IMHO, this code is still broken. On a non-DOS platform it tries to pass an integer to a function expecting a cardinal and always returns a type check error and doesn't compile. Is there some advanced switch that should be used in situations like this to turn off type-checking (I've never really looked at all of the directives and didn't see where they were in the docs after a quick look) and the type-casting section is empty.
The code in question: FUNCTION Rand:Integer; C; PROCEDURE SRand(Seed:Cardinal); C; FUNCTION GetPID:Integer; C;
Regarding portability: in my DJGPP C library, there is no `srand' function, but a `srandom' function which expects an int (=Integer) as parameter. I am not sure whether these C functions belong to the ANSI standard for the C library (I have left my copy of `The C programing language' by KR at home).
PROCEDURE Randomize; BEGIN SRand(GetPID); RandSeed:=Rand END;
I don't see how this should emulate the Randomize procedure in Borland Pascal. According the info docs for my C library, getpid returns the unique process number for the program running on the system. If you call getpid twice in the same program, you get the same integer. Why not compute a seed from the system time (somehow), as Borland Pascal does?
That is, if we need a Randomize procedure at all? My C library (DJGPP) doesn't have such a beast. Ok, there is Borland compatibility ....
I use random numbers a great deal in my work (statistical simulations), and in most cases (if not all) I specify the seed myself, since this is the only way of gettting reproducible results. This may not sound like "random numbers", but there is really no such thing as random numbers on a digital computer! What we generate is a completely deterministic sequence (called `pseudo random') which just looks random; Whatever constitutes `random' in this context depends on the particular application!
When I was programming in Turbo Pascal (the pre-GPC era), I *never* used the Random function in the system unit because I did not have source code and thus couldn't see how the pseudo-random sequence was generated. There are a lot of bad (= non-random) random number generators out there! Instead, I wrote my own random number generator, or, rather found one in books/papers on the topic [there is a huge literature].
The following piece of code represents a widely used procedure for generating uniform random numbers (reals between 0.0 and 1.0), but it could easily be changed to generating integers between 0 and 2^31-1 (just modify the last line in RanUni).
The routine is portable to all systems with a 32-bit integer type. It does not rely on controlled overflow which may not be portable. The technically inclined can consult the reference given in the comments. BTW, GPC users familiar with the book(s) `Numerical Recipes' may be interested in knowing that the `ran0´ routine in the latest (2nd) edition (C only; not Pascal) corresponds to the algorithm below.
Anyways; here's the code:
var ru_s1 : Integer; { Global variable }
{ Initialize the seed with value s1 }
procedure Init_RanUni (s1 : Integer); begin ru_s1 := s1; end;
{ Basic uniform random generator: Minimal Standard in Park and } { Miller (1988): "Random Number Generators: Good Ones Are Hard to } { Find", Comm. of the ACM, 31, 1192-1201. } { Parameters: m = 2^31-1, a=48271. }
function RanUni : double; const im = 2147483647; ia = 48271; iq = 44488; { m div a } ir = 3399; { m mod a }
var k : Integer;
begin k := ru_s1 div iq;
ru_s1 := ia * (ru_s1 - k*iq) - ir * k; if ru_s1 < 1 then ru_s1 := ru_s1 + im;
RanUni := ru_s1 / im; end;
Regards,
Jesper Lund