Path: csiph.com!usenet.pasdenom.info!news.redatomik.org!newsfeed.xs4all.nl!newsfeed1a.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail
Date: Sun, 07 Jun 2015 11:40:37 -0700
From: "C.D. Reimer" <chris@cdreimer.com>
User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0
MIME-Version: 1.0
To: python-list@python.org
Subject: Re: Python Random vs. Cython C Rand for Dice Rolls
References: <55747C96.7090006@cdreimer.com> <CAPTjJmo0Aw961q54BySZBSDdD8xqCPFjMezcytzWxrE0m6afTw@mail.gmail.com>
In-Reply-To: <CAPTjJmo0Aw961q54BySZBSDdD8xqCPFjMezcytzWxrE0m6afTw@mail.gmail.com>
Content-Type: text/plain; charset=windows-1252; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: list
Newsgroups: comp.lang.python
Message-ID: <mailman.270.1433702448.13271.python-list@python.org>
Lines: 77
NNTP-Posting-Host: 2001:888:2000:d::a6
Xref: csiph.com comp.lang.python:92287

On 6/7/2015 10:33 AM, Chris Angelico wrote:
> 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.

Here's the revised Cython code.

cdef extern from "stdlib.h":

     int c_libc_rand "rand"()

cdef int f[12]

     

def roll(int x):

     cdef:

         int a = 0, b = 0, i = 0

     for i in range(x):

         a = c_libc_rand() % 6 + 1

         b = c_libc_rand() % 6 + 1
         

         f[(a + b) - 1] += 1

     return f


Here's the new console output.

PS Z:\projects\programming\python\basic_games\fastdice> python test_fastdice.py

TOTAL SPOTS     NUMBER OF TIMES

  2                1389911

  3                2777722

  4                4168248

  5                5553632

  6                6944907

  7                8334670

  8                6945597

  9                5553557

  10               4167485

  11               2775806

  12               1388465

1.65599989891


I had to put the array definition outside of the function declaration, 
where it's automatically initialized to zero in the global space. I 
tried to initialize the array from inside the function but Cython threw 
up errors, either the C declaration wasn't correct or unable to convert 
from C to Python when updating the array elements. I'll figure out that 
problem later. The new version works as expected.

Thanks,

Chris R.