I was surprised at this result when I tried to write a couple of lines of code that would prevent an email from going out to an active mailing list, unless I (as the developer) had expressly set "is_dev_mode" to be 0 or 1 in an anonymous hash supplied as the parameter. Without expressly telling the subroutine whether or not we're in development mode, I figured, the subroutine needed to assume that we are, so as not to actually send an email to all of my important customers. So:

send_email (  {
       subject => "Test subject",
       body    => "Test body",
  } );

sub send_email {

   my $param = shift;

   my $subject     = $param->{subject};
   my $body        = $param->{body};
   my $is_dev_mode = exists $param->{is_dev_mode}
     ? $param->{is_dev_mode}
     : undef;

   $is_dev_mode = 1
       unless ( $is_dev_mode == 0 || $is_dev_mode == 1 )
       ; # In case the CGI param is ''; default is to be "safe" (is_de
+v_mode == 1)

   _blast_email_to_all_my_important_customers( $subject, $body ) unles
+s ( $is_dev_mode );
}

[download]

So I run the code, and... did all my important customers just get an embarrassing test email?

They sure did. Now, let's add "use strict;" and "use warnings;" and try it again, and throw in a warn statement to try to figure out what's happened.

use strict;
use warnings;

send_email (  {
       subject => "Test subject",
       body    => "Test body",
  } );

sub send_email {

   my $param = shift;

   my $subject     = $param->{subject};
   my $body        = $param->{body};
   my $is_dev_mode = exists $param->{is_dev_mode}
     ? $param->{is_dev_mode}
     : undef;

   $is_dev_mode = 1
       unless ( $is_dev_mode == 0 || $is_dev_mode == 1 )
       ; # In case the CGI param is ''; default is to be "safe" (is_de
+v_mode == 1)

   warn "is_dev_mode is '", $is_dev_mode, "'\n";

   _blast_email_to_all_my_important_customers( $subject, $body ) unles
+s ( $is_dev_mode );
}

[download]

Result:

Use of uninitialized value $is_dev_mode in numeric eq (==) at C:\Users
+\davel\Desktop\test-of-dev-mode-flag-revised.pl line 19.
Use of uninitialized value $is_dev_mode in warn at C:\Users\davel\Desk
+top\test-of-dev-mode-flag-revised.pl line 23.
is_dev_mode is ''

[download]

Thanks for the warning, but the customers just got ANOTHER email blast.

OK, I see how $is_dev_mode was undef, and hence the warning. But if $is_dev_mode is undef, then $is_dev_mode is not equal to zero, right? And if $is_dev_mode is undef, then $is_dev_mode is not equal to 1, right? And if neither is true, then the "unless it's true" clause means $is_dev_mode gets 1 as its value (so as to avoid the embarrassing blast to all my important customers), right? Nope! is_dev_mode is still undefined.

An interesting and surprising result to me. I don't think I'll be setting variables to undef and using them in this kind of boolean test again.

I would not have thought that "( $an_undefined_variable == 0 )" would evaluate to true because, golly, undef isn't a number and so the "==" operator is testing whether one number is the same as another number. Now, in hindsight of course, it's apparent that the "==" operator is going to convert both sides to a number in order to chug along and do the best it can, and 0 seems to be the closest thing to a number for undef. I think.

In reply to Surprising result when using undef by davebaker

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