how to read from more than one files at a time

[Perldiscuss - Perl Newsgroups And Mailing Lists]

Hi,

I need to read from more than one file at a time and do some operation on
the strings and join them together, and put into one output file.

Here is the code. But I noticed the second file never get read. It must be
sth very simple to overcome this. Can anyone give me a hint?

thanks,
Ben

# Usage: perl jointfiles.pl infile1 infile2 outfile
open (IN1, $ARGV[0]);
open (IN2, $ARGV[1]);
open (OUT, ">$ARGV[2]");
print "Input fileis : $ARGV[0], $ARGV[1]\n";
print "Output file: $ARGV[2]\n";

while(<IN1>)
{
chomp;
$in1=$_;
<IN2>;
$in2=$_;
printf OUT "$in1$in2\n";
}
close(IN1);
close(IN2);
close(OUT);

[Mark Lobue]

> -----Original Message-----

> From: [email]benjamin.zhou@mbusa.com[/email] [mailto:benjamin.zhou@mbusa.com]
> Sent: Wednesday, October 15, 2003 11:42 AM
> To: [email]beginners@perl.org[/email]
> Subject: how to read from more than one files at a time
>
>
> Hi,
>
> I need to read from more than one file at a time and do some
> operation on
> the strings and join them together, and put into one output file.
>
> Here is the code. But I noticed the second file never get
> read. It must be
> sth very simple to overcome this. Can anyone give me a hint?
>
> thanks,
> Ben
>
> # Usage: perl jointfiles.pl infile1 infile2 outfile
> open (IN1, $ARGV[0]);
> open (IN2, $ARGV[1]);
> open (OUT, ">$ARGV[2]");
> print "Input fileis : $ARGV[0], $ARGV[1]\n";
> print "Output file: $ARGV[2]\n";
>
> while(<IN1>)
> {
> chomp;
> $in1=$_;
> <IN2>;
> $in2=$_;

$_ is not automatically assigned the input in this case.
try:
$in2 = <IN2>;

> printf OUT "$in1$in2\n";

You won't need the trailing \n, because you aren't chomping it from file IN2

-Mark

[John W. Krahn]

Perldiscuss - Perl Newsgroups And Mailing Lists wrote:

>
> Hi,

Hello,

> I need to read from more than one file at a time and do some operation on
> the strings and join them together, and put into one output file.
>
> Here is the code. But I noticed the second file never get read. It must be
> sth very simple to overcome this. Can anyone give me a hint?

use warnings;
use strict;

> # Usage: perl jointfiles.pl infile1 infile2 outfile
> open (IN1, $ARGV[0]);
> open (IN2, $ARGV[1]);
> open (OUT, ">$ARGV[2]");

You should _ALWAYS_ verify that the files opened correctly.

open (IN1, $ARGV[0]) or die "Cannot open $ARGV[0]: $!";
open (IN2, $ARGV[1]) or die "Cannot open $ARGV[1]: $!";
open (OUT, ">$ARGV[2]") or die "Cannot open $ARGV[2]: $!";

> print "Input fileis : $ARGV[0], $ARGV[1]\n";
> print "Output file: $ARGV[2]\n";
>
> while(<IN1>)
> {
> chomp;
> $in1=$_;
> <IN2>;

You are reading from IN2 in a void context. In other words, you are not
assigning the line you read from IN2 to a variable. It only gets
assigned to $_ in a while expression like "while(<IN1>)" above.

$in2 = <IN2>;

> $in2=$_;
> printf OUT "$in1$in2\n";
> }
> close(IN1);
> close(IN2);
> close(OUT);

John
--
use Perl;
program
fulfillment

[Rob Dixon]

<benjamin.zhou@mbusa.com> wrote:

>
> I need to read from more than one file at a time and do some
> operation on the strings and join them together, and put into
> one output file.
>
> Here is the code. But I noticed the second file never get
> read. It must be sth very simple to overcome this. Can anyone
> give me a hint?
>
> thanks,
> Ben
>
> # Usage: perl jointfiles.pl infile1 infile2 outfile

use strict;
use warnings;

> open (IN1, $ARGV[0]);
> open (IN2, $ARGV[1]);
> open (OUT, ">$ARGV[2]");

As John says,

open IN1, $ARGV[0] or die $!;
open IN2, $ARGV[1] or die $!;
open OUT, ">$ARGV[2]" or die $!;

> print "Input fileis : $ARGV[0], $ARGV[1]\n";
> print "Output file: $ARGV[2]\n";
>
> while(<IN1>)
> {
> chomp;
> $in1=$_;
> <IN2>;
> $in2=$_;
> printf OUT "$in1$in2\n";
> }

You need tp think about what happens when the two files are
different lengths. You may not expect that to happen but your
code should still handle that case. Some symmetry between the
handling of the two files would also be nice. Finally you
should really use 'print' here instead of 'printf'.

This code reads a line from each file and outputs them both
until one or other file is exhausted.

my ($in1, $in2);

while (defined ($in1 = <IN1>) and defined ($in2 = <IN2>)) {
chomp ($in1, $in2);
print OUT "$in1$in2\n";
}

> close(IN1);
> close(IN2);
> close(OUT);

HTH,

Rob

[Perldiscuss - Perl Newsgroups And Mailing Lists]

thanks John and Rob for the great enlightenment, they taught me a lot.

I wonder why the following is wrong:
while (($in1=<IN1>) && ($in2=<IN2>))
{
...
}

Ben


Rob Dixon wrote:

> <benjamin.zhou@mbusa.com> wrote:

> >
> > I need to read from more than one file at a time and do some
> > operation on the strings and join them together, and put into
> > one output file.
> >
> > Here is the code. But I noticed the second file never get
> > read. It must be sth very simple to overcome this. Can anyone
> > give me a hint?
> >
> > thanks,
> > Ben
> >
> > # Usage: perl jointfiles.pl infile1 infile2 outfile

> use strict;
> use warnings;

> > open (IN1, $ARGV[0]);
> > open (IN2, $ARGV[1]);
> > open (OUT, ">$ARGV[2]");

> As John says,

> open IN1, $ARGV[0] or die $!;
> open IN2, $ARGV[1] or die $!;
> open OUT, ">$ARGV[2]" or die $!;

> > print "Input fileis : $ARGV[0], $ARGV[1]n";
> > print "Output file: $ARGV[2]n";
> >
> > while(<IN1>)
> > {
> > chomp;
> > $in1=$_;
> > <IN2>;
> > $in2=$_;
> > printf OUT "$in1$in2n";
> > }

> You need tp think about what happens when the two files are
> different lengths. You may not expect that to happen but your
> code should still handle that case. Some symmetry between the
> handling of the two files would also be nice. Finally you
> should really use 'print' here instead of 'printf'.

> This code reads a line from each file and outputs them both
> until one or other file is exhausted.

> my ($in1, $in2);

> while (defined ($in1 = <IN1>) and defined ($in2 = <IN2>)) {
> chomp ($in1, $in2);
> print OUT "$in1$in2n";
> }

> > close(IN1);
> > close(IN2);
> > close(OUT);

> HTH,

> Rob

[Rob Dixon]

<benjamin.zhou@mbusa.com> wrote:

>
> thanks John and Rob for the great enlightenment, they taught me a lot.
>
> I wonder why the following is wrong:
> while (($in1=<IN1>) && ($in2=<IN2>))
> {
> ..
> }

In the special case where <FH> returns a null string (when the last
record of a file is empty and has no terminating record separator)
that blank record will evaluate as 'false' and prematurely exit
the loop.

HTH,

Rob