Chance

Introduction

Turbo's Random(6) gave a number from 0 to 5, and a program started the same way every time until it called Randomize. HiSoft's RANDOM is different on both counts - and it has a weakness the Turbo course did not have to warn you about.

RANDOM

RANDOM is a function with no brackets and nothing to give it. It returns a whole number from 0 to 255. For a dice throw, take the remainder after dividing by 6 and add 1:

A:=RANDOM MOD 6+1;

RANDOM(6) is refused, and so is RANDOMIZE - there is none, and none needed. RANDOM does not start from a fixed point: its numbers depend on the exact moment it is called, so a program run twice starts differently.

Where Chance Really Comes From

That same dependence on the moment is its weakness. A loop that does the same work between one call and the next calls RANDOM at the same rhythm each time round, and the numbers fall into step: a loop printing twenty of them ended 34 43 52 61 70 79, nine apart. Two runs of a loop throwing ten pairs of dice differed only in the first two throws; the last eight were identical. The manual warns of it too.

The one thing on the Einstein that is never in step is you. Take the number at a moment the player chose - just after a key is pressed - and it is as good as chance gets here. The code below throws the dice only when a key is pressed.

The Code

10 (*$C-*)
20 PROGRAM DICE;
30 VAR A,B:INTEGER; C:CHAR;
40 BEGIN
50 WRITELN('ANY KEY THROWS, Q STOPS');
60 REPEAT
70 REPEAT C:=INCH UNTIL C<>CHR(0);
80 A:=RANDOM MOD 6+1;
90 B:=RANDOM MOD 6+1;
100 WRITE(A,B);
110 IF A=B THEN WRITE('DOUBLE');
120 WRITELN
130 UNTIL C='Q'
140 END.

Line 10 is there for INCH (S15).

Starting from

DICE.ASC saved from XBAS; HPEIN DICE.

What you should see

A pair of numbers from 1 to 6 for each key you press, with DOUBLE after a pair that match - for example:

ANY KEY THROWS, Q STOPS
2 4
4 4 DOUBLE
2 1

Q throws once more, then stops. Run it again and the throws are different.

Change One Thing

• Change line 80 to A:=RANDOM(6)+1;, the Turbo way. Will it compile?
• Put RANDOMIZE; in front of line 50. And that?
• Change line 70 to C:=INCH;, so the loop no longer waits. What happens to the throws - and how do you stop it?

Exercises

16.1 Throw one die 600 times, count how often each number comes up, and print the six counts.

16.2 A guessing game: the program picks a number from 1 to 100 (after waiting for a key, so that it is a fair pick) and tells you "HIGHER" or "LOWER" until you guess it.

Worked solutions are in Appendix II.

When It Goes Wrong

Summary

RANDOM gives 0 to 255 and needs no seeding; RANDOM MOD n + 1 gives 1 to n. Called in a steady loop its numbers fall into patterns - so take them at moments the player chooses, after a key press.

Next

S17, Pictures - something Turbo could not do at all.