if( <HANDLE>) vs WHILE ( <HANDLE> )

[p5pRT]

Migrated from rt.perl.org#1190 (status was 'resolved')

Searchable as RT1190$

[p5pRT]

From bbense@networking.stanford.edu

The following program illustrates my problem. Why does

if ( <FOO> )

leave $_ empty ?

Using -w gives this warning
Use of uninitialized value at test.pl line 14, <FOO> chunk 1.

#/usr/bin/perl

open(FOO,'>./foo') ;

print FOO "valueA\tvalueB\n" ;

close (FOO);

# this does not work

open(FOO,"<./foo") or die "open foo failed" ;

if ( <FOO> ) {
  print "​:$_​:" ;
} else {
  die "bad foo" ;
}

close (FOO) ;

# this does work

open(FOO,"<./foo") or die "open foo failed" ;

while ( <FOO> ) {
  print "​:$_​:" ;
}

close (FOO) ;

Perl Info

Site configuration information for perl 5.00503:

Configured by bbense at Thu Apr  1 09:45:08 PST 1999.

Summary of my perl5 (5.0 patchlevel 5 subversion 3) configuration:
  Platform:
    osname=solaris, osvers=2.5.1, archname=sun4-solaris-thread
    uname='sunos telemark.stanford.edu 5.5.1 generic_103640-21 sun4u sparc sunw,ultra-1 '
    hint=previous, useposix=true, d_sigaction=define
    usethreads=define useperlio=define d_sfio=define
  Compiler:
    cc='gcc', optimize='-O -g', gccversion=2.8.1
    cppflags='-D_REENTRANT -DDEBUGGING -I/usr/local/include -I/usr/pubsw/include'
    ccflags ='-D_REENTRANT -DDEBUGGING -I/usr/local/include -I/usr/pubsw/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='gcc', ldflags ='-L/usr/local/lib -L/usr/pubsw/lib'
    libpth=/usr/local/lib /usr/pubsw/lib /lib /usr/lib /usr/ccs/lib
    libs=-lsfio -lsocket -lnsl -lgdbm -ldb -ldl -lm -lposix4 -lpthread -lc -lcrypt -lbsd
    libc=/lib/libc.so, so=so, useshrplib=false, libperl=libperl.a
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags=' '
    cccdlflags='-fPIC', lddlflags='-G -L/usr/local/lib -L/usr/pubsw/lib'

Locally applied patches:
    


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


Environment for perl 5.00503:
    HOME=/opt/users/bbense
    LANG (unset)
    LANGUAGE (unset)
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/opt/users/bbense/bin:/usr/bin:.:/usr/local/bin:/usr/newsw/bin:/usr/pubsw/bin:/usr/pubsw/X/bin:/usr/newsw/X/bin:/networking/@sys/bin:/networking/share/bin:/usr/ccs/bin
    PERL_BADLANG (unset)
    SHELL=/bin/tcsh

[p5pRT]

From [Unknown Contact. See original ticket]

Booker Bense <bbense@​networking.stanford.edu> writes​:

This is a bug report for perl from bbense@​telemark.stanford.edu,
generated with the help of perlbug 1.26 running under perl 5.00503.

-----------------------------------------------------------------
[Please enter your report here]

The following program illustrates my problem. Why does

if ( <FOO> )

leave $_ empty ?

Because you have not to assigned anything to $_.
From perlop.pod​:

"In scalar context, evaluating a filehandle in angle brackets yields
the next line from that file (the newline, if any, included), or
C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
(sometimes known as file-slurp mode) and the file is empty, it
returns C<''> the first time, followed by C<undef> subsequently.

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.

"
--
Nick Ing-Simmons

[p5pRT]

From [Unknown Contact. See original ticket]

Booker Bense writes​:

The following program illustrates my problem. Why does
if ( <FOO> )
leave $_ empty ?

Because while() is the only place that <> is special. See the
perlop manpage​:

  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 while statement
  (even if disguised as a for(;;) loop), the value is
  automatically assigned to the global variable $_,

Nat