Re: [osdldbt-general] dbt-1: datagen fixes

[<ma...@os...>]

Rats.  So it's the int casting that's causing all the trouble.  I will
look into making a better random number generator for ints then.  Anyone
have any good ones? :)

Mark

On 18 Apr, Motoyuki KAWABA wrote:
> Hi,
> 
> To decrease values of the 1st argument of get_random() doesn't look
> good fixing, because (int)get_random(MAX_VALUE) rarely returns
> MAX_VALUE.
> 
> Here is the definition of (int)get_random().
> 
>   (int)get_random(MAX_VALUE) 
>          = (int)(get_percentage() * MAX_VALUE)
>          = (int)((double)rand() / (double)RAND_MAX * MAX_VALUE)
> 
>          where rand() returns [0 .. RAND_MAX].
> 
> Only when rand() returns RAND_MAX, (int)get_random(MAX_VALUE) can return
> MAX_VALUE, because casting to integer is equivalent to a
> floor-function in this case. (That probability is 1/2147483647 under
> RedHat9.)
> 
> I've tried the next program which just counts up the frequencies of
> generated numbers.
> -------------
> #include <stdio.h>
> #include <stdlib.h>
> 
> double get_percentage();
> long long get_random(long long max);
> 
> #define MAX_VALUE 5                  /* Maximum value */
> 
> unsigned int hist[MAX_VALUE+1];
> 
> main() {
>   unsigned long  i = 0;
> 
>   for (i = 0; i <= MAX_VALUE; i++)
>     hist[i] = 0;
> 
>   for (i = 0; i < 100000; i++)
>     hist[(int)get_random(MAX_VALUE)]++;
> 
>   for (i = 0; i <= MAX_VALUE; i++)
>     printf("%d %d\n", i, hist[i]);
> }
> 
> /* Derived from common/common.c */
> long long get_random(long long max)
> {
> 	return (long long) (get_percentage() * (double) max);
> }
> 
> double get_percentage()
> {
> 	return (double) rand() / (double) RAND_MAX;
> }
> -------------
> Here is the output of the sample program.
> $ ./xxx
> 0 19925
> 1 20205
> 2 19952
> 3 19884
> 4 20034
> 5 0                          <- MAX_VALUE
> -------------
> I don't know whether get_percentage() should really return [0.0
> .. 1.0]. But at least it may be necessary to make another function for
> integer random number generation.
> 
> For example,
> 
> /* returns [0 .. max] */
> long long get_random2(long long max)
> {
> 	return (long long) (get_percentage2() * (double) (max + 1));
> }
> 
> /* returns [0 .. 1.0) */
> double get_percentage2()
> {
> 	return (double) rand() / ((double) RAND_MAX + 1.0);
> }
> -------------
> 
> Thanks.
> ----------
> Motoyuki KAWABA
> FUJITSU LABORATORIES LTD.
> ka...@la...