Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

monks, i was wondering if any of you could tell me why a while loop over the default filehandle with $/ set to undef when using inplace editing runs twice instead of once. for example: 
open(X,"< test");
undef $/;
while(<X>) { print "hello" }

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prints only once as expected, but 
$ARGV[0]="test";
$^I="";
undef $/;
while(<>) { print "hello" }

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prints twice regardless of how many lines in 'test' thanks

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Re: inplace file editing
by ZZamboni (Curate) on Jun 11, 2001 at 05:38 UTC
    I think the difference does not have to do with the use of $^I, but with how $/=undef behaves when reading from the magic <> file handle. The following program: 
    $^I="";
undef $/;
while(<>) { print "hello:\n" }

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    Behaves the same if I comment out the $^I line: it prints "hello" twice, except that when $^I="" it is printed to the file I give as argument, obliterating it. Printing $_ on each iteration shows that $_ contains '' (empty string) on the second iteration, so my guess is that setting $/ to undef somehow changes the way in which perl reads from <>, but I don't really have an answer...

    --ZZamboni

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      sorry, what i meant to say was 
      $ARGV[0]="test";
$^I="";
undef $/;

while(<>) { print STDOUT "hello\n" }

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      (which stops hello getting printed to the file)...but it still prints twice. can ANYONE help?
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Re: inplace file editing
by Abigail (Deacon) on Jun 11, 2001 at 11:51 UTC
    Are you sure that that's all the code you have? Because I cannot reproduce what you are getting. 
        $ cat p.pl
    #!/opt/perl/bin/perl -w

    use strict;

    $ARGV [0] = $0;
    $^I = "";
    undef $/;

    while (<>) {print STDOUT "Hello\n"}

    __END__
    $ ./p.pl
    Hello
    $

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    Only one time....

    -- Abigail

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Re: inplace file editing
by mattr (Curate) on Jun 11, 2001 at 17:58 UTC
    It took me a while to figure out what was going on. Here's my program: 
    #!/usr/bin/perl

$ARGV[0]="test";
$^I="";
undef $/;
while(<>) { print STDOUT "hello\n" }

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    $/ seems to work correctly, reading to the end of the file if set to undef. However, by setting $^I to the null string, Perl seems to think it is running as if at the command line. the "while(<>)" reads from the file named "test", but the print statement writes to that same file. I find that without STDOUT, file test is erased and filled with a single "hello". Using the STDOUT filehandle, the file is erased and set to 0 bytes. Do you see the same activity?

    What I don't get is by keeping the same program as above but inserting $^I=undef; just before the print statement inplace editing does not seem to get disabled, i.e. the file is overwritten anyway.

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Re: inplace file editing
by Anonymous Monk on Jun 11, 2001 at 20:41 UTC
    abigail: yes, im sure thats the only code. with the same code as you on my box (perl 5.6.0) i get different output. here is a transcript. 
    gotham:~# cat p.pl
#!/usr/bin/perl -w
use strict;
$ARGV[0]=$0;
$^I="";
undef $/;
while(<>) { print STDOUT "hello\n" }

gotham:~# perl p.pl
hello
hello
gotham:~#

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    weird, huh?
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Re: inplace file editing
by Anonymous Monk on Jun 11, 2001 at 21:00 UTC
    UPDATE: btw, it prints only once if the file 'test' is empty but if it has any lines in it, it prints twice (which it shouldnt)
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Re: inplace file editing
by mattr (Curate) on Jun 12, 2001 at 10:51 UTC
    (re Abigail your comment) ..really wierd, since when I paste in your last program verbatim Abigail the program erases itself (visible in directory but 0 bytes), this when run as perl p.pl or as ./p.pl.

    Somebody else figure this out? Or is this just attributed to bugs that got fixed in Perl..? 

    bash-2.02# perl -V
Summary of my perl5 (5.0 patchlevel 5 subversion 2) configuration:
  Platform:
    osname=linux, osvers=2.2.5, archname=i586-linux
    uname='linux fatou 2.2.5 #2 smp mon mar 29 17:28:58 mest 1999 i686
+ unknown '
    hint=recommended, useposix=true, d_sigaction=define
    usethreads=undef useperlio=undef d_sfio=undef
  Compiler:
    cc='cc', optimize='-O2 -pipe', gccversion=egcs-2.91.66 19990314 (e
+gcs-1.1.2 release)
    cppflags='-Dbool=char -DHAS_BOOL -I/usr/local/include'
    ccflags ='-Dbool=char -DHAS_BOOL -I/usr/local/include'
    stdchar='char', d_stdstdio=define, usevfork=false
    intsize=4, longsize=4, ptrsize=4, doublesize=8
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=1
+2
    alignbytes=4, usemymalloc=n, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -L/usr/local/lib'
    libpth=/usr/local/lib /lib /usr/lib
    libs=-lnsl -lndbm -lgdbm -ldb -ldl -lm -lc -lposix -lcrypt
    libc=, so=so, useshrplib=false, libperl=libperl.a
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-rdynami
+c'
    cccdlflags='-fpic', lddlflags='-shared -L/usr/local/lib'


Characteristics of this binary (from libperl):
  Built under linux
  Compiled at Apr 15 1999 05:11:26
  @INC:
    /usr/lib/perl5/5.00502/i586-linux
    /usr/lib/perl5/5.00502
    /usr/lib/perl5/site_perl/5.005/i586-linux
    /usr/lib/perl5/site_perl/5.005

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