Elementary Array Question

Rails
1

*From Chris P., 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][x]. Can anyone explain to
me why they are switched? If you make them [x][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[x][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[x][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

WORLD_VALUES = {:o => :water, :X => :land}

WORLD = [[:o,:o,:o,:o,:o,:o,:o,:o,:o,:o],
[:o,:o,:o,:o,:X,:X,:o,:o,:o,:o],
[:o,:o,:o,:o,:o,:o,:o,:o,:X,:X],
[:o,:o,:o,:X,:o,:o,:o,:o,:o,:X],
[:o,:o,:o,:X,:o,:X,:X,:o,:o,:o],
[:o,:o,:o,:o,:X,:X,:X,:X,:o,:o],
[:o,:o,:o,:X,:X,:X,:X,:X,:X,:X],
[:o,:o,:o,:X,:X,:o,:X,:X,:X,:o],
[:o,:o,:o,:o,:o,:o,:X,:X,:o,:o],
[:X,:X,:o,:o,:o,:X,:o,:o,:o,:o],
[:o,:o,:o,:o,:o,:o,:o,:o,:o,:o]]

def continent_size(x, y)
size = 0

begin
    WORLD.first(x).each do |row|
        row.first(y).each do |col|
            size += 1 if WORLD_VALUES[col] == :land
        end
    end
rescue
    return "invalid continent size"
end

return size

end

continent_size 10, 10 #should be 30

HTH,
Aldo S.

jordantheous wrote:

*From Chris P., 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][x]. Can anyone explain to
me why they are switched? If you make them [x][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[x][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[x][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)

3

On Jan 23, 8:34 pm, jordantheous [email protected] wrote:

*From Chris P., 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][x]. Can anyone explain to
me why they are switched? If you make them [x][y] it still outputs
the correct number (23).
Thanks in advance,
Jordan

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

Fred

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[x][y] and world[y][x] 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[x][y] and then try it with world[y][x]
you get two different values. I’m still not sure why the code starts
with x, y and then get’s switched to [y][x] (I’ve pasted the original
below showing the [y][x] 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][x] != ’ 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][x] = ’ 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)

On Jan 23, 1:38 pm, Frederick C. [email protected]

5

On 24 Jan 2009, at 17:45, jordantheous wrote:

that not true?

world[x][y] and world[y][x] 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[x][y] and then try it with world[y][x]
you get two different values. I’m still not sure why the code starts
with x, y and then get’s switched to [y][x] (I’ve pasted the original
below showing the [y][x] 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