Edit Question - KnowledgeHub

Title Body

Consider an algorithm to test the probability that a certain number is picked from a set of N unique numbers after a specific number of tries (for example, with N=2, what's the probability in Roulette (without 0) that it takes X tries for Black to win?). The correct distribution for this is pow(1-1/N,X-1)*(1/N). However, when I test this using the following code, there is always a deep ditch at X=31, independently from N, and independently from the seed. Is this an intrinsic flaw that cannot be prevented due to the implementation specifics of the PRNG in use, is this a real bug, or am I overlooking something obvious? // C #include <sys/times.h> #include <math.h> #include <stdio.h> int array[101]; void main(){ int nsamples=10000000; double breakVal,diffVal; int i,cnt; // seed, but doesn't change anything struct tms time; srandom(times(&time)); // sample for(i=0;i<nsamples;i++){ cnt=1; do{ if((random()%36)==0) // break if 0 is chosen break; cnt++; }while(cnt<100); array[cnt]++; } // show distribution for(i=1;i<100;i++){ breakVal=array[i]/(double)nsamples; // normalize diffVal=breakVal-pow(1-1/36.,i-1)*1/36.; // difference to expected value printf("%d %.12g %.12g\n",i,breakVal,diffVal); } } Tested on an up-to-date Xubuntu 12.10 with libc6 package 2.15-0ubuntu20 and Intel Core i5-2500 SandyBridge, but I discovered this already a few years ago on an older Ubuntu machine. I also tested this on Windows 7 using Unity3D/Mono (not sure which Mono version, though), and here the ditch happens at X=55 when using System.Random, while Unity's builtin Unity.Random has no visible ditch (at least not for X<100). The distribution: The differen

Tags (comma-separated) Cancel