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Substr ref anomoly (revisited)

by BrowserUk (Pope)
on Jun 30, 2004 at 05:58 UTC ( #370685=perlquestion: print w/ replies, xml ) Need Help??
BrowserUk has asked for the wisdom of the Perl Monks concerning the following question:

Can anyone understand why the two lists output by this test program are different? Note the first word of both lists.

Update: Added the references to the outputs to show they hadn't changed.

Update2: This code won't do anything useful before 5.8.3.

#! perl -slw use strict; my $s = 'the quick brown fox jumps over the lazy dog'; my @wordRefs; print do{ my $ref = \substr( $s, $-[ 0 ], $+[ 1 ] - $-[ 0 ] ); push @wordRefs, $ref; "$ref : $$ref"; } while $s =~ m[(\S+\s*)]g; print $/, '---', $/; print "$_ : $$_" for @wordRefs; __END__ P:\test>subrefs LVALUE(0x182caf8) : the LVALUE(0x1831624) : quick LVALUE(0x18315d0) : brown LVALUE(0x1824334) : fox LVALUE(0x18243f4) : jumps LVALUE(0x1831660) : over LVALUE(0x1831678) : the LVALUE(0x1831690) : lazy LVALUE(0x18316a8) : dog --- LVALUE(0x182caf8) : dog LVALUE(0x1831624) : quick LVALUE(0x18315d0) : brown LVALUE(0x1824334) : fox LVALUE(0x18243f4) : jumps LVALUE(0x1831660) : over LVALUE(0x1831678) : the LVALUE(0x1831690) : lazy LVALUE(0x18316a8) : dog

The first list is produced as the references are stacked. The second is produced from those same references. Why are they different?


Examine what is said, not who speaks.
"Efficiency is intelligent laziness." -David Dunham
"Think for yourself!" - Abigail
"Memory, processor, disk in that order on the hardware side. Algorithm, algoritm, algorithm on the code side." - tachyon

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Re: Substr ref anomoly (revisited)
by davido (Archbishop) on Jun 30, 2004 at 06:15 UTC

    I get the same output from both prints (in other words, expected behavior), using ActiveState's Perl version 5.8.4 for Win32.


    Dave

      Thanks. Loks like the bugs fix (5.8.3) required a bug fix:) Time to upgrade again.


      Examine what is said, not who speaks.
      "Efficiency is intelligent laziness." -David Dunham
      "Think for yourself!" - Abigail
      "Memory, processor, disk in that order on the hardware side. Algorithm, algoritm, algorithm on the code side." - tachyon
Re: Substr ref anomoly (revisited)
by tachyon (Chancellor) on Jun 30, 2004 at 06:34 UTC

    AS 5.6.1 behaves in a different manner. Note how the ref is always the same.....

    C:\>perl test.pl LVALUE(0x1aae9d0) : the LVALUE(0x1aae9d0) : quick LVALUE(0x1aae9d0) : brown LVALUE(0x1aae9d0) : fox LVALUE(0x1aae9d0) : jumps LVALUE(0x1aae9d0) : over LVALUE(0x1aae9d0) : the LVALUE(0x1aae9d0) : lazy LVALUE(0x1aae9d0) : dog --- LVALUE(0x1aae9d0) : dog LVALUE(0x1aae9d0) : dog LVALUE(0x1aae9d0) : dog LVALUE(0x1aae9d0) : dog LVALUE(0x1aae9d0) : dog LVALUE(0x1aae9d0) : dog LVALUE(0x1aae9d0) : dog LVALUE(0x1aae9d0) : dog LVALUE(0x1aae9d0) : dog C:\>perl -v This is perl, v5.6.1 built for MSWin32-x86-multi-thread (with 1 registered patch, see perl -V for more detail) Copyright 1987-2001, Larry Wall Binary build 635 provided by ActiveState Corp. http://www.ActiveState. +com Built 15:34:21 Feb 4 2003

    cheers

    tachyon

      Good point. I forgot to mention that it wouldn't work at all before 5.8.3, but I thought the "one lvalue per substr" bug was fixed in 5.8.3. From davidos reponse it looks like it was nearly fixed in 5.8.3 and finally fixed in 5.8.4.


      Examine what is said, not who speaks.
      "Efficiency is intelligent laziness." -David Dunham
      "Think for yourself!" - Abigail
      "Memory, processor, disk in that order on the hardware side. Algorithm, algoritm, algorithm on the code side." - tachyon

      I noticed that under 5.6.1 when doing some DBI work. It took me forever to realize that every element in the @data array was actually being set to the same address.

      while( my $row = $sth->fetchrow_hashref() ) { push @data, $row; }

      I thought it had something funky to do with the $row ref being outside of the while loop's curly-brace scope (is this a closure?). I solved the problem thusly:

      while( my $ref = $sth->fetchrow_hashref() ) { my $row = $ref; push @data, $row; }

      I never saw a problem with row ordering. And, from my logic, this really shouldn't work. It reads (to me) that I'm copying the $ref reference into the $row variable, instead of creating a new reference. But, for whatever reason, the array started working the way I figured it should.

      I had chalked this up as my not completely understanding the logic and scoping of references.

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