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

Hello,

I'm perplexed on this line, from the camel book, second edition, pg 540:

Similarily, set default values with:

 
$pi || = 3;

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Which, doesn't really makes sense, and doesn't well, work anyways, am I missing something?

I personally am trying to shorten this piece of code: 

 
unless($list_info{subscribed_message} ne "" || exists $list_info{subsc
+ribed_message})

{$list_info{subscribed_message} = $subscribed_message}

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Which is trying to set a default variable to something (a copy of a tied hash) that may not be there. There's about 10 of those nasty lines, so a for loop looks inticing.

Any help will be apreciated :)

Replies are listed 'Best First'.
Re: Error in Camel Book Ver2?
by redcloud (Parson) on Dec 21, 2000 at 10:14 UTC

    Well, look at that example line this way:

    $pi ||= 3;

    is like saying:

    $pi = $pi || 3;

    Now, the || operator short circuits if the first operand is true, so if $pi has any value that is true (i.e. it is not 0, "", or undef -- any others?) then the value of the right hand side will be $pi, otherwise it'll be 3. So, this is a way to say that the default value of $pi is 3, because that's what it'll be set to if it hasn't been set to something else already.

    So, in your case, you might do:

    $list_info{subscribed_message} ||= $subscribed_message;

    Make sense?

    Update:This doesn't work, though if "" is a valid value.

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Re: Error in Camel Book Ver2?
by chipmunk (Parson) on Dec 21, 2000 at 10:23 UTC
    Nope, no error there. $pi ||= 3 is basically equivalent to $pi = ($pi || 3) which could be described as "If $pi doesn't already have a true value, then set $pi's value to 3." (The 'true' part should not be overlooked.)

    So, you could write your assignment as: $list_info{subscribed_message} ||= $subscribed_message; That will set the value of $list_info{subscribed_message} to the value of $subscribed_message, if $list_info{subscribed_message} doesn't exist, or is undef, "", or 0.

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      if I make a script that says:

       
$pi ||= 3
print $pi;

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      I'll get back something like

       
syntax error at or.pl line 1, near "|| ="

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      So the way its written won't work

      I wonder if this will be different if your testing a hash, since you use exists() instead of defined();

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        syntax error at or.pl line 1, near "|| ="

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        That's a Perl4 (or earlier) error message. Please upgrade to a version of Perl that was released sometime after the Spice Girls hit the pop charts.

        -- Randal L. Schwartz, Perl hacker

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        The code that you provide there is not the code giving you that error message. The error message you list indicates that you have an extraneous space in ||=. ||= should not have any spaces.

        The code you provided will give you an entirely different error, namely syntax error at - line 2, near "print" since you didn't have a semicolon at the end of the first line.

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Re: Error in Camel Book Ver2?
by Dominus (Parson) on Dec 22, 2000 at 08:31 UTC
    Says nobody in particular: 
    unless($list_info{subscribed_message} ne "" 
       || exists $list_info{subscribed_message}) {
    $list_info{subscribed_message} = $subscribed_message
}

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    The first thing I notice here is that your test is redundant. If x ne "" is true, then exists x must also be true. (Or equivalently, if exists $h{k} is not true, then k is not in the hash, so $h{k} must be equal to "".) So you should rewrite this as: 

    if ($list_info{subscribed_message} eq "") {
  $list_info{subscribed_message} = $subscribed_message;
}

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    which is something of an improvement already.

    Now, perhaps this would be a good place to use a mutator subroutine?

      sub set_if_not_set_already {
    $_[0] = $_[1] if $_[0] eq ""; 
  }

  set_if_not_set_already($list_info{subscribed_message},
                         $subscribed_message);

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    Or you could abbreviate a little futher and get rid of the mutator: 
      sub initialize_list_info {
    $list_info{$_[0]} = $_[1]
        if list_info{$_[0]} eq ""; 
  }

  initialize_list_info('subscribed_message', $subscribed_message);

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    This is neither a recommendation nor a disrecommendation. Just something to consider.

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