Hi,
Is there any difference in reading a file using a while or a
foreach in perl?

If I do :
foreach(<filehandle>) {
my $local = <filehandle>; # I assumed I will increment the file
descriptor here
print " local $local\n";
}

But if I do :
while(<filehandle>) {
my $local = <filehandle>; # I assumed I will increment the file
descriptor here
print " local $local\n";
}
It works fine....
Is there something wrong or some difference in the two operations? Or
am I missing something?

Thanks and Regards,
Saurabh

Small example to replicate the issue
my file name is test.pl

#!/usr/bin/perl

open (handle,"test.pl")||die "\n $0 Could not open $! \n";

my $line = <handle>;#read a line till \n or eof
print " line $line";
#foreach(<handle>){ # Not as expected
while(<handle>){ # Works as expected
$line =<handle>;#read a line till \n or eof

print " in side $line";
$line =<handle>;#read a line till \n or eof
print " in side $line";
$line =<handle>;#read a line till \n or eof
}
close handle;

Posted by Ben Bullock on March 25, 2008, 4:06 am

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> Hi,

> Is there any difference in reading a file using a while or a

> foreach in perl?

The foreach version seems to first read the whole of the file into an
array, and then go through it line by line:

#!/usr/bin/perl
#use warnings;
use strict;
open (handle,"testangleop.pl") or die "$0 Could not open $!";

my $line = <handle>;                #read a line till \n or eof
print "0 line $line";
foreach (<handle>) {                # Not as expected
#while(<handle>){ # Works as expected
print $_;
$line =<handle>;                #read a line till \n or eof
print "1 in side $line";
$line =<handle>;                #read a line till \n or eof
print "2 in side $line";
$line =<handle>;                #read a line till \n or eof
print "3 in side $line";
}

The while seems to increment through the loop.

See also

http://www.unix.org.ua/orelly/perl/prog3/ch02_11.htm

Posted by John W. Krahn on March 25, 2008, 7:59 am

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Ben Bullock wrote:

>> Hi,

>> Is there any difference in reading a file using a while or a

>> foreach in perl?

> The foreach version seems to first read the whole of the file into an

> array, and then go through it line by line:

perldoc -q "What is the difference between a list and an array"

John
--
Perl isn't a toolbox, but a small machine shop where you
can special-order certain sorts of tools at low cost and
in short order. -- Larry Wall

Posted by Ben Bullock on March 25, 2008, 11:30 am

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On Tue, 25 Mar 2008 11:59:23 +0000, John W. Krahn wrote:

> Ben Bullock wrote:

>> The foreach version seems to first read the whole of the file into an

>> array, and then go through it line by line:

> perldoc -q "What is the difference between a list and an array"

Found in /usr/local/lib/perl5/5.10.0/pod/perlfaq4.pod
What is the difference between a list and an array?

An array has a changeable length. A list does not.

If I had written "the foreach version reads the whole of the file into a
list", I would have contradicted this FAQ entry, which says I can't read
things into a list, because reading things into a list would change the
list's length, and "a list does not" have a changeable length.

Posted by Ben Morrow on March 25, 2008, 12:37 pm

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> On Tue, 25 Mar 2008 11:59:23 +0000, John W. Krahn wrote:

> > Ben Bullock wrote:

> >> The foreach version seems to first read the whole of the file into an

> >> array, and then go through it line by line:

> >

> > perldoc -q "What is the difference between a list and an array"

> Found in /usr/local/lib/perl5/5.10.0/pod/perlfaq4.pod

> What is the difference between a list and an array?

> An array has a changeable length. A list does not.

> If I had written "the foreach version reads the whole of the file into a

> list", I would have contradicted this FAQ entry, which says I can't read

> things into a list, because reading things into a list would change the

> list's length, and "a list does not" have a changeable length.

No, you're misunderstanding. A list is immutable *after it has been
created*. Obviously you can create lists with any contents, otherwise
you would be limited to using only lists compiled into perl. foreach
accepts a list as argument and iterates over it; <> in list context
(which is the real problem here) reads the entire file, splits it on
newline (or rather $/), and returns a (newly created) list with the
results. You can't modify the list after that: for an example where you
can, see Tie::File, which reads a file into an *array* instead.

Ben

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