Re: Localization method of $1

[Jeff Pinyan <jeffp@crusoe.net>]

On Jan 15, Peter Scott said:

> sub pref { \$_[0], " = ", shift || '(undef)', "\n" }
> 
> $_ = "abc";
> print   "Before setting:  ", pref $1;
> s/(ab)/bar($1)/e;
> print   "After s///:      ", pref $1;
> 
> sub bar {
>    $_[0] =~ /(b)/;
>    print "Inside /e sub:   ", pref $1;
>    print "Original \$1 now: ", pref $_[0];
> # Note: attempt to assign to $_[0] here generates
> # "Modification of a read-only value"
> }

That's because $_[0] is /really, truly/ $1.

  @_ = ("new thing") ;

Will not affect $1.  Assigning to a specific index of @_ will modify the
variable that was passed to the function in that position (see the code
below).

Elements of @_ are aliases to the actual variables.  If, however, you
assign to @_ by a means other than specific indices, this bond is broken.

  $var = 1;
  foo($var);
  sub foo {
    print "$_[0], $var\n";
    $var = 2;
    print "$_[0], $var\n";
    $_[0] = 3;
    print "$_[0], $var\n";
    @_ = (4);
    print "$_[0], $var\n";
    $_[0] = 5;
    print "$_[0], $var\n";
    $var = 6;
    print "$_[0], $var\n";
  }

  1, 1
  2, 2
  3, 3
  4, 3
  5, 3
  5, 6

This is with Perl 5.005_02, just so you know.

-- 

  MIDN 4/C PINYAN, USNR, NROTCURPI     http://www.pobox.com/~japhy/
  jeff pinyan: japhy@pobox.com     perl stuff: japhy+perl@pobox.com
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Follow-Ups from:

Peter Scott <Peter@PSDT.com>

References to:

Peter Scott <Peter@PSDT.com>

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