Good practice: Variable declaration

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

Further to emilford's node today about tidy code , and my own query the other day about OO inheritance, here's a new question:

Say I've got a parent class, Word, with subclasses, Noun, Verb & Adjective, and I want to create a reference to one of the subclasses, whose type depends on a file input. To comply with use strict on lexical scope, I reckon I need to start off with a variable declaration, which *doesn't* include an assignment: my $word;. You see this in other languages but I've never seen it in Perl, hence I'm raising it now:

my $word;  #this is it--are you supposed to do this?
if ($part eq 'n')
{   $word = new Noun(@blah);
}
elsif ($part eq 'v')
{   $word = new Verb(@blah);
}
else
{   $word = new Adjective(@blah);
}
print $word->get_english();
#etc etc
[download]

If this isn't the done thing, please could you tell me how you *should* do it.

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Re: Good practice: Variable declaration by japhy (Canon) on Jun 27, 2002 at 16:43 UTC
That's fine. You could be spiffy and write it like: `my %class = qw( n Noun v Verb ); my $word = ($class{$part} \|\| "Adjective")->new(...);` [download] if you feel like being spiffy. _____________________________________________________ Jeff`[japhy]`Pinyan: Perl, regex, and perl hacker, who'd like a job (NYC-area) `s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;`	[reply] [d/l]
Re: Re: Good practice: Variable declaration by Juerd (Abbot) on Jun 27, 2002 at 17:07 UTC
That's fine. You could be spiffy and write it like: my %class = qw( n Noun v Verb ); my $word = ($class{$part} \|\| "Adjective")->new(...); Spiffier. `my $word = ({ qw/n Noun v Verb/ }->{$part} \|\| 'Adjective')->new(...);` [download] - Yes, I reinvent wheels. - Spam: Visit eurotraQ.	[reply] [d/l]
Re: Re: Re: Good practice: Variable declaration by japhy (Canon) on Jun 27, 2002 at 17:13 UTC
I changed my code from that because that doesn't lend itself to repetition. Easier to change one data structure than many lines of code. Although I un-abstracted from what I had originally: `my %class = qw( n Noun v Verb ); my $default = 'Adjective'; ...` [download] _____________________________________________________ Jeff`[japhy]`Pinyan: Perl, regex, and perl hacker, who'd like a job (NYC-area) `s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;`	[reply] [d/l]
Re: Good practice: Variable declaration by mfriedman (Monk) on Jun 27, 2002 at 16:40 UTC
Yes, that's exactly how you're supposed to do it. :) If you want to be really anal, you could always write `my $word = undef;` [download] Which accomplishes the same thing.	[reply] [d/l]
Re: Deciding a class at runtime. by frankus (Priest) on Jun 27, 2002 at 16:40 UTC
This seems correct. Perhaps better to use: `$word = Verb->new(@blah);` [download] But I guess that's a matter of preference unless you're doing multi-inheritence. -- Brother Frankus. ¤	[reply] [d/l]
Re: Good practice: Variable declaration by Aristotle (Chancellor) on Jun 27, 2002 at 17:19 UTC
To comply with use strict on lexical scope, I reckon I need to start off with a variable declaration, which doesn't include an assignment: my $word; No, you only need the `my` in there. Whether you assign something or not doesn't matter any. May I suggest though, that in your case it would be more readable to do it this way: `my $word = $part eq 'n' ? new Noun(@blah) : $part eq 'v' ? new Verb(@blah) : new Adjective(@blah); print $word->get_english();` [download] Update: DOH! Ack. I don't know where I left my brain while I was posting the initial version of this. japhy++ Makeshifts last the longest.	[reply] [d/l]
Re: Re: Good practice: Variable declaration by japhy (Canon) on Jun 27, 2002 at 17:32 UTC
Ouch. Remove those extra `$word =`'s. Not only are they assigning to a package-scoped `$word`, they're screwing up your `?:` code. And you have a ":" where you want a "?", beneath the first "?". `japhy% perl -MO=Deparse,-p my $word = $part eq 'n' ? $word = new Noun(@blah) : $part eq 'v' ? $word = new Verb(@blah) : $word = new Adjective(@blah);` [download] Check that out and you'll see that `$word` ends up getting assigned as an Adjective object no matter what, because of the precedence of `?:` compared with that of `=`. _____________________________________________________ Jeff`[japhy]`Pinyan: Perl, regex, and perl hacker, who'd like a job (NYC-area) `s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;`	[reply] [d/l] [select]


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