Kozz has asked for the wisdom of the Perl Monks concerning the following question:

Esteemed monks:

I've got data in an n-by-n two-dimensional array. Assuming that my n is divisible by two, I need to be able to iterate over the items in sub-matrices. Let me explain, and see if you can help me with this algorithm. I've tried, but can't quite wrap my head around it, and the code I DO make always tends to be far uglier than I'd expect it to be.

Disclosure: This is homework related. However, iterating over the data in this manner is not the main thrust of it. My "work" was done in the assignment of the 2-dim array elements' values. I just chose to break up the output into more manageable blocks (perhaps badly?).

Supposing n = 4, then I've got 4 "rows" of indices 0..3, each row having a "column" with indices 0..3, like this (vertical bars and dashed line to represent a line dividing matrix into quadrants):

[ 00 01 | 02 03 ] [ 10 11 | 12 13 ] ----------------- [ 20 21 | 22 23 ] [ 30 31 | 32 33 ]
I want to output the four elements in the upper-left quadrant of the matrix (00, 01, 10, 11, in that order), then upper right, lower left, lower right.

What trips me up is that I iterate over the first two cols of the first row, then increment row but reset cols. Then row gets reset and cols adjusted.... it's just a very awkward traversal of the matrix.

I know this is more of an algorithms question than a perl question, so smack me down with a -- if you must. But I know that there are monks that can likely whip up a very elegant solution for this.

Thanks for any insight you can provide.

Update: Code per request. This script was made just for a "simple" (ha-ha) base case to see if I could figure out how to iterate over pairs of indices in this manner. I know I've got an infinite loop, too, while I'm trying to figure out when to increment/decrement my rows & columns.

#!/usr/bin/perl my $n = 2; my $max_exp = 1; use strict; my $max_part = 2**$max_exp; my $size = 2**$n; print "Matrix is $size by $size.\n"; print "Max size I'm allowed to print is $max_part at a time.\n"; my $max_part_tiles = $max_part**2; my $total_tiles = ($size)**2; my $tiles_counted = 0; my $part_tiles_counted = 0; my ($row_num, $col_num, $end_col) = (0, 0); my ($cols_displayed, $rows_displayed) = (0, 0); while($tiles_counted < $total_tiles){ $col_num=0; $end_col=$max_part; while($row_num < $size){ while($col_num < $end_col){ print "$row_num $col_num\n"; $col_num++; $part_tiles_counted++; } if($col_num < ($size-1) && (($row_num + 1) % $size > 0 +) ){ $row_num++; $col_num-=$max_part; } if($col_num < ($max_part-1) && $row_num == ($size-1)){ # $col_num++; $row_num-=$max_part; $end_col+=$max_part; } if($col_num == ($size-1) && $row_num < ($size-1)){ $row_num++; $col_num-=$max_part; } if($col_num == ($max_part-1) && $row_num == ($size-1)) +{ $row_num++; $col_num=0; $end_col=$max_part; } $tiles_counted += $part_tiles_counted; } }