🚀 go-pugleaf

lovens weche wrote:
> 
> Hi, for a lotto program I'm working on I want to code a random number
> generator *but*  numbers generated must be always different.
> 
> ex:
> 
> 1st number genereated: 2
> 2nd number generated: 6
> 3rd number generated: 2 (WRONG!!! bad "period" as they say I think)
> 
> ok so we can forget the rand() funtion (please don't tell me to code a
> routine that checks numbers generated and if they repeat re-draw a number,
> bad performance code...)
> 
> ok here's the code I have to genereate the random numbers.. the variable a,
> c and m must be set by me the programmer.. hummm thats where my problems
> begin...
> 
> --- the code (don't you love the C programming language?
> ;) ------------------
> 
> a = 2 ^ 31;

This sets a to 2.

> c = 3;
> m = (2 ^ 31) -1;

This sets m to 1.

> 
> // wait 1 millisecond
> 
> VectCurrPtr[0] = (a * time() + c) % m;
> 
> for (i = 1; i < NumElem; i++)
>     VectCurrPtr[i] = (a * VectCurrPtr[i - 1] + c) % m;
> 

This is a linear congruential generator. With the values you've given
above it will do a very bad job of producing random numbers. But even
with good values it will repeat. And it should. Think about it: if you
threw a die and in every six throws you got all six faces exactly once
you'd be very suspicious. And you'd have a game that was very easy to
beat.

You need a second step beyond the random number generator, to supply a
random sequence:

for (i = 0; i < NumElem; i++)
	VectCurrPtr[i] = i;
for (i = NumElem; 1 < i; i--)
	{
	int j = rand() % (i - 1);
	int temp = VectCurrPtr[j];
	VectCurrPtr[j] = VectCurrPtr[i - 1];
	VectCurrPtr[i - 1] = temp;
	}

-- 

Pete Becker
Dinkumware, Ltd. (http://www.dinkumware.com)