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From: composer@chem.bu.edu (Jeff Kellem)
Newsgroups: comp.lang.perl
Subject: Re: defined() - bug or feature?
Message-ID: <COMPOSER.91Mar21145231@chem.bu.edu>
Date: 21 Mar 91 19:52:31 GMT
References: <1991Mar21.142935.11333@bmers145.bnr.ca>
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Reply-To: composer@chem.bu.edu
Organization: Boston University Chemistry Department
Lines: 37
In-reply-to: friedman@chekov.UU.NET's message of 21 Mar 91 14:29:35 GMT

In article <1991Mar21.142935.11333@bmers145.bnr.ca> friedman@chekov.UU.NET (Barry Friedman) writes:
 > I've noticed the following non-intuitive behaviour of 'defined()':
 > When a variable is used in a string it becomes defined.
 >
 > I don't recall any discussion on this.
 >
 > #!/usr/bin/perl
 >
 > $a='$b is undefined';
 > $a='$b is defined' if defined ($b);
 > print $a,"\n";
 > print "b is now <$b>","\n";
 > $a='$b is defined' if defined ($b);
 > print $a,"\n";
 > __END__
 >
 > produces
 > $b is undefined
 > b is now <>
 > $b is defined

From the man page (actually my Texinfo conversion of the man page) under
the section titled "Data Types and Objects",

    "...An undefined null string may become defined the first time you
    access it, but prior to that you can use the `defined()' operator to
    determine whether the value is defined or not."

Therefore, accessing "$b" in the line `print "b is now <$b>", "\n";'
makes $b defined instead of undefined.

Cheers...

			-jeff

Jeff Kellem
Internet: composer@chem.bu.edu