Factorials

Did you ever wonder how many ways to arrange n distinct objects? The answer is the factorial or the product of all positive integers less than or equal to n. For example, if there are three objects, a, b and c they can be arranged in 3 x 2 x 1 = 6 ways. abc, bca, cab, cba, acb, and bac.

These arrangements are called the permutations of the set of objects. Try a permutation of four objects. (You should have 4 x 3 x 2 x1 = 24 arrangements.) Here are variants of a Logo procedure that will tell you the permutations of any number of objects up to about 150. For those of you into graphics, you should be able experimentally check the theory of the math by arranging the permutations of a more modest number of objects from the toolbox.

FACTORIAL.lgo

TO FACTORING :N
    ; Limit the number factored to a maximum of 150
    LOCAL "ANSWER
    LOCAL "COUNTER
    MAKE "ANSWER 1
    MAKE "COUNTER :N
    REPEAT :N [MAKE "ANSWER :ANSWER * :COUNTER MAKE "COUNTER :COUNTER - 1]
    OUTPUT :ANSWER
END

TO FACTORIAL.FOR :N
    LOCAL "ANSWER
    MAKE "ANSWER 1
    FOR "I 1 :N [MAKE "ANSWER :ANSWER * :I]
    OUTPUT :ANSWER
END

TO FACTORIAL.WHILE :N
    LOCAL "ANSWER
    LOCAL "I
    MAKE "ANSWER 1
    MAKE "I :N
    WHILE [:I > 0] [MAKE "ANSWER :ANSWER * :I MAKE "I :I - 1]
    OUTPUT :ANSWER
END

TO FACTORIAL.GO :N
    LOCAL "ANSWER
    LOCAL "I
    MAKE "ANSWER 1
    MAKE "I :N
    LABEL "LOOP
    MAKE "ANSWER :ANSWER * :I
    MAKE "I :I - 1
    IF :I > 0 THEN GO "LOOP
    OUTPUT :ANSWER
END

TO FACTORIAL.RECUR :N
    IF :N = 0 THEN OUTPUT 1
    OUTPUT :N * FACTORIAL.RECUR :N - 1
END

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Procedure	FACTORING, FACTORIAL.FOR, FACTORIAL.WHILE, FACTORIAL.GO, FACTORIAL.RECUR
Description	Various ways to calculate the factorial of a number
Level	Beginner
Tags	Math, Factorial