Won’t You Be My (Grid) Neighbor

I’m working on an update to Froggy Went A Hoppin’ and want to make some things generic that I hardcoded in the first version. One of those is being able to see the “neighbors” of a given lily pad in the pond. For example, in the following level map:

1,1,0,4,1,0,1,1,0,4,1,
1,2,1,2,0,1,1,2,1,2,0,
1,1,0,1,1,0,1,1,0,1,1,
2,0,1,1,0,2,2,0,1,1,0,
1,1,0,4,1,0,1,1,0,4,1,
1,2,1,2,0,1,1,2,1,2,0,
1,1,0,1,1,0,1,1,0,1,1

…I wanted to know which lily pads were next to (left, right, up, down, and diagonal) any specified lily pad. (Note: I don’t want to know the values of those locations, I want to know the indexes into that grid — if I have that I can get the value, set it, etc.)

I ended up with this code:

--==============================================================
-- pass in a lily pad number and get back a
-- table containing all the neighbors for
-- that lily pad.

local numCols = 11
local numRows = 7

local function getNeighbors(padNum)

	local neighbors = {}

	local function inLeftColumn()
		return ((padNum-1) % numCols) == 0
	end
	local function inRightColumn()
		return (padNum % numCols) == 0
	end

	local inLeft = inLeftColumn() 
	local inRight = inRightColumn()

	-- number above, above left/right
	if padNum > numCols then
		neighbors[#neighbors+1] = padNum - numCols
		if not inLeft then
			neighbors[#neighbors+1] = padNum - numCols - 1
		end
		if not inRight then
			neighbors[#neighbors+1] = padNum - numCols + 1
		end
	end

	-- number below &amp; below left/right
	if padNum <= (numCols * (numRows-1)) then
		neighbors[#neighbors+1] = padNum + numCols -- number below
		if not inLeft then
			neighbors[#neighbors+1] = padNum + numCols - 1
		end
		if not inRight then
			neighbors[#neighbors+1] = padNum + numCols + 1
		end
	end

	-- number to right
	if not inRight then
		neighbors[#neighbors+1] = padNum + 1
	end

	-- number to left
	if not inLeft then
		neighbors[#neighbors+1] = padNum - 1
	end

	return neighbors
end

--==============================================================

-- pass in a lily pad number and get back a

-- table containing all the neighbors for

-- that lily pad.

local numCols = 11

local numRows = 7

local function getNeighbors(padNum)

local neighbors = {}

local function inLeftColumn()

return ((padNum-1) % numCols) == 0

end

local function inRightColumn()

return (padNum % numCols) == 0

end

local inLeft = inLeftColumn()

local inRight = inRightColumn()

-- number above, above left/right

if padNum > numCols then

neighbors[#neighbors+1] = padNum - numCols

if not inLeft then

neighbors[#neighbors+1] = padNum - numCols - 1

end

if not inRight then

neighbors[#neighbors+1] = padNum - numCols + 1

end

-- number below & below left/right

if padNum <= (numCols * (numRows-1)) then

neighbors[#neighbors+1] = padNum + numCols -- number below

if not inLeft then

neighbors[#neighbors+1] = padNum + numCols - 1

end

if not inRight then

neighbors[#neighbors+1] = padNum + numCols + 1

end

-- number to right

if not inRight then

neighbors[#neighbors+1] = padNum + 1

end

-- number to left

if not inLeft then

neighbors[#neighbors+1] = padNum - 1

end

return neighbors

end

If I pass a 1 into that function (using a grid that’s 11 wide by 7 tall) I get back a table containing 2, 12, and 13 — the neighbors for that location. If I pass in 12, I get back a table containing 1, 2, 13, 23, 24.

Showing the neighbors for 8, 48, and 77.

I was hoping to find a “clever” way to find the neighbors, but that’s what I’ve ended up with for now. I’ve only tested it with a couple different sized grids, but I think it should work for just about any.

One comment on “Won’t You Be My (Grid) Neighbor”

Jay says:

February 15, 2014 at 1:20 am

Thanks to Mark in beautiful West Wellow — he pointed me in the right direction for the modulo thing I was thinking of. So the two functions toward the top now look like this:

	local function inLeftColumn()
		return ((padNum-1) % numCols) == 0
	end

	local function inRightColumn()
		return (padNum % numCols) == 0
	end

local function inLeftColumn()

return ((padNum-1) % numCols) == 0

end

local function inRightColumn()

return (padNum % numCols) == 0

end

So much nicer. Just the one line in each function.

Just for the sake of history, this is what they looked like before Mark stepped in.

	local function inLeftColumn()
		local inLeft = false
		for x = 1, numCols * numRows, numCols do
			if x == padNum then
				inLeft = true
			end
		end
		return inLeft
	end
	local function inRightColumn()
		local inRight = false
		for x = numCols, numCols * numRows, numCols do
			if x == padNum then
				inRight = true
			end
		end
		return inRight
	end