On Mon, Jun 8, 2015 at 3:17 AM, C.D. Reimer <ch...@cdreimer.com> wrote:
> The Python random shows a uniform bell curve with low numbers at the ends
> and the peak in the middle, which is similar to the text in the book for the
> BASIC program. The Cython C rand is over all the place (especially with a
> negative number), which reminds me how bad the random number generator was
> on my 1MHz C64 in the day.
The negative result is a strong indicator that you're not seeing the
results of rand() here. While there is a potential for bias (check out
RAND_MAX, and consider that there may be some small bias there;
although on most modern systems, RAND_MAX is going to be high enough
that the bias from modulo 6 won't be highly visible), it's much MUCH
more likely that you're looking at uninitialized memory. I'm not
overly familiar with Cython, but can you simply add "={0}" to the
declaration to force it to be initialized to all zeroes? That's what
I'd do in raw C. Here's the same thing done in straight-forward C:
#include <stdlib.h>
#include <stdio.h>
void roll(int *f,int x)
{
int i, a, b;
for (i=0; i<x; ++i)
{
a = rand() % 6 + 1;
b = rand() % 6 + 1;
f[(a + b) - 1] += 1;
}
}
int main()
{
int f[12];
int i;
roll(f, 50000000);
for (i=1;i<12;++i)
printf("%-15d %d\n", i+1, f[i]);
return 0;
}
rosuav@sikorsky:~$ time ./a.out
2 1387782
3 6972536
4 4170215
5 21326281
6 6945600
7 8334121
8 6943983
9 9750611
10 4168554
11 6972254
12 1389371
real 0m1.089s
user 0m1.088s
sys 0m0.000s
Similar weird results to yours. Replacing "int f[12];" with "int
f[12]={0};" produces a much more plausible bell curve:
rosuav@sikorsky:~$ time ./a.out
2 1387782
3 2776523
4 4170215
5 5552330
6 6945600
7 8334121
8 6943983
9 5554675
10 4168554
11 2776846
12 1389371
real 0m1.093s
user 0m1.088s
sys 0m0.000s
But I don't know Cython, so I don't know whether this would work as-is
or not. Worst case, toss in a quick initialize-to-zero loop above your
main loop, and you can at least verify correctness.
ChrisA
--
https://mail.python.org/mailman/listinfo/python-list
