Looping "foreach my $thing(@things)" places a reference not a scalar into $thing

Thread: Looping "foreach my $thing(@things)" places a reference not a scalar into $thing

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Looping "foreach my $thing(@things)" places a reference not a scalar into $thing
	2007-08-30 01:07:39
# New Ticket Created by Worik # Please include the string: [perl #45041] # in the subject line of all future correspondence about this issue. # <URL: h ttp://rt.perl.org/rt3/Ticket/Display.html?id=45041 > This is a bug report for perl from worik.stantongmail.com, generated with the help of perlbug 1.35 running under perl v5.8.8. ------------------------------------------------------------ ----- [Please enter your report here] This little programme demonstrates the problem my test = (1..12); print join(", ", test)."n"; foreach my $t(test){ if($t % 2 ){ $t *= -1; } } print join(", ", test)."n"; The output is: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 -1, 2, -3, 4, -5, 6, -7, 8, -9, 10, -11, 12 The output should be: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 [Please do not change anything below this line] ------------------------------------------------------------ ----- --- Flags: category=core severity=high --- Site configuration information for perl v5.8.8: Configured by Debian Project at Tue Mar 6 01:52:23 UTC 2007. Summary of my perl5 (revision 5 version 8 subversion 8) configuration: Platform: osname=linux, osvers=2.6.15.7, archname=i486-linux-gnu-thread-multi uname='linux rothera 2.6.15.7 #1 smp sat sep 30 10:21:42 utc 2006 i686 gnulinux ' config_args='-Dusethreads -Duselargefiles -Dccflags=-DDEBIAN -Dcccdlflags=-fPIC -Darchname=i486-linux-gnu -Dprefix=/usr -Dprivlib=/usr/share/perl/5.8 -Darchlib=/usr/lib/perl/5.8 -Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 -Dvendorarch=/usr/lib/perl5 -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl/5.8.8 -Dsitearch=/usr/local/lib/perl/5.8.8 -Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1 -Dsiteman3dir=/usr/local/man/man3 -Dman1ext=1 -Dman3ext=3perl -Dpager=/usr/bin/sensible-pager -Uafs -Ud_csh -Uusesfio -Uusenm -Duseshrplib -Dlibperl=libperl.so.5.8.8 -Dd_dosuid -des' hint=recommended, useposix=true, d_sigaction=define usethreads=define use5005threads=undef useithreads=define usemultiplicity=define useperlio=define d_sfio=undef uselargefiles=define usesocks=undef use64bitint=undef use64bitall=undef uselongdouble=undef usemymalloc=n, bincompat5005=undef Compiler: cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DTHREADS_HAVE_PIDS -DDEBIAN -fno-strict-aliasing -pipe -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64', optimize='-O2', cppflags='-D_REENTRANT -D_GNU_SOURCE -DTHREADS_HAVE_PIDS -DDEBIAN -fno-strict-aliasing -pipe -I/usr/local/include' ccversion='', gccversion='4.1.2 (Ubuntu 4.1.2-0ubuntu4)', gccosandvers='' intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234 d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=12 ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8 alignbytes=4, prototype=define Linker and Libraries: ld='cc', ldflags =' -L/usr/local/lib' libpth=/usr/local/lib /lib /usr/lib libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt perllibs=-ldl -lm -lpthread -lc -lcrypt libc=/lib/libc-2.5.so, so=so, useshrplib=true, libperl=libperl.so.5.8.8 gnulibc_version='2.5' Dynamic Linking: dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E' cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib' Locally applied patches: --- INC for perl v5.8.8: /etc/perl /usr/local/lib/perl/5.8.8 /usr/local/share/perl/5.8.8 /usr/lib/perl5 /usr/share/perl5 /usr/lib/perl/5.8 /usr/share/perl/5.8 /usr/local/lib/site_perl . --- Environment for perl v5.8.8: HOME=/home/worik LANG=en_NZ.UTF-8 LANGUAGE (unset) LD_LIBRARY_PATH (unset) LOGDIR (unset) PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin :/bin:/usr/games PERL_BADLANG (unset) SHELL=/bin/bash -- They laughed at Einstein. They laughed at the Wright Brothers. They laughed at Bozo the Clown.

Re: Looping "foreach my $thing(@things)" places a reference not a scalar into $thing
	2007-08-30 10:02:24
Worik (via RT) wrote: > This little programme demonstrates the problem > > my test = (1..12); > print join(", ", test)."n"; > foreach my $t(test){ > if($t % 2 ){ > $t *= -1; > } > } > print join(", ", test)."n"; > > The output is: > 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 > -1, 2, -3, 4, -5, 6, -7, 8, -9, 10, -11, 12 > > The output should be: > 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 > 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 Thank you for your report. This is a feature for the foreach loop. The foreach iterator is aliased to each element of the array so modifying $t will modify each element in the array in turn. This is documented (somewhat obtusely) in perlsyn... If any element of LIST is an lvalue, you can modify it by modifying VAR inside the loop. Conversely, if any element of LIST is NOT an lvalue, any attempt to modify that element will fail. In other words, the "foreach" loop index variable is an implicit alias for each item in the list that you're looping over. Since I was under the illusion that aliasing only happened when $_ is used, I've added an entry on the wiki about this trap. http://www.perlfoundation.org/perl5/index.cgi?fore ach_aliasing -- Stabbing you in the face so you don't have to.

Re: Looping "foreach my $thing(@things)" places a reference not a scalar into $thing
	2007-08-30 10:20:44
On 8/30/07, via RT Worik <perlbug-followupperl.org> wrote: > # New Ticket Created by Worik > # Please include the string: [perl #45041] > # in the subject line of all future correspondence about this issue. > # <URL: h ttp://rt.perl.org/rt3/Ticket/Display.html?id=45041 > > > > This is a bug report for perl from worik.stantongmail.com, > generated with the help of perlbug 1.35 running under perl v5.8.8. > > > ------------------------------------------------------------ ----- > [Please enter your report here] > This little programme demonstrates the problem > > my test = (1..12); > print join(", ", test)."n"; > foreach my $t(test){ > if($t % 2 ){ > $t = -1; > } > } > print join(", ", test)."n"; As Michael Schwern said, this turns out to be the way the for/foreach loop is documented to work. Using aliasing, makes it more efficient (but dangerous for the unaware as you have discovered for your self). To loop through an array, without worrying about changing the contents of the array elements, you may use an idiom like that foreach (test){ my $t = $_; # makes a copy if($t % 2 ){ $t = -1; } } > The output is: > 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 > -1, 2, -3, 4, -5, 6, -7, 8, -9, 10, -11, 12 > > The output should be: > 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 > 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 > > > > [Please do not change anything below this line] > ------------------------------------------------------------ ----- > --- > Flags: > category=core > severity=high > --- > Site configuration information for perl v5.8.8: > > Configured by Debian Project at Tue Mar 6 01:52:23 UTC 2007. > > Summary of my perl5 (revision 5 version 8 subversion 8) configuration: > Platform: > osname=linux, osvers=2.6.15.7, archname=i486-linux-gnu-thread-multi > uname='linux rothera 2.6.15.7 #1 smp sat sep 30 10:21:42 utc 2006 > i686 gnulinux ' > config_args='-Dusethreads -Duselargefiles -Dccflags=-DDEBIAN > -Dcccdlflags=-fPIC -Darchname=i486-linux-gnu -Dprefix=/usr > -Dprivlib=/usr/share/perl/5.8 -Darchlib=/usr/lib/perl/5.8 > -Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 > -Dvendorarch=/usr/lib/perl5 -Dsiteprefix=/usr/local > -Dsitelib=/usr/local/share/perl/5.8.8 > -Dsitearch=/usr/local/lib/perl/5.8.8 -Dman1dir=/usr/share/man/man1 > -Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1 > -Dsiteman3dir=/usr/local/man/man3 -Dman1ext=1 -Dman3ext=3perl > -Dpager=/usr/bin/sensible-pager -Uafs -Ud_csh -Uusesfio -Uusenm > -Duseshrplib -Dlibperl=libperl.so.5.8.8 -Dd_dosuid -des' > hint=recommended, useposix=true, d_sigaction=define > usethreads=define use5005threads=undef useithreads=define > usemultiplicity=define > useperlio=define d_sfio=undef uselargefiles=define usesocks=undef > use64bitint=undef use64bitall=undef uselongdouble=undef > usemymalloc=n, bincompat5005=undef > Compiler: > cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DTHREADS_HAVE_PIDS > -DDEBIAN -fno-strict-aliasing -pipe -I/usr/local/include > -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64', > optimize='-O2', > cppflags='-D_REENTRANT -D_GNU_SOURCE -DTHREADS_HAVE_PIDS -DDEBIAN > -fno-strict-aliasing -pipe -I/usr/local/include' > ccversion='', gccversion='4.1.2 (Ubuntu 4.1.2-0ubuntu4)', > gccosandvers='' > intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234 > d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=12 > ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='off_t', > lseeksize=8 > alignbytes=4, prototype=define > Linker and Libraries: > ld='cc', ldflags =' -L/usr/local/lib' > libpth=/usr/local/lib /lib /usr/lib > libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt > perllibs=-ldl -lm -lpthread -lc -lcrypt > libc=/lib/libc-2.5.so, so=so, useshrplib=true, > libperl=libperl.so.5.8.8 > gnulibc_version='2.5' > Dynamic Linking: > dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E' > cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib' > > Locally applied patches: > > > --- > INC for perl v5.8.8: > /etc/perl > /usr/local/lib/perl/5.8.8 > /usr/local/share/perl/5.8.8 > /usr/lib/perl5 > /usr/share/perl5 > /usr/lib/perl/5.8 > /usr/share/perl/5.8 > /usr/local/lib/site_perl > . > > --- > Environment for perl v5.8.8: > HOME=/home/worik > LANG=en_NZ.UTF-8 > LANGUAGE (unset) > LD_LIBRARY_PATH (unset) > LOGDIR (unset) > > PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin :/bin:/usr/games > PERL_BADLANG (unset) > SHELL=/bin/bash > > -- > They laughed at Einstein. They laughed at the Wright Brothers. They > laughed at Bozo the Clown. > >

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