This was the bulk of my logic for coding Conway's Game of Life as I was first learning to code (last week, that is).

game_of_life_old.rb

# setup code whereby the world of gridsize "x" by "x" cells is
# generated has been omitted for brevity, but know that the world
# argument below refers to an array of arrays giving us a 2D 'map'
# of sorts full of cells; if a cell's value == 1, that cell is alive;
# if its value == 0, that cell is dead. whether a cell lives or dies
# in the next generation of life depends solely on its # of neighbors
# and that logic is contained below.

def run_generation_of_life(world)
  new_world = []
  world.each_with_index do |row, row_index|
    new_world_row = []
    row.each_with_index do |cell, column_index|
      if cell == 1
        if neighbors(row_index, column_index, world).size > 3 || neighbors(row_index, column_index, world).size < 2
          cell = 0
          new_world_row << cell
        elsif [2,3].include? neighbors(row_index,column_index,world).size
          cell = 1
          new_world_row << cell
        end
      elsif cell == 0  # if cell is dead...
        if neighbors(row_index,column_index,world).size == 3
          cell = 1
          new_world_row << cell
        else
          cell = 0
          new_world_row << cell
        end
      end
    end
    new_world << new_world_row
  end
  new_world
end