Re: problem with RandomInteger

*To*: mathgroup at smc.vnet.net*Subject*: [mg112196] Re: problem with RandomInteger*From*: Bill Rowe <readnews at sbcglobal.net>*Date*: Sat, 4 Sep 2010 04:00:56 -0400 (EDT)

On 9/3/10 at 6:10 AM, izrailsk at iacp.dvo.ru (Yuri Izrailsky) wrote: >I have recently encountered a strange behavior of RandomInteger >function when used inside Module construct. Here is an example: >str1[] := Module[{l, i, j}, >l = {0, 0, 0}; >i = RandomInteger[{1, 3}]; >l[[i]] = 1; >j = RandomInteger[{1, 3}]; >l[[j]] >] >In[3]:= N[Total[Table[str1[],{200}]]]/200 Out[3]= 0.345 >which is reasonably close to 1/3, but >In[5]:= N[Total[Table[str1[],{1000}]]]/1000 Out[5]= 0.997 >looks very strange. What's happening here? It looks like a bug. I can confirm I get essentially the same result with your code using In[21]:= $Version Out[21]= 7.0 for Mac OS X x86 (64-bit) (February 19, 2009) My testing indicates the point of failure occurs at n = 250. That is, N[Total[Table[str1[],{n}]]]/n returns a value of approximately 1 for n greater than 249 I would also note your code seems inefficient. Effectively you are generating two random integers between 1 and 3 and giving the result 1 when they are equal and 0 if they are not equal. Then you construct a list of these values and compute the mean. The same can be done with the following: g[n_Integer] := Mean[Unitize[Subtract @@ RandomInteger[{1, 3}, {2, n}]] - 1] But this code does not exhibit the problem.