Question

Can someone please guide me or explain how to perform backtracking in LISP ? Any examples or links would be appreciated.
I did try to google , however none of them had simple example enough for me to understand.

Thanks

Answer 1

The typical way is to have non-mutable state passed down the call-stack, with helper functions taking the current state-returning a new state to “fake” mutation.

A possible (although rather suboptimal) sudoku-solver would then be:

;;; Use a list of 81 integers to represent a sudoku board,
;;; each number 1-9 represents itself, 0 represents a blank
(defun sudoku-solver (board)
  (cond ((notany #'zerop board)
     (if (sudoku-solved-p board)
         board
         nil))
    (t (let ((positions (sudoku-all-blanks board)))
         (loop for position in positions
          do (loop for number in '(1 2 3 4 5 6 7 8 9)
              do (let ((result (sudoku-solver
                        (sudoku-set board
                            position
                            number))))
                   (when result
                 (return-from sudoku-solver result)))))))))

This will automatically backtrack until a solution is found. I have skipped obscuring the demonstration with the support code that would turn it from a demonstration to actual working code.