|Perl: the Markov chain saw|
Iterating over Blocks of 2-Dim Arrayby Kozz (Friar)
|on Apr 01, 2004 at 02:49 UTC||Need Help??|
Kozz has asked for the
wisdom of the Perl Monks concerning the following question:
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):
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.