MODULE AvgLoopLen;
(* Oxford Oberon-2 *)
IMPORT Random, Out;

PROCEDURE Fac(n: INTEGER; f: REAL): REAL;
BEGIN
	IF n = 0 THEN
		RETURN f
	ELSE
		RETURN Fac(n - 1,n*f)
	END
END Fac;

PROCEDURE Power(n,i: INTEGER): REAL;
VAR
	p: REAL;
BEGIN
	p := 1.0;
	WHILE i > 0 DO p := p * n; DEC(i) END;
	RETURN p
END Power;

PROCEDURE Abs(x: REAL): REAL;
BEGIN
	IF x < 0 THEN RETURN -x ELSE RETURN x END
END Abs;

PROCEDURE Analytical(n: INTEGER): REAL;
VAR
	i: INTEGER;
	res: REAL;
BEGIN
	res := 0.0;
	FOR i := 1 TO n DO
		res := res + (Fac(n,1.0) / Power(n,i) / Fac(n - i,1.0));
	END;
	RETURN res
END Analytical;

PROCEDURE Averages(n: INTEGER): REAL;
CONST
	times = 100000;
VAR
	rnds: SET;
	r,count,i: INTEGER;
BEGIN
	count := 0; i := 0;
	WHILE i < times DO
		rnds := {};
		LOOP
			r := Random.Roll(n);
			IF r IN rnds THEN EXIT ELSE INCL(rnds,r); INC(count) END
		END;
		INC(i)
	END;

	RETURN count / times
END Averages;

VAR
	i: INTEGER;
	av,an,df: REAL;
BEGIN
	Random.Randomize;
	Out.String("        Averages  Analytical  Diff%     ");Out.Ln;
	FOR i := 1 TO 20 DO
		Out.Int(i,3); Out.String(": ");
		av := Averages(i);an := Analytical(i);df := Abs(av - an) / an * 100.0;
		Out.Fixed(av,10,4);Out.Fixed(an,11,4);Out.Fixed(df,10,4);Out.Ln
	END
END AvgLoopLen.