One Die Game (#203)


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One Die Game (#203)

Hej Rubyists,

This week’s quiz was submitted by Siep K. through the
suggestions page

Imagine a two player game with the following rules:

  • Playing material: 1 ordinary die; opposing sides sum up to 7.
  • The first move consists of the first player rolling the die and
    initializing the total to the amount on the face shown.
  • All further moves consist of turning the die one quarter and adding
    the face value of the new side to the current total. Example: If the
    die is facing 2 after the initial throw, then possible moves are 1,3,4
    and 6, with totals 3,5,6 and 8.
  • A player wins when that player’s move results in a total of 31.
  • A player loses when that player overshoots 31.

Let’s say it’s your turn. The current total is 24 and the die is
facing 3. You can win by turning the die to face 6, totaling 30. Your
opponent cannot play 1 because it’s at the bottom of the die, so he is
forced to overshoot the goal of 31.

Build a program which plays this game against a human opponent and
blunders occasionally (to keep it fun).

P.S. I did not invent this game; I’m pretty sure I found this game in
a chapter about nim-like games1 in one of Martin Gardner’s books.

Have Fun!

Why do I feel like I’m doing someone’s homework assignment ?

Here’s my attempt at the quiz:

I know Daniel asked that the computer sometimes make a mistake (for
fun), but my computer is not really into having fun. It’s a type-a,
aggressive, winner takes all computer. Don’t mess with it.

I’m throwing the gauntlet down. Write you own computer opponent and we
can have them battle it out!

Subclass Player:
class MyRadAI < Player

Create a method next move:
def next_move(dice, opponents)

opponents is an array of the other players and each opponent has name
and score accessors.
the dice class has some convenient methods available to you to figure
out which moves are available:
def available_nums
def valid?(choice) is also useful.


My AI is simple, but I think it is still pretty playable.


class Game
class Player
attr_reader :name
def initialize(die,name)
@die,@name = die,name
def choose_move(score)

class AI < Player
def choose_move(score)
if score < 25
while !@die.legal_move?( (r = rand(6)+1) )
return r
#simple win
return 31-score if @die.legal_move?(31-score)
return rand(6)+1

class Die
attr_reader :current_side
def initialize
@opposites = [[1,6],[2,5],[3,4]]
def roll
@current_side = rand(6)+1
def legal_move?(side)
return false if side == @current_side
return false if side > 6 or side < 1
a = [@current_side,side].sort!
return !@opposites.include?(a)
def flip(side)
if legal_move?(side)
@current_side = side
private :roll
def initialize
print "enter your name: "
name = gets.chomp
@d =
@bust, @score = 31,0
@players = [,“the computer”),,name)]
@players = rand(10) < 5 ? @players : @players.reverse
puts “#{} rolled the die to
@score += @d.current_side
def update
@players.each do |p|
puts “current score = #{@score}”
while @d.flip(p.choose_move(@score)) == 1 ? true : false
puts “invalid move”
puts “#{} flipped the die to
@score += @d.current_side
if @score > @bust
puts “#{} lost!”
return false
if @score == @bust
puts “#{} won!”
return false
return true
def play
while update
private :update

g =

Luke C. and Chris Cacciatore both submitted solutions for this
week’s quiz. There was some ambiguity in the way the quiz was posted,
so the implementations were slightly different from one another. Luke
had a running total for each player and Chris had a single total.
Despite that difference, both solutions tackled the problem similarly
and are both interesting.

The Dice classes in both solutions have methods for rolling a random
side and validating moves by keeping track of opposites. Chris keeps
an array of explicitly defined opposites: @opposites = [[1,6],[2,5],[3,4]]. Luke’s method for determining available moves
makes use of the fact that opposites are defined by the faces adding
up to seven:

def available_nums
(1…@total_sides).reject {|n| (@top + n == (@total_sides + 1) || n
== @top)}

Each of these implementations has a different strength when it comes
to future changes: Chris’s is easier to rearrange the configuration of
the dice and Luke’s is easier to expand to dice with a different
number of faces. One caveat for dice of more faces is that additional
adjacency information would need to be added (for example a
dodecahedron or icosahedron would have different adjacencies).

Both solutions require the definition of only one method to create a
new player or different AI. For Luke’s code the only method that needs
to be implemented is next_move. Let’s take a look at the Human and
Computer Player implementations:

class Human < Player
def next_move(dice, opponents = nil)
done = false
while(done == false)
choice = gets.to_i
if(dice.valid?(choice) == true)
done = true
print “!!!you can’t choose #{choice}!!!”



For the human player it gets the choice from the prompt, prompting
again if the player selects an invalid choice.

class Computer < Player
def next_move(dice, opponents = nil)
if(@score == 30)
return 1
elsif((@score % 6) == 0) && dice.available_nums.include?(6)
return 6
return dice.available_nums[rand(4)]

The computer player tries to set up a path for a win. If the score is
a multiple of 6 and the computer is able to choose 6 on the die, then
it will choose 6 each time until the score reaches 30, and then will
choose 1 on the next turn to secure victory. If it can’t then it makes
a random choice. It would be fairly easy to implement different
computer AI by providing different ways to compute the next move.

Chris uses a similar technique for choosing the next move from the
computer player. It checks for a winning move and takes it if
available, otherwise it makes a random move.

def choose_move(score)
if score < 25
while !@die.legal_move?( (r = rand(6)+1) )
return r
#simple win
return 31-score if @die.legal_move?(31-score)
return rand(6)+1

The play consists of players alternating moves. Both programs check
for the validity of the move outside of the player’s implementation,
so you are unable to cheat by having your player return a roll of `(31

  • current_value)`.

An interesting element of Luke’s program is that he allows you to set
up a game between two human players or two computer players rather
than just between a human and a computer.

Though the specific game mechanics were slightly different in each
program, they both provided interesting solutions to the problem.

Thank you Luke and Chris for your submissions to this week’s quiz!

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