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Id: 132538
Status: resolved
Priority: 0/
Queue: perl5

Owner: jkeenan <jkeenan [at] cpan.org>
Requestors: calestyo [at] scientia.net
Cc:
AdminCc:

Operating System: (no value)
PatchStatus: HasPatch
Severity: low
Type: docs
Perl Version: (no value)
Fixed In: (no value)

Attachments
132538-0001-Clarify-different-cases-of-assignment-to-list-of-sca.patch



To: perlbug [...] perl.org
CC: root [...] localhost.perl.org
Date: Thu, 7 Dec 2017 05:00:20 +0100 (CET)
From: calestyo [...] scientia.net
Subject: perldata doesn't tell what happens on list assignment, with less/more elements
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This is a bug report for perl from calestyo@scientia.net, generated with the help of perlbug 1.40 running under perl 5.26.1. ----------------------------------------------------------------- perldata explains most things related to lists, arrays, etc. and their assignments. However, I feel it misses one crucial point, namely what happens when one assigns a list/array with more respectively less elements to a list, e.g. @more=(1, 2, 3, 4); @less=(1, 2); $a="a"; $b="b"; $c="c"; and with that each of the following then the cases: ($a, $b, $c) = @more; ($a, $b, $c) = @less; ($a, $b, $c) = (1, 2, 3, 4); ($a, $b, $c) = (1, 2); Seems the result is, that if there is - as above - no array or hash in the list like in: ($a, $b, @X) = ... any additional elements on the right-hand side (be it from a list or an array) are discarded... while if there are not enough elements (again from a list or an array on the right-hand side), their corresponding elements on the left-hand side list become undef. Especially the later, i.e. that after the assignment of a too "short" list/array, $c doesn't retain the previous value of "c" isn't necessarily obvious and should thus be documented :) Cheers, Chris. ----------------------------------------------------------------- --- Flags: category=docs severity=wishlist --- Site configuration information for perl 5.26.1: Configured by Debian Project at Tue Nov 28 17:44:14 UTC 2017. Summary of my perl5 (revision 5 version 26 subversion 1) configuration: Platform: osname=linux osvers=4.9.0 archname=x86_64-linux-gnu-thread-multi uname='linux localhost 4.9.0 #1 smp debian 4.9.0 x86_64 gnulinux ' config_args='-Dusethreads -Duselargefiles -Dcc=x86_64-linux-gnu-gcc -Dcpp=x86_64-linux-gnu-cpp -Dld=x86_64-linux-gnu-gcc -Dccflags=-DDEBIAN -Wdate-time -D_FORTIFY_SOURCE=2 -g -O2 -fdebug-prefix-map=/build/perl-w2lU16/perl-5.26.1=. -fstack-protector-strong -Wformat -Werror=format-security -Dldflags= -Wl,-z,relro -Dlddlflags=-shared -Wl,-z,relro -Dcccdlflags=-fPIC -Darchname=x86_64-linux-gnu -Dprefix=/usr -Dprivlib=/usr/share/perl/5.26 -Darchlib=/usr/lib/x86_64-linux-gnu/perl/5.26 -Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 -Dvendorarch=/usr/lib/x86_64-linux-gnu/perl5/5.26 -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl/5.26.1 -Dsitearch=/usr/local/lib/x86_64-linux-gnu/perl/5.26.1 -Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1 -Dsiteman3dir=/usr/local/man/man3 -Duse64bitint -Dman1ext=1 -Dman3ext=3perl -Dpager=/usr/bin/sensible-pager -Uafs -Ud_csh -Ud_ualarm -Uusesfio -Uusenm -Ui_libutil -Ui_xlocale -Uversiononly -DDEBUGGING=-g -Doptimize=-O2 -dEs -Duseshrplib -Dlibperl=libperl.so.5.26.1' hint=recommended useposix=true d_sigaction=define useithreads=define usemultiplicity=define use64bitint=define use64bitall=define uselongdouble=undef usemymalloc=n default_inc_excludes_dot=define bincompat5005=undef Compiler: cc='x86_64-linux-gnu-gcc' ccflags ='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-strict-aliasing -pipe -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64' optimize='-O2 -g' cppflags='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-strict-aliasing -pipe -I/usr/local/include' ccversion='' gccversion='7.2.0' gccosandvers='' intsize=4 longsize=8 ptrsize=8 doublesize=8 byteorder=12345678 doublekind=3 d_longlong=define longlongsize=8 d_longdbl=define longdblsize=16 longdblkind=3 ivtype='long' ivsize=8 nvtype='double' nvsize=8 Off_t='off_t' lseeksize=8 alignbytes=8 prototype=define Linker and Libraries: ld='x86_64-linux-gnu-gcc' ldflags =' -fstack-protector-strong -L/usr/local/lib' libpth=/usr/local/lib /usr/lib/gcc/x86_64-linux-gnu/7/include-fixed /usr/include/x86_64-linux-gnu /usr/lib /lib/x86_64-linux-gnu /lib/../lib /usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt perllibs=-ldl -lm -lpthread -lc -lcrypt libc=libc-2.25.so so=so useshrplib=true libperl=libperl.so.5.26 gnulibc_version='2.25' Dynamic Linking: dlsrc=dl_dlopen.xs dlext=so d_dlsymun=undef ccdlflags='-Wl,-E' cccdlflags='-fPIC' lddlflags='-shared -L/usr/local/lib -fstack-protector-strong' Locally applied patches: DEBPKG:debian/cpan_definstalldirs - Provide a sensible INSTALLDIRS default for modules installed from CPAN. DEBPKG:debian/db_file_ver - https://bugs.debian.org/340047 Remove overly restrictive DB_File version check. DEBPKG:debian/doc_info - Replace generic man(1) instructions with Debian-specific information. DEBPKG:debian/enc2xs_inc - https://bugs.debian.org/290336 Tweak enc2xs to follow symlinks and ignore missing @INC directories. DEBPKG:debian/errno_ver - https://bugs.debian.org/343351 Remove Errno version check due to upgrade problems with long-running processes. DEBPKG:debian/libperl_embed_doc - https://bugs.debian.org/186778 Note that libperl-dev package is required for embedded linking DEBPKG:fixes/respect_umask - Respect umask during installation DEBPKG:debian/writable_site_dirs - Set umask approproately for site install directories DEBPKG:debian/extutils_set_libperl_path - EU:MM: set location of libperl.a under /usr/lib DEBPKG:debian/no_packlist_perllocal - Don't install .packlist or perllocal.pod for perl or vendor DEBPKG:debian/fakeroot - Postpone LD_LIBRARY_PATH evaluation to the binary targets. DEBPKG:debian/instmodsh_doc - Debian policy doesn't install .packlist files for core or vendor. DEBPKG:debian/ld_run_path - Remove standard libs from LD_RUN_PATH as per Debian policy. DEBPKG:debian/libnet_config_path - Set location of libnet.cfg to /etc/perl/Net as /usr may not be writable. DEBPKG:debian/mod_paths - Tweak @INC ordering for Debian DEBPKG:debian/perlivp - https://bugs.debian.org/510895 Make perlivp skip include directories in /usr/local DEBPKG:debian/deprecate-with-apt - https://bugs.debian.org/747628 Point users to Debian packages of deprecated core modules DEBPKG:debian/squelch-locale-warnings - https://bugs.debian.org/508764 Squelch locale warnings in Debian package maintainer scripts DEBPKG:debian/patchlevel - https://bugs.debian.org/567489 List packaged patches for 5.26.1-3 in patchlevel.h DEBPKG:fixes/document_makemaker_ccflags - https://bugs.debian.org/628522 [rt.cpan.org #68613] Document that CCFLAGS should include $Config{ccflags} DEBPKG:debian/find_html2text - https://bugs.debian.org/640479 Configure CPAN::Distribution with correct name of html2text DEBPKG:debian/perl5db-x-terminal-emulator.patch - https://bugs.debian.org/668490 Invoke x-terminal-emulator rather than xterm in perl5db.pl DEBPKG:debian/cpan-missing-site-dirs - https://bugs.debian.org/688842 Fix CPAN::FirstTime defaults with nonexisting site dirs if a parent is writable DEBPKG:fixes/memoize_storable_nstore - [rt.cpan.org #77790] https://bugs.debian.org/587650 Memoize::Storable: respect 'nstore' option not respected DEBPKG:debian/makemaker-pasthru - https://bugs.debian.org/758471 Pass LD settings through to subdirectories DEBPKG:debian/makemaker-manext - https://bugs.debian.org/247370 Make EU::MakeMaker honour MANnEXT settings in generated manpage headers DEBPKG:debian/kfreebsd-softupdates - https://bugs.debian.org/796798 Work around Debian Bug#796798 DEBPKG:fixes/autodie-scope - https://bugs.debian.org/798096 Fix a scoping issue with "no autodie" and the "system" sub DEBPKG:fixes/memoize-pod - [rt.cpan.org #89441] Fix POD errors in Memoize DEBPKG:debian/hurd-softupdates - https://bugs.debian.org/822735 Fix t/op/stat.t failures on hurd DEBPKG:fixes/math_complex_doc_great_circle - https://bugs.debian.org/697567 [rt.cpan.org #114104] Math::Trig: clarify definition of great_circle_midpoint DEBPKG:fixes/math_complex_doc_see_also - https://bugs.debian.org/697568 [rt.cpan.org #114105] Math::Trig: add missing SEE ALSO DEBPKG:fixes/math_complex_doc_angle_units - https://bugs.debian.org/731505 [rt.cpan.org #114106] Math::Trig: document angle units DEBPKG:fixes/cpan_web_link - https://bugs.debian.org/367291 CPAN: Add link to main CPAN web site DEBPKG:fixes/time_piece_doc - https://bugs.debian.org/817925 Time::Piece: Improve documentation for add_months and add_years DEBPKG:fixes/extutils_makemaker_reproducible - https://bugs.debian.org/835815 https://bugs.debian.org/834190 Make perllocal.pod files reproducible DEBPKG:fixes/file_path_hurd_errno - File-Path: Fix test failure in Hurd due to hard-coded ENOENT DEBPKG:debian/hppa_op_optimize_workaround - https://bugs.debian.org/838613 Temporarily lower the optimization of op.c on hppa due to gcc-6 problems DEBPKG:debian/installman-utf8 - https://bugs.debian.org/840211 Generate man pages with UTF-8 characters DEBPKG:fixes/file_path_chmod_race - https://bugs.debian.org/863870 [rt.cpan.org #121951] Prevent directory chmod race attack. DEBPKG:fixes/extutils_file_path_compat - Correct the order of tests of chmod(). (#294) DEBPKG:fixes/getopt-long-2 - [rt.cpan.org #120300] Withdraw part of commit 5d9947fb445327c7299d8beb009d609bc70066c0, which tries to implement more GNU getopt_long campatibility. GNU DEBPKG:fixes/getopt-long-3 - provide a default value for optional arguments DEBPKG:fixes/getopt-long-4 - https://bugs.debian.org/864544 [rt.cpan.org #122068] Fix issue #122068. DEBPKG:fixes/test-builder-reset - https://bugs.debian.org/865894 Reset inside subtest maintains parent DEBPKG:debian/hppa_opmini_optimize_workaround - https://bugs.debian.org/869122 Lower the optimization level of opmini.c on hppa DEBPKG:debian/sh4_op_optimize_workaround - https://bugs.debian.org/869373 Also lower the optimization level of op.c and opmini.c on sh4 DEBPKG:fixes/json-pp-example - [rt.cpan.org #92793] https://bugs.debian.org/871837 fix RT-92793: bug in SYNOPSIS DEBPKG:debian/perldoc-pager - https://bugs.debian.org/870340 [rt.cpan.org #120229] Fix perldoc terminal escapes when sensible-pager is less DEBPKG:debian/prune_libs - https://bugs.debian.org/128355 Prune the list of libraries wanted to what we actually need. DEBPKG:debian/configure-regen - https://bugs.debian.org/762638 Regenerate Configure et al. after probe unit changes DEBPKG:fixes/rename-filexp.U-phase1 - regen-configure: rename filexp.U to filexp_path.U, phase 1 DEBPKG:fixes/rename-filexp.U-phase2 - regen-configure: rename filexp.U to filexp_path.U, phase 2 DEBPKG:fixes/packaging_test_skips - Skip various tests if PERL_BUILD_PACKAGING is set --- @INC for perl 5.26.1: /etc/perl /usr/local/lib/x86_64-linux-gnu/perl/5.26.1 /usr/local/share/perl/5.26.1 /usr/lib/x86_64-linux-gnu/perl5/5.26 /usr/share/perl5 /usr/lib/x86_64-linux-gnu/perl/5.26 /usr/share/perl/5.26 /usr/local/lib/site_perl /usr/lib/x86_64-linux-gnu/perl-base --- Environment for perl 5.26.1: HOME=/home/calestyo LANG=en_DE.UTF-8 LANGUAGE (unset) LD_LIBRARY_PATH (unset) LOGDIR (unset) PATH=/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games PERL_BADLANG (unset) SHELL=/bin/bash
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On Thu, 07 Dec 2017 04:00:45 GMT, calestyo@scientia.net wrote: Show quoted text
> > This is a bug report for perl from calestyo@scientia.net, > generated with the help of perlbug 1.40 running under perl 5.26.1. > > > ----------------------------------------------------------------- > perldata explains most things related to lists, arrays, etc. and > their assignments. > > However, I feel it misses one crucial point, namely what happens > when one assigns a list/array with more respectively less elements > to a list, e.g. > > @more=(1, 2, 3, 4); > @less=(1, 2); > $a="a"; $b="b"; $c="c"; > > and with that each of the following then the cases: > ($a, $b, $c) = @more; > ($a, $b, $c) = @less; > ($a, $b, $c) = (1, 2, 3, 4); > ($a, $b, $c) = (1, 2); > > > Seems the result is, that if there is - as above - no array or hash > in the list like in: > ($a, $b, @X) = ... > any additional elements on the right-hand side (be it from a list > or an array) are discarded... while if there are not enough > elements (again from a list or an array on the right-hand side), > their corresponding elements on the left-hand side list become > undef. > Especially the later, i.e. that after the assignment of a too "short" > list/array, $c doesn't retain the previous value of "c" isn't > necessarily obvious and should thus be documented :) > > Cheers, > Chris. >
Please review the patch attached. Thank you very much. -- James E Keenan (jkeenan@cpan.org)
Subject: 132538-0001-Clarify-different-cases-of-assignment-to-list-of-sca.patch
From e01ff6d79771f0c623f6be7a477289c8edd26d35 Mon Sep 17 00:00:00 2001 From: James E Keenan <jkeenan@cpan.org> Date: Thu, 7 Dec 2017 16:09:54 -0500 Subject: [PATCH] Clarify different cases of assignment to list of scalars. Prepared in response to RT #132538. --- pod/perldata.pod | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 50 insertions(+) diff --git a/pod/perldata.pod b/pod/perldata.pod index 9d714f2..b093e3a 100644 --- a/pod/perldata.pod +++ b/pod/perldata.pod @@ -778,6 +778,56 @@ As of Perl 5.22, you can also use C<(undef)x2> instead of C<undef, undef>. (You can also do C<($x) x 2>, which is less useful, because it assigns to the same variable twice, clobbering the first value assigned.) +Note that assignment of one list of scalars to another list of scalars is +B<not> the same as assigning that first list to an array. + +When you assign a list of scalars to an array, all previous values in that +array are wiped out and number of elements in the array will now be equal to +the number of elements in the right-hand list -- the list from which +assignment was made. The array will automatically resize itself to precisely +accommodate each element in the right-hand list. + + use warnings; + my (@xyz, $x, $y, $z); + + @xyz = (1, 2, 3); + print "@xyz\n"; # 1 2 3 + + @xyz = ('al', 'be', 'ga', 'de'); + print "@xyz\n"; # al be ga de + + @xyz = (101, 102); + print "@xyz\n"; # 101 102 + +When, however, you assign a list of scalars to another list of scalars, the +results differ according to whether the left-hand list -- the list being +assigned to -- has the same, more or fewer elements than the right-hand list. + + ($x, $y, $z) = (1, 2, 3); + print "$x $y $z\n"; # 1 2 3 + + ($x, $y, $z) = ('al', 'be', 'ga', 'de'); + print "$x $y $z\n"; # al be ga + + ($x, $y, $z) = (101, 102); + print "$x $y $z\n"; # 101 102 + # Use of uninitialized value $z in concatenation (.) + # or string at [program] line [line number]. + +If the number of scalars in the left-hand list is less than that in the +right-hand list, the "extra" scalars in the right-hand list will simply not be +assigned. + +If the number of scalars in the left-hand list is greater than than in the +left-hand list, the "missing" scalars will become undefined. + + ($x, $y, $z) = (101, 102); + for my $el ($x, $y, $z) { + (defined $el) ? print "$el " : print "<undef>"; + } + print "\n"; + # 101 102 <undef> + List assignment in scalar context returns the number of elements produced by the expression on the right side of the assignment: -- 2.7.4
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On Thu, 07 Dec 2017 13:18:52 -0800, jkeenan wrote: Show quoted text
> Please review the patch attached.
Show quoted text
> +Note that assignment of one list of scalars to another list of scalars is > +B<not> the same as assigning that first list to an array.
I find this sentence confusing. I think the rest of your patch clarifies things sufficiently that you can just leave out this sentence. Show quoted text
> +array are wiped out and number of elements in the array will now be equal to
*the* number Show quoted text
> +If the number of scalars in the left-hand list is greater than than in the
s/than tha\Kn/t/ -- Father Chrysostomos
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On Fri, 08 Dec 2017 00:55:01 GMT, sprout wrote: Show quoted text
> On Thu, 07 Dec 2017 13:18:52 -0800, jkeenan wrote:
> > Please review the patch attached.
>
> > +Note that assignment of one list of scalars to another list of > > scalars is > > +B<not> the same as assigning that first list to an array.
> > I find this sentence confusing. I think the rest of your patch > clarifies things sufficiently that you can just leave out this > sentence. >
> > +array are wiped out and number of elements in the array will now be > > equal to
> > *the* number >
> > +If the number of scalars in the left-hand list is greater than than > > in the
> > s/than tha\Kn/t/
Pushed to blead with those corrections in commit 436908e565f0e613465123e7cb08fa54487c3b8f. Marking ticket Resolved. Thank you very much. -- James E Keenan (jkeenan@cpan.org)
Date: Sun, 10 Dec 2017 05:24:56 +0100
To: perlbug-followup [...] perl.org
Subject: Re: [perl #132538] perldata doesn't tell what happens on list assignment, with less/more elements
From: Christoph Anton Mitterer <calestyo [...] scientia.net>
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Hi. Thanks for the patch :) My comments below: +Note that assignment of one list of scalars to another list of scalars is +B<not> the same as assigning that first list to an array. => I also think this is confusing... +When you assign a list of scalars to an array, all previous values in that +array are wiped out and number of elements in the array will now be equal to +the number of elements in the right-hand list -- the list from which +assignment was made. The array will automatically resize itself to precisely +accommodate each element in the right-hand list. => I would add here (just for convenience): When you assign a list of scalars _(or an array)_ to an array + use warnings; => The documentation somewhere says this is always implicitly assumed. + my (@xyz, $x, $y, $z); + + @xyz = (1, 2, 3); + print "@xyz\n"; # 1 2 3 + + @xyz = ('al', 'be', 'ga', 'de'); + print "@xyz\n"; # al be ga de + + @xyz = (101, 102); + print "@xyz\n"; # 101 102 => could also fit in 3 lines, but I'm fine with it :) @a = (1, 2, 3); # array of 3 elements holds: 1 2 3 @a = ("a", "b", "c", "d"); # array of 3 elements holds: a b c d @a = (4, 5); # array of 2 elements holds: 4 5 + +When, however, you assign a list of scalars to another list of scalars => "... or an array" , the +results differ according to whether the left-hand list -- the list being +assigned to -- has the same, more or fewer elements than the right-hand list. + + ($x, $y, $z) = (1, 2, 3); + print "$x $y $z\n"; # 1 2 3 + + ($x, $y, $z) = ('al', 'be', 'ga', 'de'); + print "$x $y $z\n"; # al be ga + + ($x, $y, $z) = (101, 102); + print "$x $y $z\n"; # 101 102 + # Use of uninitialized value $z in concatenation (.) + # or string at [program] line [line number]. + +If the number of scalars in the left-hand list is less than that in the +right-hand list, the "extra" scalars in the right-hand list will simply not be +assigned. => one might add here, that 'de' is of course still evaluated, but just discarded => and I'd rather write either "not be assigned to anything" or "will be simply discarded". + +If the number of scalars in the left-hand list is greater than than in the +left-hand list, the "missing" scalars will become undefined. => there are two times left-hand list... the 2nd should be right hand list => I'd add after the "undefined" something like "loosing it's previous value" + + ($x, $y, $z) = (101, 102); + for my $el ($x, $y, $z) { + (defined $el) ? print "$el " : print "<undef>"; + } + print "\n"; + # 101 102 <undef> + Thanks :-)


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