In addition to facts and rules, CLIPS also offers a full set of the programming functions associated with 'normal' procedural languages. We're now going to demonstrate some of them by programming CLIPS badly to set up the cards for a game of pontoon. We'll use the following snippet of code to get started:
(defglobal ?*shuffleswaps* = 150) (deffacts cards (card-names ace two three four five six seven eight nine ten jack queen king) (card-values 1 2 3 4 5 6 7 8 9 10 10 10 10) (card-suits hearts clubs diamonds spades) )
The first job is to set up the pack of cards we will us to play the game. This is done with a two rules, thus:
(defrule go (initial-fact) => (assert (state create-pack)) ) (defrule create-cards ?old-state <- (state create-pack) (card-names $?names) (card-suits $?suits) => (bind ?number 0) (loop-for-count (?suit 1 4) do (loop-for-count (?name 1 13) do (bind ?number (+ ?number 1)) (assert (draw-pile (nth$ ?name ?names) (nth$ ?suit ?suits) ?number)) ) ) (assert (top-card 1)) (retract ?old-state) (assert (state shuffle-pack)) )
The first rule is of course only a control rule; the second does the work. It uses a construct you've not seen before - the loop-for-count iterator. This is very similar to a for loop in C or BASIC. You give it a variable, a lower bound and an upper bound, and it repeats everything between the do and the closing bracket the appropriate number of times, incrementing the variable from the lower to the upper bounds. You might also want to look at the while function, which is similar. Another new function in this rule is nth$, which returns a single value at a given position in a multifield variable. So, if we had a variable ?a which had the multiple value (dog cat fish), then (nth$ 2 ?a) would return the value (fish). So, by the time this rule has finished, we have a pack of cards, but they are in a perfect order. We need to shuffle them. The following two rules do just that:
(defrule start-shuffle (state shuffle-pack) (not (swap-count ?)) => (seed (round (time))) (assert (swap-count 1)) (assert (swap-position1 (round (+ (* (/ (random) 32767) 51) 1)))) (assert (swap-position2 (round (+ (* (/ (random) 32767) 51) 1)))) ) (defrule shuffle-pack (state shuffle-pack) ?swapcard1 <- (swap-position1 ?cp1) ?swapcard2 <- (swap-position2 ?cp2) ?swapcount <- (swap-count ?cc) (test (< ?cc ?*shuffleswaps*)) ?card1 <- (draw-pile ?name1 ?suit1 ?cp1) ?card2 <- (draw-pile ?name2 ?suit2 ?cp2) => (retract ?card1) (retract ?card2) (retract ?swapcount) (retract ?swapcard1) (retract ?swapcard2) (assert (draw-pile ?name1 ?suit1 ?cp2)) (assert (draw-pile ?name2 ?suit2 ?cp1)) (assert (swap-count (+ ?cc 1))) (assert (swap-position1 (round (+ (* (/ (random) 32767) 51) 1)))) (assert (swap-position2 (round (+ (* (/ (random) 32767) 51) 1)))) ) (defrule pack-shuffled ?state <- (state shuffle-pack) ?swapcount <- (swap-count ?cc) (test (= ?cc ?*shuffleswaps*)) => (retract ?swapcount) (retract ?state) (assert (state print-deck)) )
The shuffling algorithm isn't brilliant. All it does is pick random pairs of cards and swap their places in the pack. There is no guarantee that the pack will be well shuffled. (How could you do this better?) The only new things in this rule are (seed (round (time))) and (random).
The next part of the program prints out the deck of cards in the order they would be dealt.
(defrule start-printdeck (state print-deck) (not (next-card ?)) (top-card ?topcard) => (assert (next-card ?topcard)) ) (defrule printdeck (state print-deck) ?nextcard <- (next-card ?number) (draw-pile ?name ?suit ?number) => (printout t ?number ": " ?name " of " ?suit " has value " (card-value ?name) crlf) (retract ?nextcard) (assert (next-card (+ ?number 1))) )
This rule uses a function (card-value) which is not part of CLIPS, but is user-defined. The definition is below:
(deffunction card-value (?card-name) (switch ?card-name (case ace then (bind ?return-value 1)) (case two then (bind ?return-value 2)) (case three then (bind ?return-value 3)) (case four then (bind ?return-value 4)) (case five then (bind ?return-value 5)) (case six then (bind ?return-value 6)) (case seven then (bind ?return-value 7)) (case eight then (bind ?return-value 8)) (case nine then (bind ?return-value 9)) (case ten then (bind ?return-value 10)) (case jack then (bind ?return-value 10)) (case queen then (bind ?return-value 10)) (case king then (bind ?return-value 10)) (default (bind ?return-value 0)) ) (return ?return-value) )
The function takes the name of a card (ace, two, king etc) and returns its numeric value. It uses the switch statement to do this.