How to flip a matrix upside down

So recently we had some problem about flipping a matrix upside down, sounds easy, but not that simple if the matrix is stored as an array. For example, you have a 5*4 matrix like this:

and it’s written like this 11112222333344445555 The question is how to flip the matrix upside down without transforming the array to the 2D matrix. After flipping, the array should be like this: 55554444333322221111 So I wrote a Perl script to do this:

#!/usr/bin/perl -w

# this script convert an array to an n*m matrix and then 
# flip the matrix upside down then restore it back to an array

use strict;
my @div = (1,1,1,1,2,2,2,2,3,3,3,3,4,4,4,4,5,5,5,5);
print "@div\n";

my $row = 2;
my $col = 10;

my @out = map{0} 0..$#div; # define an empty array
print "@out\n";

for (my $i = 1 ; $i &lt;= $row; $i++) {
    for (my $j = 1; $j &lt;= $col; $j++) {
        $out[($col*($row-$i)+$j)-1] = $div[(($i-1)*$col+$j)-1];
    }
}

print "@out\n";

It’s an interesting exercise and it’s useful sometimes when you’re dealing with NetCDF files and Arcmap ascii outputs or converting files between binary and ascii.

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