princepawn has asked for the wisdom of the Perl Monks concerning the following question:

What a well-done title :-) 1: I have noticed that if hash values are not defined that the hash definition can become askew: example: 
%H = ( 'x' => 'y', 'y' => $undef_var, 'z' = 'a' );

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then sometimes $H{y} might equal 'z' because the elements in the list simply shift up. 2: I tried to use a ternary operator to insure that each value of a hash had some value to avoid the above as follows: 
#!/usr/bin/perl

%p = (
      'a' => 'B',
      'z' => defined($x) ? $x : '',
      'c' => 'D');

use Data::Dumper;
#warn Data::Dumper->Dump([\%p],['p']);
warn $p{z};

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But I am told that something is wrong with my warn line...

Replies are listed 'Best First'.
Re: Undefined Hash Values and Ternary Operators
by KM (Priest) on May 25, 2000 at 21:12 UTC
    I have never come across any random shifting in a hash like you state. If the keys value is undef, then it is undef. I have never seen Perl decide to suddenly change the values of keys. 
    $a = undef;

%H = ( 'x' => 'y', 'y' => $a, 'z' => 'a' );
for (1..10000) { print "FOO\n" if $H{x} ne 'y'; }

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    Nothing prints. The ternary operator work how you are using it. You are likely getting a warning because you are calling warn(), with no conditional:

    warn "foo" unless defined($a);

    Is how you want to use that.

    Cheers,
    KM

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RE: Undefined Hash Values and Ternary Operators
by mdillon (Priest) on May 25, 2000 at 21:19 UTC
    i'm not sure what the problem is. i tried your code above for initializing %H and $H{y} was never 'z'. the following code also works for me: 
    my $undef_var = undef;

my %hash;
@hash{foo, bar, baz} = (1, $undef_var, 2);

for (sort keys %hash)
{
        printf "%s => %s\n", $_, defined $hash{$_} ? $hash{$_} : 'unde
+f';
}

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Re: Undefined Hash Values and Ternary Operators
by Adam (Vicar) on May 25, 2000 at 21:07 UTC
    I used the following script with no problems. 
    #!/usr/bin/perl
$x='y'; # comment this line out to test undef case.

%p = (
      'a' => 'B',
      'z' => defined($x) ? $x : 'n',
      'c' => 'D');

print $p{z}

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    (BTW:When you call warn it will generate a warning with the input you give it, so I'm not surprised by the warning you got. If you intended it as a test clause then you should say warn 'err' if( !defined($x) ); or something like that.)
    Oh, and yes... if you include an undef in an array it will get dropped, which is why your hash was acting funny.
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      It has been said:
      Oh, and yes... if you include an undef in an array it will get dropped, which is why your hash was acting funny.
      This is not true. undef is a valid element of a list, and therefore an array.
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