Perl while loop using read()

Question

I want to read a text file data.txt using read. I would like to read the first four integers of each line at a time.

data.txt

line1 0101020102
line2 2010201001
line3 0201010201

The line1 etc. are part of the file.

This is what I could do up to now

Perl

my $data;
my $IN;
my $n = 0;

$line = 1;

open($IN, '<', "data.txt") or die " Error";

while ( $line != 0 ) {
    seek($IN, 6, SEEK_SET);
    $line = read($IN, $data, 4);
    seek($IN, 0, SEEK_SET);
}

The problem is that using this while loop, Perl keeps reading the first line in an infinite loop.

How can I deal with this problem?

Why would it do anything else, when you seek() to the beginning of the file after every read? — hobbs
– hobbs, Commented Sep 1, 2018 at 15:15
Are you thinking that using read is a good idea here (it isn't) or are you simply experimenting with the operator? — Borodin
– Borodin, Commented Sep 1, 2018 at 20:52

Dave Cross · Accepted Answer · 2018-09-01 16:02:07Z

2

I think you misunderstand what seek() does. With SEEK_SET as the third parameter, the second parameter is interpreted as the position to move to as measured from the start of the file. So every time you run this line:

seek($IN, 6, SEEK_SET);

It always moves to the seventh position in the file (which is, obviously, always on the same line). Then when you run this line:

seek($IN, 0, SEEK_SET);

It always moves back to the start of the file.

Initially, I thought the solution was to change SEEK_SET to SEEK_CUR (which interprets the second parameter as the amount to move relative to the current position) but it's a bit more complex than that. Here's how I got it to work:

seek($IN, 6, SEEK_CUR); # Move forward six spaces
$line = read($IN, $data, 4);
seek($IN, 7, SEEK_CUR); # Move forward seven places

But really, a regex is the best approach here.

while(<$IN>) {
    ($data) = /\s(\d{4})/;
    print "$data\n";
}

answered Sep 1, 2018 at 16:02

Dave Cross

69.5k3 gold badges55 silver badges101 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Borodin Over a year ago

Note that this won't work on Windows, because you assume a single additional line terminator character whereas Windows uses two

Collectives™ on Stack Overflow

Perl while loop using read()

data.txt

Perl

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

data.txt

Perl

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related