Beefy Boxes and Bandwidth Generously Provided by pair Networks
Do you know where your variables are?
 
PerlMonks  

Comment on

( #3333=superdoc: print w/ replies, xml ) Need Help??

What you are observing is exactly what dynamic scoping is about: Code called from where the variable is defined can see it, even though it's not in the same lexical scope.

This can be demonstrated without recursion:

use 5.010; use strict; use warnings; sub sayit { say $1 // 'undef'; } do { '42' =~ /(\d+)/ and sayit(); }; sayit(); __END__ 42 undef

Subroutine sayit reads $1 outside of the lexical scope of the block where $1 is set. But since it's in the dynamic scope, it can still see the value.

The sayit call outside the block prints undef\n, which demonstrates that $1 isn't merely a global variable.

As to your actual question:

However, $1, set to '1' by the last successful match at the lowest-but-one level of recursion, is propagated upward unchanged through several levels of subroutine 'blocks'

I see no evidence for that. After $1 is set to '1', exactly one more recursive call happens, and there it is printed out. Then the recursion ends, and you don't print $1 anymore.

Update: after experimenting a bit, I can provide evidence of the phenomen you mentioned:

sub R { printf qq{before: \$_ is '$_'}; printf qq{ \$1 is %s \n}, defined($1) ? qq{'$1'} : 'undefined'; s/(\d+)// ? $1 + R() : 0; printf qq{after: \$_ is '$_'}; printf qq{ \$1 is %s \n}, defined($1) ? qq{'$1'} : 'undefined'; } $_ = 'x55x666x7777x1x'; R(); __END__ before: $_ is 'x55x666x7777x1x' $1 is undefined before: $_ is 'xx666x7777x1x' $1 is '55' before: $_ is 'xxx7777x1x' $1 is '666' before: $_ is 'xxxx1x' $1 is '7777' before: $_ is 'xxxxx' $1 is '1' after: $_ is 'xxxxx' $1 is '1' after: $_ is 'xxxxx' $1 is '1' after: $_ is 'xxxxx' $1 is '1' after: $_ is 'xxxxx' $1 is '1' after: $_ is 'xxxxx' $1 is '1'

This is because there is only one variable $1. It is dynamically scoped, so once it is set in an inner scope, an outer scope sees the modification too.


In reply to Re: 'Dynamic scoping' of capture variables ($1, $2, etc.) by moritz
in thread 'Dynamic scoping' of capture variables ($1, $2, etc.) by AnomalousMonk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":



  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • Outside of code tags, you may need to use entities for some characters:
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.
  • Log In?
    Username:
    Password:

    What's my password?
    Create A New User
    Chatterbox?
    and the web crawler heard nothing...

    How do I use this? | Other CB clients
    Other Users?
    Others musing on the Monastery: (6)
    As of 2014-08-31 08:28 GMT
    Sections?
    Information?
    Find Nodes?
    Leftovers?
      Voting Booth?

      The best computer themed movie is:











      Results (294 votes), past polls