«

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;

On Mar 25, 4:25 pm, Saurabh Jain <hundredr...@gmail.com> 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:

#!/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

Saurabh Jain wrote:
> Is there any difference in reading a file using a while or a
> foreach in perl?

Yes.

foreach (<FILEHANDLE>)

reads the whole file at once, and creates a list in memory of all the
lines, so that method is inefficient and not recommended in most cases.

while (<FILEHANDLE>)

reads one line at a time.

Please study "perldoc perlsyn" for more comprehensive descriptions.

--
Gunnar Hjalmarsson
Email: http://www.gunnar.cc/cgi-bin/contact.pl

Ben Bullock wrote:
> On Mar 25, 4:25 pm, Saurabh Jain <hundredr...@gmail.com> 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

Saurabh Jain <> wrote:
> Is there any difference in reading a file using a while or a
>foreach in perl?

Yes. Reading a file line by line works only with while. foreach reads the
whole file at once.

>If I do :
> foreach(<filehandle>) {
> my $local = <filehandle>; # I assumed I will increment the file

You just tried to read from a file that is at EOF already.

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

You are alternating between reading one line into $_ (in the while
condition) and one line into $local. Is this what you meant to do?

>descriptor here
> print " local $local\n";
>}

>
>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

And here you are reading one line into $_ (in the while condition) and then
successively three lines into $line. This may make sense if you know that a
data set has a fixed format of always 4 lines. But in 99% of all cases it's
a bug.
As for the foreach version: it already slurps the whole file into a list,
therefore there is nothing left that could be read into any of the $line.

jue

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.

Quoth Ben Bullock <>:
> 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

Quoth Frank Seitz <>:
> Ben Morrow wrote:
> > Quoth Ben Bullock <>:
> >>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"
>
> Hm. Why is this distinction relevant here?

I'm not at all sure it is, except in the 'variable vs. value' sense.

> > 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;
>
> use strict;
> use warnings;
>
> my @a = qw/a b c/;
> for my $v (@a) {
> push @a,'d' if $v eq 'c';
> print "$v\n";
> }

Good point. for is a little weird in this respect...

Ben

On Mar 25, 1:14*pm, Frank Seitz <devnull4...@web.de> wrote:
> Ben Morrow wrote:
> > Quoth Ben Bullock <benkasminbull...@gmail.com>:
> >>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"
>
> Hm. Why is this distinction relevant here?
>
> > 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;
>
> use strict;
> use warnings;
>
> my @a = qw/a b c/;
> for my $v (@a) {
> * * push @a,'d' if $v eq 'c';
> * * print "$v\n";}
>
> __END__
> a
> b
> c
> d
>

http://groups.google.com/group/comp....2ebca559578bcf

On Mar 25, 1:50*pm, nolo contendere <simon.c...@fmr.com> wrote:
> On Mar 25, 1:14*pm, Frank Seitz <devnull4...@web.de> wrote:
>
>
>
> > Ben Morrow wrote:
> > > Quoth Ben Bullock <benkasminbull...@gmail.com>:
> > >>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"
>
> > Hm. Why is this distinction relevant here?
>
> > > 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;
>
> > use strict;
> > use warnings;
>
> > my @a = qw/a b c/;
> > for my $v (@a) {
> > * * push @a,'d' if $v eq 'c';
> > * * print "$v\n";}
>
> > __END__
> > a
> > b
> > c
> > d
>
> http://groups.google.com/group/comp....2ebca559578bcf

also, see 'Perl: modifying an array in a loop'

http://blog.plover.com/prog/perl/undefined.html#3