What is the purpose of '||=' in the example code for Spreadsheet::XLSX

[vt220]

I tried a modified version of the example code given in http://search.cpan.org/~dmow/Spreadsheet-XLSX-0.13-withoutworldwriteables/lib/Spreadsheet/XLSX.pm. It works very well and now I am trying to understand it.

I am not sure what these two lines are doing.

$sheet -> {MaxRow} ||= $sheet -> {MinRow};
$sheet -> {MaxCol} ||= $sheet -> {MinCol};

My guess is that if MaxRow or MaxCol are undefined then they are set to equal MinRow or MinCol respectfully. If that is true then under what circumstances is MAX not defined?

Also, I Data::Dumper::Simple'd $excel and it appears MinCol and MaxCol does not vary by row i.e., it is set once for each worksheet. So why execute it on each row? Or did I read the dump incorrectly?

Following is the code I ran:

use strict;
use warnings;
use Data::Dumper::Simple;
use Spreadsheet::XLSX;
#   use Text::Iconv;
#   my $converter = Text::Iconv -> new ("utf-8", "windows-1251");

# Text::Iconv is not really required.
# This can be any object with the convert method. Or nothing.

#   my $excel = Spreadsheet::XLSX -> new ('test.xlsx', $converter);

my $excel  = Spreadsheet::XLSX -> new ($ARGV[0]);

print Dumper($excel);

foreach my $sheet (@{$excel -> {Worksheet}}) {

    printf("Sheet: %s\n", $sheet->{Name});

    $sheet -> {MaxRow} ||= $sheet -> {MinRow};

    foreach my $row ($sheet -> {MinRow} .. $sheet -> {MaxRow}) {

        $sheet -> {MaxCol} ||= $sheet -> {MinCol};

        foreach my $col ($sheet -> {MinCol} ..  $sheet -> {MaxCol}) {

            my $cell = $sheet -> {Cells} [$row] [$col];

            if ($cell) {
                printf("( %s , %s ) => %s\n", $row, $col, $cell -> {Va
+l});
            }
        }
    }
}

Thanks in advance!