A Case Study

1
A Case Study

• Some Advice on How to Go about Solving Problems
  in Prolog
• A Program to Play Connect-4 Game

2
Some Advice on How to Go about Solving Problems
in Prolog

• 1. Have a very clear understanding of the problem
• what are the initial conditions?
• what is the goal state?
• what are the rules and the constraints?
• 2. Find suitable data structures to represent the
  problem states

3
Some Advice on How to Go about Solving Problems
in Prolog (cont)

• 3. Try to abstract the problem
• can it be divided into several sub-problems?
• is it similar to some problem that we already
  know how to solve?
• 4. When using recursion, to think about
• base case how to terminate the program
• general case how to define N from N-1
• 5. Write the code in a top-down fashion

4
Connect-4 Game

• Two players
• A 6x7 vertical board
• Each player drops one of his/her counters into
  any columns in turns
• The first player to get four of his/her counters
  in a row (vertically, horizontally or diagonally)
  wins the game.

5
Find suitable data structures to represent the
problem states

• 6x7 board use a list of lists
• _,_,_,_,_,_,_,
• _,_,_,_,_,_,_,
• _,_,_,_,_,_,_,
• _,_,_,_,_,_,_,
• _,_,_,_,_,_,_,
• _,_,_,_,_,_,_
• Variables represent empty slots
• o represents computers counters.
• x represents human player's counters.

6
Find suitable data structures to represent the
problem states

• An optional consideration
• To keep track of every step of a game, we can use
  a list to remember all the moves and the states
  of boards
• init_board, move1, board1, move2, board2,
• board is described as the above
• move is the form of Who, Move

7
How to access the board

• get_col(1, Board, Col)-
• Board X6,_,_,_,_,_,_,
• X5,_,_,_,_,_,_,
• X4,_,_,_,_,_,_,
• X3,_,_,_,_,_,_,
• X2,_,_,_,_,_,_,
• X1,_,_,_,_,_,_,
• Col X1,X2,X3,X4,X5,X6.
• get_row(1, Board, Row)- Board Row_.

• get_diagonal(3, Board, D)-
• Board X6,_,_,_,_,_,_,
• _,X5,_,_,_,_,_,
• _,_,X4,_,_,_,_,
• _,_,_,X3,_,_,_,
• _,_,_,_,X2,_,_,
• _,_,_,_,_,X1,_,
• D X1,X2,X3,X4,X5,X6.
• get_row(2, Board, Row)- Board _,Row_.

8
How to update the board

• Q. How to add a new counter to the nth column of
  a board?
• Go through the nth column (from bottom), add the
  counter at the first empty slot.
  
• x add(C, H_)-
• x x o -gt x x o var(H), H C.
• o x o o o x o o add(C, HT)-
• x o x x o x o x x o nonvar(H),
  add(C, T).

9
The main loop for playing the game

• One termination case
• If we can find a connect 4, then game is over.
• Two non-terminal cases
• if it is human players turn,
• get a move from the player
• make the move, let computer carry on
• if it is computers turn
• computer selects a move
• make the move, let human player carry on

10
The main loop for playing the game

• c4_loop(Turn, CurrentBoard)-
• check4(CurrentBoard). found a connect-4, game
  is over
• print_result(Turn).
• c4_loop(x, CurrentBoard)- it is human player's
  turn
• get_move_fm_human(CurrentBoard, Move),
• move(x, Move, CurrentBoard, NewBoard),
• print_move('You', x, Move), print_board(NewBoard)
  ,
• c4_loop(o, NewBoard).
• c4_loop(o, CurrentBoard)- otherwise
• computer_selects_a_move(CurrentBoard, Move),
• move(o, Move, CurrentBoard, NewBoard),
• print_move('I', o, Move), print_board(NewBoard),
• c4_loop(x, NewBoard).