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>I have a problem with the following perl script:

> #!/usr/local/bin/perl -w
> $ip="213.234.124.46";
> $mask = "255.255.255.0";
> @mask=split ('\.', $mask);
> @ip=split ('\.', $ip);
> $x = $mask[0];
> $y = $ip[0];
> $z = $x & ($y);
> $u = $x & ($y+0);
> print ("x = $x, y = $y, x&y = $z ($u)\n");


Are you understanding string versus numeric anding? Here's a good
article on the matter... but I suppose I'd better more strongly
document it.

Date: 07 Nov 1995 06:18:01 -0800
From: merlyn@stonehenge.com (Randal L. Schwartz)
Subject: Re: '&' operator on strings
To: h.kim@intelsat.int
Reply-to: merlyn@stonehenge.com
In-reply-to: h.kim@intelsat.int's message of Tue, 7 Nov 1995 00:53:52
Organization: Stonehenge Consulting Services; Portland, Oregon, USA
Newsgroups: comp.lang.perl.misc
Article 10877 of comp.lang.perl.misc:
X-Newsreader: Gnus v5.0.12


>>>>> "Hong" == Hong S Kim <h.kim@intelsat.int> writes:

Hong> Hi,
Hong> I have installed perl5.001m on our HP715 box and the installation
Hong> went smoothly.

Hong> My colleague was trying an example from "Teach Yourself Perl in 21
Hong> Days" which states that strings are the treated the same as
Hong> numbers. Specifically that both

Hong> 1. $answer = 124.3 & 99;
Hong> 2. $answer = "124.3" & "99";

Hong> should yield 96. Number (2) however gives the value 10. Why?

Unfortunately, there are two ways to think of bit operators on strings:

(1) interpret the strings as numbers, convert the numbers to integers,
perform the operation, and then re-float that back to a number, or
(2) interpret the strings as *bit patterns*, and perform the bit operation
directly on strings.

For many moons (perl 2 to perl 4), Perl used interpretation #1
*unless* there was a vec() present in the program, and then it used
interpretation #2. In an effort to eliminate "unexpected action at a
distance" from perl 5, Larry made the bit-ops *always* use
interpretation #2. If you want to bit-and the numeric value of the strings,
just make them a number yourself:

$answer = (0+"124.3") & (0+"99");

Sure, it's a little more hassle, but well worth the elimination of
"action at a distance".

Yes, this means that:

print "01" & "1";

will print 1 in perl4 (in absence of a vec), and 0 in perl5. Oh well.

print "Just another Perl hacker," # but not what the media calls "hacker!" :-)
# legal fund: $14811.84 collected, $129659.85 spent; email fund@stonehenge.com
for details--
Name: Randal L. Schwartz / Stonehenge Consulting Services (503)777-0095
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