Chris Pine's, "Learning to Program" sample exercise for calculating land mass in a Civilization game. Page 73.

landmass.rb

# 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,o],
         [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],
         [o,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 world[y][x] != 'land'
# Either it's water or we already counted it,
# but either way, we don't want to count it now. 
  return 0
end
  #  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)