overwriting with scalar $_ using foreach over array and inner wh ile loop

[p5pRT]

Migrated from rt.perl.org#1828 (status was 'resolved')

Searchable as RT1828$

[p5pRT]

From Bernd.Koester@m.dasa.de

Using default scalar $_ (implicit) in an foreach loop over an array
and having an inner while loop reading a file we noticed that perl is
overwriting the scalar of the outer loop inserting read lines
from the file into the array.

my @​array=(1,2,3);
foreach (@​array) {
  open FILE, "<test";
  while <FILE> {
  ...
  }
  close FILE;
  }

FILE contains just one character (a).
After this procedure @​array contains (a,a,a)!
Best regards,

Bernd and Dirk

Perl Info

Site configuration information for perl 5.00503:

Configured by root at Mon May 10 10:54:22 MET DST 1999.

Summary of my perl5 (5.0 patchlevel 5 subversion 3) configuration:
  Platform:
    osname=solaris, osvers=2.5, archname=sun4-solaris
    uname='sunos sunny2 5.5 generic sun4m sparc sun4m '
    hint=recommended, useposix=true, d_sigaction=define
    usethreads=undef useperlio=undef d_sfio=undef
  Compiler:
    cc='cc', optimize='-O', gccversion=
    cppflags='-I/usr/local/include'
    ccflags ='-I/usr/local/include'
    stdchar='unsigned char', d_stdstdio=define, usevfork=false
    intsize=4, longsize=4, ptrsize=4, doublesize=8
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
    alignbytes=8, usemymalloc=y, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -L/usr/local/lib'
    libpth=/usr/local/lib /lib /usr/lib /usr/ccs/lib
    libs=-lsocket -lnsl -ldl -lm -lc -lcrypt
    libc=/lib/libc.so, so=so, useshrplib=false, libperl=libperl.a
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags=' '
    cccdlflags='-KPIC', lddlflags='-G -L/usr/local/lib'



@INC for perl 5.00503:
	/usr/local/lib/perl5/5.00503/sun4-solaris
	/usr/local/lib/perl5/5.00503
	/usr/local/lib/perl5/site_perl/5.005/sun4-solaris
	/usr/local/lib/perl5/site_perl/5.005
	.


Environment for perl 5.00503:
    HOME=/usr/users/sunny6/ko26619
    LANG (unset)
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
 
PATH=/usr/users/sunny6/ko26619:/usr/users/sunny6/ko26619/tools:/disks/disk1/
oracle/733/bin:/usr/users/sunny6/csdb/v0.2/lib:/disks/disk1/adobe/Acrobat3/b
in:/usr/users/sunny6/ko26619:/otn/sunny11/d7/a701/adept/bin:/otn/sunny11/d7/
a701/adept/bin/sun4r5:/otn/sunny11/d7/a701/adept/bin/sun4r5/dasa:/otn/sunny1
1/d7/a701/adept/lib/sun4r5:/usr/bin/X11:/otn/sunny11/d7/a701/csdb/lib:/otn/s
unny11/d7/csdb/idb_com:/opt/bin:/bin:/usr/bin:/usr/ccs/bin:/usr/local/bin:/u
sr/local:/usr/openwin/bin:/usr/new:/usr/hosts:/opt/SUNWspro/bin:/usr/users/s
unny6/ko26619:/otn/sunny11/d7/o40l/csdb/lib:/otn/sunny11/d7/o40l/omle40/bin:
/usr/users/sunny6/ko26619:/usr/users/sunny6/ko26619/tools:/disks/disk1/oracl
e/733/bin:/usr/users/sunny6/csdb/v0.2/lib:/disks/disk1/adobe/Acrobat3/bin:/u
sr/users/sunny6/ko26619:/otn/sunny11/d7/a701/adept/bin:/otn/sunny11/d7/a701/
adept/bin/sun4r5:/otn/sunny11/d7/a701/adept/bin/sun4r5/dasa:/otn/sunny11/d7/
a701/adept/lib/sun4r5:/usr/bin/X11:/otn/sunny11/d7/a701/csdb/lib:/otn/sunny1
1/d7/csdb/idb_com:/opt/bin:/bin:/usr/bin:/usr/ccs/bin:/usr/local/bin:/usr/lo
cal:/usr/openwin/bin:/usr/new:/usr/hosts:/opt/SUNWspro/bin:/usr/users/sunny6
/ko26619:/otn/sunny11/d7/o40l/csdb/lib:/otn/sunny11/d7/o40l/omle40/bin:/usr/
bin::/otn/sunny11/d7/o40l/omni40/bin:/otn/sunny11/d7/o40l/flexlm-5.12a/bin:/
usr/local/bin/netscape:.:/otn/sunny11/d7/o40l/omni40/bin:/otn/sunny11/d7/o40
l/flexlm-5.12a/bin:/usr/local/bin/netscape:.
    PERL=/disks/disk1/csdb/www/v0.2/csdb-perl
    PERL_BADLANG (unset)
    SHELL=/usr/bin/ksh

[p5pRT]

From [Unknown Contact. See original ticket]

Koester, Bernd writes​:
| Using default scalar $_ (implicit) in an foreach loop over an array
| and having an inner while loop reading a file we noticed that perl is
| overwriting the scalar of the outer loop inserting read lines
| from the file into the array.
|
| my @​array=(1,2,3);
| foreach (@​array) {
| open FILE, "<test";
| while <FILE> {
| ...
| }
| close FILE;
| }
|
|
| FILE contains just one character (a).
| After this procedure @​array contains (a,a,a)!

This is not a bug​:

man perlsyn ------------------------------------------------------------

The foreach modifier is an iterator​: For each value in EXPR, it aliases
$_ to the value and executes the statement.

---

man perlop -------------------------------------------------------------

If and ONLY if the input symbol is the only thing inside the conditional
of a while or for(;;) loop, the value is automatically assigned to the
variable $_.

---

Hence the while loop assigns to $_ which is an alias into @​array. No
surprise.

Mx.

[p5pRT]

From [Unknown Contact. See original ticket]

Martyn Pearce <m.pearce@​inpharmatica.co.uk> wrote

Hence the while loop assigns to $_ which is an alias into @​array. No
surprise.

But you haven't pointed out the key issue, that while never localises
any variable, unlike foreach. And this point isn't stressed in
the current (5.005_03) documentation.

The development version (5.005_62) makes this more explicit​:

---------------------pod/perlop.pod--------------------
Ordinarily you must assign the returned value to a variable, but
there is one situation where an automatic assignment happens. If
and only if the input symbol is the only thing inside the conditional
of a C<while> statement (even if disguised as a C<for(;;)> loop),
the value is automatically assigned to the global variable $_,
destroying whatever was there previously. (This may seem like an
odd thing to you, but you'll use the construct in almost every Perl
script you write.) The $_ variables is not implicitly localized.
You'll have to put a C<local $_;> before the loop if you want that
to happen.

---

I think the attached patch to perlsyn would also be useful.

Mike Guy

Inline Patch

--- ./pod/perlsyn.pod.orig	Fri Sep 17 21:23:08 1999
+++ ./pod/perlsyn.pod	Fri Nov 26 16:02:15 1999
@@ -163,6 +163,8 @@
 refers to the innermost enclosing loop.  This may include dynamically
 looking back your call-stack at run time to find the LABEL.  Such
 desperate behavior triggers a warning if you use the B<-w> flag.
+Unlike a C<foreach> statement, a C<while> statement never implicitly
+localises any variables.
 
 If there is a C<continue> BLOCK, it is always executed just before the
 conditional is about to be evaluated again, just like the third part of a

End of patch