Editing an index only if it exists

Question

I need to create a 2D grid of 0's and 1's representing an image, and create a blur method which would change any 0 next to a 1 (left, right, above or below) into a 1, like so:

0000 => 0000
0000    0010
0010    0111
0000    0010

Next I have to allow the user to pass in a number that would allow the blur to extend multiple spaces in each direction. If I call image.blur(2), it would extend 2 spaces in each direction, but each first step would have to call blur again to account for diagonals. For example:

00000000 => 00010000
00000000    00111000
00010000    01111100
00000000    00111000
00000000    00010000

Here is my code.

class Image
  attr_accessor :picture
  def initialize(*rows)
    @picture = *rows
  end
  def output_image
    @picture.each_index do |i|
      puts @picture[i].join
    end
  end
  def blur(distance=1)
    @blurred_image = Array.new(@picture.length, 0) {Array.new(@picture[0].length, 0)} #create new array of zeroes the size of @picture
    @picture.each_index do |i|
      @picture[i].each_index do |j|
        if @picture[i][j] == 1
          @blurred_image[i][j]   = 1 if @blurred_image[i][j] 
          @blurred_image[i][j-1] = 1 if @blurred_image[i][j-1] 
          @blurred_image[i][j+1] = 1 if @blurred_image[i][j+1] 
          @blurred_image[i-1][j] = 1 if @blurred_image[i-1][j] 
          @blurred_image[i+1][j] = 1 if @blurred_image[i+1][j] 
        end
      end       
    end
    if distance > 1
      @picture = @blurred_image #save progress of image blur in @picture so we can continue in recursive call 
      blur(distance-1) #iterate as long as distance > 1
      elsif distance == 1 #necessary so blurred image isn't printed 'distance' times
      @blurred_image.each_index do |i|
        puts @blurred_image[i].join
      end
    end
  end
end

pic = Image.new(
  [0,0,0,0,0,0,0,0,0],
  [1,0,0,0,0,0,0,0,0],
  [0,0,0,0,0,0,0,0,0],
  [0,0,0,0,1,0,0,0,0],
  [0,0,0,0,0,0,0,0,0],
  [0,0,0,0,0,0,0,0,0],
  [0,0,0,0,0,0,0,0,0],
  [0,0,0,0,0,0,0,0,0],
  [0,0,0,0,0,0,0,0,0]
)
pic.blur(3)

My function works, but only if the 1's don't extend past the bounds of the array. If I put a 1 in a corner, it looks like my function attempts to edit the value of an index that doesn't exist, and I get the following message:

image_blur.rb:28:in `block (2 levels) in blur': undefined method `[]' for nil:NilClass (NoMethodError)
from image_blur.rb:22:in `each_index'
from image_blur.rb:22:in `block in blur'
from image_blur.rb:21:in `each_index'
from image_blur.rb:21:in `blur'
from image_blur.rb:35:in `blur'
from image_blur.rb:35:in `blur'
from image_blur.rb:47:in `<main>'

I'm trying to tell it to only assign a 1 to an index if the index exists. I'd appreciate any help.

Answer 1

Looks like you're trying to check two levels deep before you check one level deep.

eg it's possible that picture[1] doesn't exist... so if you then try to check picture[i][j] it fails.

You can try checking if each level is present before indexing into it...

if @picture[i] && @picture[i][j] && @picture[i][j] == 1
   if @blurred_image[i].present?
      @blurred_image[i][j]   = 1 if @blurred_image[i][j] 
      @blurred_image[i][j-1] = 1 if @blurred_image[i][j-1] 
      @blurred_image[i][j+1] = 1 if @blurred_image[i][j+1]
   end
   @blurred_image[i-1][j] = 1 if @blurred_image[i-1] && @blurred_image[i-1][j] 
   @blurred_image[i+1][j] = 1 if @blurred_image[i+1] && @blurred_image[i+1][j] 

Answer 2

My first thought would be to drop a lookup based solution like this in:

adjacents = [[-1, 0], [0, -1], [0, 0], [1, 0], [0, 1]].freeze
@blurred_image = Array.new(@picture.length, 0) { Array.new(@picture[0].length, 0) }

@picture.each_index do |i|
  @picture[i].each_index do |j|
    if @picture[i][j] == 1
      adjacents.each { |x, y| (a = @blurred_image[i+x]) && a[j+y] &&= 1 }
    end
  end
end

Note the &&= operator will perform the assignment only if the left hand side is truthy. So it would fail, for instance, if you initialised your inner arrays with nil values.