accomplish since it is often awkward to test if
a variable is defined, empty string or zero, so it's not only legal
but in a sense the right way to do things.
Indeed, your code is in a sense not only *inelegant* but *wrong* since
it will spit out a nasty warning about $parfile being uninitialized if
$parfile is unset and you use the (recommended) '-w' flag, since
neither 'length' nor comparisons are expected to operate on
uninitialized variables. Thus, your code is almost certainly less
preferable to the simpler:
if($parfile)
I think that most Perl coders would understand and expect the above to
only return true if the variable $parfile is set to a
non-zero/non-empty string value.
>
> Of course, if you want elegant you could use:
>
> if ( $parfile ) {
Not just elegant but *working* --- your code spits out warnings with
the '-w' flag and it's sloppy programming to write Perl code that
doesn't work with -w flag (indeed, the Perl manpage goes so far as to
say that not making '-w' mandatory is a *bug* in Perl -- i.e., all
Perl code should be written to pass the '-w' (warning) tests).
>
> because Perl will evaulate an undefined value as an empty string, and an
> empty string as a false, and a zero as a false. It's certainly concise...
>
> But like I said, my perl-fu is not the greatest, and I prefer explicit to
> implicit. If this is somehow offensive to the sense of style of Perl
> hackers, then so be it.
It's not about explicit/implicit, it's about code that works, is
transportable, and is (relatively) understandable. I think just about
any non-totally novice PERL programmer understands how undefined
variables are evaluated (and when they give errors) -- it's one of the
key benefits of Perl.
>
> And let me know: I'd truly like to know the "right" way to construct an
> if statement that handles a variable that is unset, set to a null string
> or set to zero. Bonus points for avoiding the warning when using
> -w...
As above, simpler is actually better code and avoids the -w warnings.
i.e, just use:
if($parfile)
> The only way I've been able to handle this in other projects is something
> like:
>
> $variable=0 if not defined $variable;
> if ( $variable != 0 ) {
>
Not sure this is necessary and it will give warning if $variable is
non-numeric i.e. a string.
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