curious behavior with shift (perl 5.8)?

[s_p_a_m_mob]

Hi there,


[]$ perl
$a = [1,2,3,4,5,6];
foreach(@{$a}){
my $b = unshift @{$a};
my $c = shift @{$a};
print $b,$c;
}
615243

That is ok but:

[]$ perl
$a = [1,2,3,4,5,6];
foreach(@{$a}){
my $b = shift @{$a};
my $c = shift @{$a};
print $b,$c;
}
1234


Why are 5 and 6 missing?

Marc-Olivier Bernard

########################
perl -V says:

Summary of my perl5 (revision 5.0 version 8 subversion 0)
configuration:
Platform:
osname=linux, osvers=2.4.21-1.1931.2.382.entsmp,
archname=i386-linux-thread-multi
uname='linux stripples.devel.redhat.com 2.4.21-1.1931.2.382.entsmp
#1 smp wed aug 6 17:18:52 edt 2003 i686 i686 i386 gnulinux '
config_args='-des -Doptimize=-O2 -g -pipe -march=i386 -mcpu=i686
-Dmyhostname=localhost -Dperladmin=root@localhost -Dcc=gcc -Dcf_by=Red
Hat, Inc. -Dinstallprefix=/usr -Dprefix=/usr -Darchname=i386-linux
-Dvendorprefix=/usr -Dsiteprefix=/usr
-Dotherlibdirs=/usr/lib/perl5/5.8.0 -Duseshrplib -Dusethreads
-Duseithreads -Duselargefiles -Dd_dosuid -Dd_semctl_semun -Di_db
-Ui_ndbm -Di_gdbm -Di_shadow -Di_syslog -Dman3ext=3pm -Duseperlio
-Dinstallusrbinperl -Ubincompat5005 -Uversiononly -Dpager=/usr/bin/less
-isr'
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='gcc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DTHREADS_HAVE_PIDS
-DDEBUGGING -fno-strict-aliasing -I/usr/local/include
-D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -I/usr/include/gdbm',
optimize='-O2 -g -pipe -march=i386 -mcpu=i686',
cppflags='-D_REENTRANT -D_GNU_SOURCE -DTHREADS_HAVE_PIDS
-DDEBUGGING -fno-strict-aliasing -I/usr/local/include
-I/usr/include/gdbm'
ccversion='', gccversion='3.2.2 20030222 (Red Hat Linux 3.2.2-5)',
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='gcc', ldflags =' -L/usr/local/lib'
libpth=/usr/local/lib /lib /usr/lib
libs=-lnsl -lgdbm -ldb -ldl -lm -lpthread -lc -lcrypt -lutil
perllibs=-lnsl -ldl -lm -lpthread -lc -lcrypt -lutil
libc=/lib/libc-2.3.2.so, so=so, useshrplib=true, libperl=libperl.so
gnulibc_version='2.3.2'
Dynamic Linking:
dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-rdynamic
-Wl,-rpath,/usr/lib/perl5/5.8.0/i386-linux-thread-multi/CORE'
cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib'




Characteristics of this binary (from libperl):
Compile-time options: DEBUGGING MULTIPLICITY USE_ITHREADS
USE_LARGE_FILES PERL_IMPLICIT_CONTEXT
Locally applied patches:
MAINT18379
Built under linux
Compiled at Aug 13 2003 11:47:58
@INC:
/usr/lib/perl5/5.8.0/i386-linux-thread-multi
/usr/lib/perl5/5.8.0
/usr/lib/perl5/site_perl/5.8.0/i386-linux-thread-multi
/usr/lib/perl5/site_perl/5.8.0
/usr/lib/perl5/site_perl
/usr/lib/perl5/vendor_perl/5.8.0/i386-linux-thread-multi
/usr/lib/perl5/vendor_perl/5.8.0
/usr/lib/perl5/vendor_perl
/usr/lib/perl5/5.8.0/i386-linux-thread-multi
/usr/lib/perl5/5.8.0

 

[John W. Krahn]

[]$ perl
$a = [1,2,3,4,5,6];
foreach(@{$a}){
my $b = unshift @{$a};
my $c = shift @{$a};
print $b,$c;
}
615243

That is ok but:

[]$ perl
$a = [1,2,3,4,5,6];
foreach(@{$a}){
my $b = shift @{$a};
my $c = shift @{$a};
print $b,$c;
}
1234


Why are 5 and 6 missing?

perldoc perlsyn

[snip]

Foreach Loops

The "foreach" loop iterates over a normal list value and sets the
variable VAR to be each element of the list in turn. If the variable
is preceded with the keyword "my", then it is lexically scoped, and is
therefore visible only within the loop. Otherwise, the variable is
implicitly local to the loop and regains its former value upon exiting
the loop. If the variable was previously declared with "my", it uses
that variable instead of the global one, but it's still localized to
the loop. This implicit localisation occurs only in a "foreach" loop.

The "foreach" keyword is actually a synonym for the "for" keyword, so
you can use "foreach" for readability or "for" for brevity. (Or
because the Bourne shell is more familiar to you than csh, so writing
"for" comes more naturally.) If VAR is omitted, $_ is set to each
value.

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.

If any part of LIST is an array, "foreach" will get very confused if
you add or remove elements within the loop body, for example with
"splice". So don't do that.
^^^^^^^^^^^^^^^^^^
^^^^^^^^^^^^^^^^^^
^^^^^^^^^^^^^^^^^^


John

 

[s_p_a_m_mob]

I missed that. Thanks to you John!

Marc-Olivier BERNARD