CODEWITHSUNDEEP
arraysperl
Ben
Ben

Reputation: 153

Referencing an element in a 2D array in Perl

I have the following code which reads in a 6x6 array from STDIN and saves it as an array of anonymous arrays. I am trying to print out each element with $arr[i][j], but the code below isn't working. It just prints out the first element over and over. How am I not accessing the element correctly?

#!/user/bin/perl

my $arr_i = 0;
my @arr = ();
while ($arr_i < 6){
    my $arr_temp = <STDIN>;
    my @arr_t = split / /, $arr_temp;
    chomp @arr_t;
    push @arr,\@arr_t;
    $arr_i++;
}
foreach my $i (0..5){
    foreach my $j (0..5){
        print $arr[i][j] . "\n";
    }
}

Upvotes: 0

Views: 61

Answers (2)

Ed.
Ed.

Reputation: 2062

To demonstrate the Perlish mantra that there's "more than one way to do it":

use 5.10.0; # so can use "say"
use strict;
use warnings qw(all);

sub get_data {
  my ($cols, $rows) = @_;
  my ($line, @rows);
  my $i;
  for ($i = 1; $i <= $rows and $line = <DATA>; $i++) {
    chomp $line;
    my $cells = [ split ' ', $line ];
    die "Row $i had ", scalar(@$cells), " instead of $cols" if @$cells != $cols;
    push @rows, $cells;
  }
  die "Not enough rows, got ", $i - 1, "\n" if $i != $rows + 1;
  \@rows;
}

sub print_data {
  my ($cols, $rows, $data) = @_;
  for (my $i = 0; $i < $rows; $i++) {
    for (my $j = 0; $j < $cols; $j++) {
      say $data->[$i][$j];
    }
  }
}

my $data = get_data(6, 6);
print_data(6, 6, $data);

__DATA__
1 2 3 4 5 6
a b c d e f
6 5 4 3 2 1
f e d c b a
A B C D E F
7 8 9 10 11 12

Explanation:

  • if we use say, that avoids unsightly print ..., "\n"
  • get_data is a function that can be called and/or reused, instead of just being part of the main script
  • get_data knows what data-shape it expects and throws an error if it doesn't get it
  • [ ... ] creates an anonymous array and returns a reference to it
  • get_data returns an array-reference so data isn't copied
  • print_data is a function too
  • both functions use a conventional for loop instead of making lists of numbers, which in Perl 5 needs to allocate memory

There is also a two-line version of the program (with surrounding bits, and test data):

use 5.10.0; # so can use "say"
my @lines = map { [ split ' ', <DATA> ] } (1..6);
map { say join ' ', map qq{"$_"}, @$_ } @lines;

__DATA__
1 2 3 4 5 6
a b c d e f
6 5 4 3 2 1
f e d c b a
A B C D E F
7 8 9 10 11 12

Explanation:

  • using map is the premier way to iterate over lists of things where you don't need to know how many you've seen (otherwise, a for loop is needed)
  • the adding of " around the cell contents is only to prove they've been processed. Otherwise the second line could just be: map { say join ' ', @$_ } @lines;

Upvotes: 0

toolic
toolic

Reputation: 62236

i and j are not the same as the variables you declared in the foreach lines. Change:

print $arr[i][j] . "\n";

to:

print $arr[$i][$j] . "\n";

warnings alerted me to this issue. You should add these lines to all your Perl code:

use warnings;
use strict;

Upvotes: 4

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