The correct way to read a data file into an array

Question

I have a data file, with each line having one number, like

10
20
30
40

How do I read this file and store the data into an array?

So that I can conduct some operations on this array.

Answer 1

Just reading the file into an array, one line per element, is trivial:

open my $handle, '<', $path_to_file;
chomp(my @lines = <$handle>);
close $handle;

Now the lines of the file are in the array @lines.

If you want to make sure there is error handling for open and close, do something like this (in the snipped below, we open the file in UTF-8 mode, too):

my $handle;
unless (open $handle, "<:encoding(utf8)", $path_to_file) {
   print STDERR "Could not open file '$path_to_file': $!\n";
   # we return 'undefined', we could also 'die' or 'croak'
   return undef
}
chomp(my @lines = <$handle>);
unless (close $handle) {
   # what does it mean if close yields an error and you are just reading?
   print STDERR "Don't care error while closing '$path_to_file': $!\n";
} 

Answer 2

There is the easiest method, using File::Slurp module:

use File::Slurp;
my @lines = read_file("filename", chomp => 1); # will chomp() each line

If you need some validation for each line you can use grep in front of read_file.

For example, filter lines which contain only integers:

my @lines = grep { /^\d+$/ } read_file("filename", chomp => 1);

Answer 3

I like...

@data = `cat /var/tmp/somefile`;

It's not as glamorous as others, but, it works all the same. And...

$todays_data = '/var/tmp/somefile' ;
open INFILE, "$todays_data" ; 
@data = <INFILE> ; 
close INFILE ;

Cheers.

Answer 4

Tie::File is what you need:

Synopsis

# This file documents Tie::File version 0.98
use Tie::File;

tie @array, 'Tie::File', 'filename' or die ...;

$array[13] = 'blah';     # line 13 of the file is now 'blah'
print $array[42];        # display line 42 of the file

$n_recs = @array;        # how many records are in the file?
$#array -= 2;            # chop two records off the end


for (@array) {
  s/PERL/Perl/g;         # Replace PERL with Perl everywhere in the file
}

# These are just like regular push, pop, unshift, shift, and splice
# Except that they modify the file in the way you would expect

push @array, new recs...;
my $r1 = pop @array;
unshift @array, new recs...;
my $r2 = shift @array;
@old_recs = splice @array, 3, 7, new recs...;

untie @array;            # all finished

Answer 5

Reading from a file handle using the "diamond operator" (<>) in an array context reads all the remaining unread lines:

open(my $AAAA, '<', '/filepath/filename.txt');
my @array = <$AAAA>; # read all lines of the file into an array
close $AAAA;