null values

[fluffyvoidwarrior]

Hello Monks, Using Activestate perl, Apache 2and CGI package. I'm returning variable values from web page form user input. eg - $myvariable = $q->param( "myinput" ); I then test for a null value with eg - if ($myvariable eq ""){ #ie if user didnt fill in field exit; #don't try to work on a null value } Although my scripts work ok I'm filling up my apache error log with drivel like - "...Use of uninitialized value in string eq at ..." It seems that an uninitialised variable is not the same as a null variable. How do I test for an uninitialised variable? I've hunted through quite a lot of big fat perl books to no avail. This seems like an embarrassingly dumb question, sorry. Masters, you are my only hope .... Thanks

[monarch]

This has to be one of the most common initial questions that comes up about perl.

if ( defined( $q->param( "myinput" ) ) ) {
    print( $q->param( "myinput" ) );
}

or

my $value = $q->param( "myinput" );
print( $value ) if ( $value );

The above only works if $value is never going to be zero, because zero evaluates to false. A value of undef or zero is false. The following is safer:

my $value = $q->param( "myinput" );
print( $value ) if ( defined( $value ) );

[fluffyvoidwarrior]

Thanks People

[robartes]

perldoc -f defined

[polettix]

I sometimes find it useful to define a wrapper function to set a default value:

sub defaulted_param {
   my ($param_name, $default_value) = @_;
   return $default_value unless defined $q->param($param_name);
   return $q->param($param_name);
}

(assuming $q is global, of course).

[mrborisguy]

For future reference, I believe, if I can remember right, that usually when $q->param($param_name); is used, a person is using mod_perl. In this case, you should always pass $q to the sub as well, otherwise bad things tend to happen (at least they always did for me!).

    -Bryan

[LanceDeeply]

no one mentioned my personal favorite: setting a default value using the or operator...

$myvariable = $q->param( "myinput" ) || ""; 
if ($myvariable eq "")
{
   # no warnings
}