Compare two (potentially) undef variables in perl

Question

I am trying to compare two variables in which are usually strings. These variables are generated from a database, $var1 from one db and $var2 from another. When I compare them in a loop I use the ne operator. However there are times when I these variables are null or undef. The comparison is done as follows:

foreach my $var1 (@$varlist)
{
  if ($var1 ne $var2)
  {
    print "vars are not equal";
  }
}

The issue is that if $var1 or $var2 are undef then I get an error. However, I need to be able to compare then values as undef b/c I will have to write them. I considered converting the variables to a string 'NULL' and then back but that seemed inefficient.

Any way to fix this? Thanks!

Answer 1

Check if they are defined, too:

foreach my $var1 (@$varlist)
    if ( ! defined $var1 || ! defined $var2 || $var1 ne $var2 )
        print "vars are not equal";

This prints that they're not equal if both are undefined. If you want another behaviour, just change the if expression.

Answer 2

It's not an error to compare undefined values, it's just a warning. I like using Perl's // operator (requires >=v5.10) in cases like this to ensure the operators are defined:

if (($var1 // '') ne ($var2 // '')) { ... }

will treat an undefined string as the empty string during the comparison, for example.

Since you want the operands to have a specific value when they are printed (NULL was one possibility), you could also consider using the //= operator.

if (($var1 //= 'NULL') ne ($var2 //= 'NULL')) {
   print "$var1 and $var2 are not equal";
}

will use the value of $var1 or $var2, or 'NULL' if they are undefined, in the comparison.