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

I need some wisdom. I have routines where information is compared between data base fields and submited CGI paramiters and need to tell the difference between when a value is submitted as blank and when the parameter wasn't specified within a subroutine. For example I will use the routine 'update_info'. In the the first case the value should be set as blank and in the second it should be ignored and left alone.

$object->update_info(value=>"");

Compared to:

$object->update_info();

I'd like to do it with a small amount of code because there are many possible values to address. Please help me find my way. I've hit a wall on it.

Mark Kenney

Replies are listed 'Best First'.
Re: Hashes and keys...
by davidrw (Prior) on Jun 28, 2005 at 01:58 UTC
    It seems like you want something along these lines, where you suck all the args of the sub into a hashref, so that you can use exists to see if a param was passed in or not. 
    sub update_info {
  my $obj = shift;
  my $params = { @_ };
  my $value = exists $params->{value} ? $params->{value} : undef;
  ...
}

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      davidrw, I am sure you meant $params->{value}, but I have two questions (one on personal style): why do you prefer creating a hash reference from an incoming hash (i.e., why not %params = @_?). Also, do you know that my $value = $p->{value} does the same thing as your code? (If the key doesn't exist, the hash lookup returns undef.)

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        sucking the hash into ref would safe some speed and memory (because it has not to be copied). You're other point is correct. A simple way to make it "smarter" is 
        
sub foo
{
  my %defaults = 
  {
    param1 = 'default1',
    param2 = 'default2',
    #etc
  }

  my $self = shift;
  my $params = { @_ };

  for ( keys %defaults )
  {
    $params->{$_} = $defaults{$_} unless defined $params->{$_};
  }
}

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        Also there are parameter checking modules out there.
        I can't remember the names but i'm sure someone will mention them.


        holli, /regexed monk/
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        Yeah, thanks -- that was a typo -- i updated it. I guess it doesn't have to be a hasref .. mostly out of habit.
        I know that my $value = $params->{value}; does the same thing, but the point was to illustrate to OP that you can use exists $params->{value} to see if the parameter was provided or not. Consider a required param: 
        sub foo {
  my $obj = shift;
  my $p = { @_ };
  my $value = exists $p->{value} ? $p->{value} : die "must give 'value
+', even if undef or ''";
  if( exists $p->{optional_but_possibly_blank_key} ){
    ...
  }

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        Basically, depending on your data/requirements, if( $p->{value}) and if(exists $p->{value}) are not the same.
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