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Shawn W_ 09-15-2009 03:56 PM

Making an array wrap, where last index + 1 = first index
 
I'm trying to manipulate a 2D array. I have created a method that looks
in adjacent cells of my 2D array and returns the contents found there.
When my method tries to look for cells to the left of the far left
column, Ruby will naturally 'wrap around' and return the far right
column, because array index 0 - 1 = -1, which Ruby interprets as the
last element of an array, in this case the far right of my rectangular
2D array. Same for beyond the top row.

What I want to do is replicate this behaviour for looking beyond the
bottom row and far right column. In other words, I want my 2D array to
wrap around top and bottom, left and right (in truth I only want left
and right to wrap, and top and bottom to be blocked, but for now I'd
just settle for full wrap).

What would be the easiest solution. Perhaps modification or additions to
the Array class in my program? Perhaps some addtions to my Array2D
class?
--
Posted via http://www.ruby-forum.com/.


Fabian Streitel 09-15-2009 06:10 PM

Re: Making an array wrap, where last index + 1 = first index
 
[Note: parts of this message were removed to make it a legal post.]

Hi there!


> What would be the easiest solution. Perhaps modification or additions to
> the Array class in my program? Perhaps some addtions to my Array2D
> class?
>



You could overload the [] method, if you're only interested in array access:

class Array2D < Array
def []( index )
if index >= self.length
#wrap around
else
super
end
end

something like that... ;-)
Same goes for []=

Greetz!


Aldric Giacomoni 09-15-2009 07:27 PM

Re: Making an array wrap, where last index + 1 = first index
 
def class Array2D < Array
def []( index )
super [] index % self.length
end
end

How about that?
--
Posted via http://www.ruby-forum.com/.


Robert Klemme 09-16-2009 08:19 AM

Re: Making an array wrap, where last index + 1 = first index
 
2009/9/15 Aldric Giacomoni <aldric@trevoke.net>:
> def class Array2D < Array
> =A0def []( index )
> =A0 =A0super [] index % self.length
> =A0end
> end
>
> How about that?


10:16:29 c$ ruby -c <<EOF
> def class Array2D < Array
> def []( index )
> super [] index % self.length
> end
> end
> EOF

-:1: formal argument cannot be a constant
def class Array2D < Array
^
-:1: syntax error, unexpected '<', expecting '\n' or ';'
def class Array2D < Array
^
-:3: syntax error, unexpected tIDENTIFIER, expecting kEND
super [] index % self.length
^
-:5: syntax error, unexpected kEND, expecting $end
10:16:35 c$

Rather

10:18:27 c$ ruby19 <<EOF
> class ArrayWrap < Array
> def []( index )
> super(index % length)
> end
> end
> a =3D ArrayWrap.new(5) {|i| i}
> 7.times do |i|
> printf "%2d %2d\n", i, a[i]
> end
> EOF

0 0
1 1
2 2
3 3
4 4
5 0
6 1
10:19:08 c$

Cheers

robert

--=20
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/


Shawn W_ 09-16-2009 11:33 AM

Re: Making an array wrap, where last index + 1 = first index
 
Aldric Giacomoni wrote:
> def class Array2D < Array
> def []( index )
> super [] index % self.length
> end
> end
>
> How about that?


I thought there might be a neat mathematical way to do this but never
would have found it by myself. Finding the remainder (% in Ruby) of:

array index / array size

works nicely.

I couldn't plug that code directly into my program and make it work, but
I used the % technique. In my case it's a 2D array. I'm using the code
by James Gray from the thread
http://www.ruby-forum.com/topic/77572#850992, namely:

01 class Array2D
02
03 def initialize(width, height)
04 @data = Array.new(width) { Array.new(height) }
05 end
06
07 def [](x, y)
08 @data[x][y]
09 end
10
11 def []=(x, y, value)
12 @data[x][y] = value
13 end
14
15 end

I modified it to incorporate 'wrapping' like so:

01 class Array2D
02
03 attr_accessor :width, :height
04
05 def initialize(width, height)
06 @width = width
07 @height = height
08 @data = Array.new(@width) { Array.new(@height) }
09 end
10
11 def [](x, y)
12 x = x % @width
13 y = y % @height
14 @data[x][y]
15 end
16
17 def []=(x, y, value)
18 x = x % @width
19 y = y % @height
20 @data[x][y] = value
21 end
22
23 end

I'm not sure how impressed Rubyists would be with the neatness of this
solution but it works.

Thx very much.
--
Posted via http://www.ruby-forum.com/.


Aldric Giacomoni 09-16-2009 02:45 PM

Re: Making an array wrap, where last index + 1 = first index
 
Well, as pointed out, I'm an idiot and wrote "def class" .. It should
just be "class" on the first line :-)

Glad to be of help.
--
Posted via http://www.ruby-forum.com/.



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