Elementary Array Question

rubyonrails-talk
1

*From Chris Pine, Learn to Program

Hey All -- In the below source code they array is called in the method as x, y, but the "if" statements are looking to [y]. Can anyone explain to me why they are switched? If you make them [y] it still outputs the correct number (23). Thanks in advance, Jordan

## Start of Example

# These are just to make the map # easier for me to read. "M" is # visually more dense than "o".

M = ' land ' o = ' water '   world = [[o,o,o,o,o,o,o,o,o,o,o],            [o,o,o,o,M,M,o,o,o,o,o],            [o,o,o,o,o,o,o,o,M,M,M],            [o,o,o,M,o,o,o,o,o,M,o],            [o,o,o,M,o,M,M,o,o,o,o],            [o,o,o,o,M,M,M,M,o,o,o],            [o,o,o,M,M,M,M,M,M,M,o],            [o,o,o,M,M,o,M,M,M,o,o],            [o,o,o,o,o,o,M,M,o,o,o],            [M,M,o,o,o,M,o,o,o,o,o],            [o,o,o,o,o,o,o,o,o,o,o]]

def continent_size world, x, y   if x > 10 && y > 10    size = 0   else     if world[y] != ' land '      return 0     end   end # Either it ' s water or we already # counted it, but either way, we don ' t # want to count it now.

# So first we count this tile... size = 1 world[y] = ' counted land ' # ...then we count all of the # neighboring eight tiles (and, # of course, their neighbors by # way of the recursion). size = size + continent_size(world, x-1, y-1) size = size + continent_size(world, x , y-1) size = size + continent_size(world, x+1, y-1) size = size + continent_size(world, x-1, y ) size = size + continent_size(world, x+1, y ) size = size + continent_size(world, x-1, y+1) size = size + continent_size(world, x , y+1) size = size + continent_size(world, x+1, y+1) size end

puts continent_size(world, 5, 5)

2

What you've got there is an array of rows (rather than an actual 2D construct), so it's normal that [y] first selects a row, then an element in that row.

Fred

3

Hey Fred,

Thanks for responding.

I was assuming that because of the line:

continent_size world, x, y

x defined the row and y defined the array value within that row. Is that not true?

world[y] and world[y] only outputs the right value because 5, 5 is right smack in the middle of the array of arrays. If you change it to 4, 3 and run it with world[y] and then try it with world[y] you get two different values. I'm still not sure why the code starts with x, y and then get's switched to [y] (I've pasted the original below showing the [y] values. Am I just being dense?:

# These are just to make the map # easier for me to read. "M" is # visually more dense than "o".

M = ' land ' o = ' water '   world = [[o,o,o,o,o,o,o,o,o,o,o],            [o,o,o,o,M,M,o,o,o,o,o],            [o,o,o,o,o,o,o,o,M,M,M],            [o,o,o,M,o,o,o,o,o,M,o],            [o,o,o,M,o,M,M,o,o,o,o],            [o,o,o,o,M,M,M,M,o,o,o],            [o,o,o,M,M,M,M,M,M,M,o],            [o,o,o,M,M,o,M,M,M,o,o],            [o,o,o,o,o,o,M,M,o,o,o],            [M,M,o,o,o,M,o,o,o,o,o],            [o,o,o,o,o,o,o,o,o,o,o]]

def continent_size world, x, y   if x > 10 && y > 10    size = 0   else     if world[y] != ' land '      return 0     end   end # Either it ' s water or we already # counted it, but either way, we don ' t # want to count it now.

# So first we count this tile... size = 1 world[y] = ' counted land ' # ...then we count all of the # neighboring eight tiles (and, # of course, their neighbors by # way of the recursion). size = size + continent_size(world, x-1, y-1) size = size + continent_size(world, x , y-1) size = size + continent_size(world, x+1, y-1) size = size + continent_size(world, x-1, y ) size = size + continent_size(world, x+1, y ) size = size + continent_size(world, x-1, y+1) size = size + continent_size(world, x , y+1) size = size + continent_size(world, x+1, y+1) size end

puts continent_size(world, 5, 5)

4

Hey Fred,

Thanks for responding.

I was assuming that because of the line:

continent_size world, x, y

x defined the row and y defined the array value within that row. Is that not true?

world[y] and world[y] only outputs the right value because 5, 5 is right smack in the middle of the array of arrays. If you change it to 4, 3 and run it with world[y] and then try it with world[y] you get two different values. I'm still not sure why the code starts with x, y and then get's switched to [y] (I've pasted the original below showing the [y] values. Am I just being dense?:

The code is written so that 9,0 calculates the value for the point at
the far right of the top row. You need to permute the values, since the first row is given by
world[0], and the far right element is world[0][9] You could just not permute the x&y and say that the world must be
specified as an array of columns rather than an array of rows but that
would result in the map being less human readable.

Fred