Re: [MacPerl] FNG

[Christian Brechbuehler <brech@vision.ee.ethz.ch>]

Kevin Dowd wrote:

> $myEmail =  'dowd%40ndirect.co.uk';
> $myEmail = ~ s/%40/@/g;
> print "$myEmail";
> 
> All I get back is 4294967295.

Surprising, isn't it?  :-)  But correct. 



Excerpt of followup (sent 27 January 1999) by Geoffrey C Kinnel:

>  0) There is no string specified to search (because =~ was not used) so
> the substitution tries to operate on nothing

No, the substitution operates on $_, which in the above program has the
undefined value.

>  1) the substitution fails, returning a 0 (the number of substitutions)

It does not for me.  See below.

>  2) you assign $myEmail the value of the bitwise negation of 0,
> represented as a 32 bit integer [...] which happens to be 4294967295

Agreed.  (In hexadecimal this is 0xffffffff, which looks much less
random.)

                              *****

Ok, Kevin Kowd's problem is long solved, but leaves me with the subtle 
question, "What does s/// return on failure?".  Not quite 0 !  (I'm
using perl, version 5.004_04 built for sun4-solaris, but hope this
doesn't matter!)

Sure I get the number of substiturions performed when it succeeds.
But when it fails, I get a weird kind of empty string.  The following
program,

    1      #! /usr/local/bin/perl -w
    2      
    3      $_ = "no match";
    4      $x = s/%40/@/;
    5      $y = "";
    6      $notx = ~$x;
    7      $noty = ~$y;
    8      $str_equal = $x eq $y;
    9      $num_equal = $x == $y;
    10 
    11   print "'$x', '$y', $str_equal, $num_equal, '$notx', '$noty'\n";

warns me that Argument "" isn't numeric in eq at line 9,
and prints

    '', '', 1, 1, '4294967295', ''

Huh?  Both $x and $y contain the empty string, they are equals as
strings and numerically, but the bitwise negation operator works
differently on them!  I would have expected bitwise flip of all
characters in a string, as it happens for $y.
    But $x, the result of the substitution, seems slightly more
numeric than the genuine string $y.  Note that line 9 chages $y !!
After the comparison, ~$y would evaluate to 4294967295, no longer "".
BTW, the warning was about $y, not $x.  Perl thinks there is no side
effect, and documents its belief with the warning to the following
program:

    $x = "1234"; print ~$x, "\n";
    $x+12;       print ~$x, "\n";

I get:

    Useless use of addition in void context at -e line 2.
    лкни
    4294966061

First, $x is a string.  Then it is a number. My question: How many
kinds of "scalars" exist?


	Christian Brechbuehler
	
	Communication Technology Laboratory, Image Science Group
	Swiss Federal Institute of Technology (ETH), Zurich

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