The reason I tested all three situations: if ( $query != 0 or $query !=
undef or $query != ' ' ) {
was for knowledge and repitition that everytime I looked at the code I
would know what all 3 meant and that they had the same meaning, 0. In
addition I wanted to be anal !
> Maybe a dumb question (since I have no routine with Net::DNS::Resolver):
> Are there alway two elements in array @a if everything went fine?
No. There are actually 0-9 elements in this array without the
"svr" string search
> btw: If the 1st FOR LOOP loop failed, @a is empty, so maybe you should
move
> the last 3 lines in the 1st FOR LOOP if branch.
tkx for the pointer! So here is what you mean?
foreach ($query->answer) {
if ( $_->type eq 'NS' ) {
#next unless $_->type eq "A";
foreach ($_->nsdname) {
push (@a,$_) if $_ =~ "svr";
$e++;
} # END 2nd FOR LOOP
} else {
print F "DNS query failed: ", $res->errorstring, "\n";
} # END 2nd IF
@a = sort { $a cmp $b } @a;
print F $a[0],"\n";
print F $a[1],"\n";
} # END 1st FOR LOOP
} # END 1st IF
Derek B. Smith
OhioHealth IT
UNIX / TSM / EDM Teams
"Wiggins
d'Anconia"
<[EMAIL PROTECTED] To
.org> [EMAIL PROTECTED]
cc
03/02/2005 09:15 beginners@perl.org
AM Subject
Re: Net::DNS
John Doe wrote:
> Hi Derek,
> hi all (see below, please help)
>
>
>>[...]
>>
>>> my $res = Net::DNS::Resolver->new;
>>> my $query = $res->query(".ohnet", "NS");
>>>
>>>
>>> if ( $query != 0 or $query != undef or $query != ' ' ) {
>
>
> Please replace this condition by
>
> if ($query)
>
> I'm to confused at the moment to explain why in detail, sorry...
>
Your question below answers why.....
>
> ==================
>
> hi all
>
> while trying to prove by code that the above if condition is incorrect...
>
> My logic says that a condition in the form
> if ($a != $val1 or $a != $val2)
> is always true.
Unless both are false.... which is a basic premise of 'or'.
>
> sub test {print "condition is true\n" if $_[0] != 1 or $_[0] != 2}
> test (1); test (2); test (3);
> # this prints 3 times as expected:
> condition is true
>
> ... I struggled over something when I included "undef" in some code
tests:
>
> print "Oups!\n" if 0==undef;
> # prints:
> Oups! # <<<<<<<< ?????
>
> Can anybody please explain this result to me.
>
> print "Oups!\n" if !defined 0;
> # prints nothing as expected.
>
The key here is that the == and != operators put their operands into
numerical context. 'undef' in numerical context resolves to 0. So the
first case is true. In the second case you are *specifically* checking
for definedness (not matching a value), and 0 *is* defined.
>
> I thought that a had a clue of perl after using it intensively for years.
> I'm not shure anymore...
>
Understanding undefined vs. true/false vs. 0 vs. some other value is
definitely a difficult thing to grasp and unusual to HLLs where context
can automagically switch values.
So back to your confusion above, you want to check,
if ($query)
Because 'undef' resolves to false. However, if $query could contain a
false value (namely 0) then you would have to check specifically for
undefinedness,
if (defined $query)
and this becomes a nasty bug when not being paid special attention, but
is also one reason why turning of initialization warnings is often a bad
idea.
Just to blow your mind, look around for "zero but true" ;-)....
>
>
> greetings joe
>
http://danconia.org
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