Question

I would like to write a predicate for tic-tac-toe game, that generate possible successor board configuration for given player and board. After typing this input:

 next_board([[x,o,x],[x,x,o],[o,e,e]],x,N).

the output should be displayed as following:

 N=[[x,o,x],[x,x,o],[o,e,e]];
 N=[[x,o,x],[x,e,o],[o,x,e]];
 N=[[x,o,x],[x,e,o],[o,e,x]];

Answer 1

To generate possible successor boards you basically have to “replace” single empty cells (marked with e) with your player chip.

At first sight you might be tempted to use select/4 but it would replace every empty cell in a row with your player chip. So, you might be better off using append/3 predicate with something like this:

next_board(Board, Player, NextBoard):-
  append(Head, [Row|Tail], Board),  % Take a single row from the board
  append(RowHead, [e|RowTail], Row), % Take a single empty cell
  append(RowHead, [Player|RowTail], NRow), % and replace it with your player chip
  append(Head, [NRow|Tail], NextBoard). % Now build a next possible board

You might as well make recursive procedures to achieve the same logic without using append/3.