具有Undef的Perl三元运算符

Question

I am having issues with getting a ternary operator to work using undef. My original code that works:

my $qr = [
    {IP=>'x.x.x.51',Testnet=>'bos-portal-legacy',Owner=>'Amund', Email => 'bosemail'},
    {IP=>'x.x.x.52',Testnet=>'bos-portal-2',Owner=>'Amund', Email => 'boemail2'},
    {IP=>'x.x.x.53',Testnet=>'bos-portal-legacy',Owner=>'Amund', Email => 'bosemail'},
    {IP=>'x.x.x.54',Testnet=>'sqa',Owner=>'Richard', Email => 'sqaemail'},
    {IP=>'x.x.x.55',Testnet=>'sqa',Owner=>'Richard', Email => 'sqaemail'},
    {IP=>'x.x.x.56',Testnet=>'fll-pro',Owner=>'Larry', Email => 'fllemail'},
    {IP=>'x.x.x.57',Testnet=>'fll-pro', Owner=>'', Email => 'fllemail'},
    {IP=>'x.x.x.58',Testnet=>'fll-pro2', Owner=>'', Email => 'flemail2'},
];
my $len = scalar @$qr;
print "Starting of array: $len\n";
my $l = $len;
my $a = @$qr[0]->{Owner};
func ($a);
my @ip;
my %test;
my $name;
my $manager;
my $ip_ref;
my $test_ref;
sub func{
foreach my $emp (@$qr) {
        if ($l > 1 && $emp->{Owner} eq $a) {
            $name = $emp->{Owner};   #to use with email as $a will change as cycle thru
            $manager = $emp->{Manager};
            push (@ip, $emp->{IP});                  #capture all IPs related to owner $name
            $test{$emp->{Testnet}} = $emp->{Email};              #capture unique testnets only related to owner $name
            $l--;                                    #to cycle thru array until reach last row
            print "Finished line: $l\n";
        }

I have some elseif as well, but didn't want to bog this all down. Everything works, and if Owner is undefined in my hash, then when it goes to print it is a blank spot as expected. So it prints like this:

Starting of array: 8
Finished line: 7
Finished line: 6
Finished line: 5
Amund
x.x.x.51, x.x.x.52, x.x.x.53
bos-portal-2 
bos-portal-legacy 
Finished with Amund
Moving on to Richard: 4
Finished line: 3
Richard
x.x.x.54, x.x.x.55
sqa 
Finished with Richard
Moving on to Larry: 2
Larry
x.x.x.56
fll-pro 
Finished with Larry
Moving on to : 1
                     #Blank Spot want to have $name printed as 'Undef' instead
x.x.x.57, x.x.x.58
fll-pro 
fll-pro2 

I want to now make it so that $name is $emp->{Owner} unless Owner is undefined then instead of a blank spot it would say 'Undef.' So I wanted to change all the parts of my code that say $name = $emp->{Owner} to a ternary operation.

I have tried many different uses of the () with undef as that is what the error stated. A couple of similar posts talked about the defined-or usage, which I tried, and other posts stated to use defined, but if I do, then all names become 'Undef' and not just the blank ones. Some of these do nothing and I still get blanks. Others give errors where $a is now uninitialized.

$a = undef ? $name = 'Undefined' : $name = $emp->{Owner};
($a = undef) ? $name = 'Undefined' : $name = $emp->{Owner};
$a = undef($a) ? $name = 'Undefined' : $name = $emp->{Owner};
$name = undef($a) ? 'Undefined' : $emp->{Owner};
$name = $emp->{Owner} // 'Undefined';
defined($a) ? $name = $emp->{Owner} : $name = 'Undef';
$name = defined($a) ? $emp->{Owner} : 'Undef';

Any help would be greatly appreciated. Thank you.

Answer 1

Owner=>''

Empty is not undefined, so

$name = defined($a) ? $emp->{Owner} : 'Undef';

will always return $emp->{Owner} content, even if it's empty. However

$name = $a ? $emp->{Owner} : 'Undef';

Will substitute 'Undef' if $a is empty and $emp->{Owner} if not, because $a is defined, but empty, ie 'False'

upd: there is sort of an uglier form of a ternary operator. it should be used only for fun, because it makes code quite a bit less readable:

$name =  $a && $emp->{Owner} || 'Undef';

Answer 2

The answer you want is:

$name = $emp->{Owner} || 'No name provided';

If $emp->{Owner} is not a false value (example: undefined, zero, empty string...) then it will be assigned. Otherwise, Perl will assign whatever is to the right of the operator.

Also, let me add that undef is a function that changes the argument to an undefined value and always returns an undefined value. For example, a very common mistake is to do something like this:

if (undef $var) {
   # do something
}

This is not testing if $var is undefined. Rather, it is making $var undefined and then testing the expression---which will be false-ish, so this if block will never be executed.

That's why defined is recommended instead:

if (!defined $var) {
  # do something
}

This will test $var without changing it.

However, as others have noted, your variable was not really undefined to begin with.

Answer 3

You need to test for defined , not undef .

assuming $a is undef , your last example should work:

$name = defined($a) ? $emp->{Owner} : 'Undef';

However, maybe you need to test for an empty string as well. undef is not the same as an empty string; if you want the same action for both, you'll need to test each condition explicitly.