Re: Random choice

*To*: mathgroup at smc.vnet.net*Subject*: [mg99840] Re: [mg99831] Random choice*From*: Leonid Shifrin <lshifr at gmail.com>*Date*: Fri, 15 May 2009 06:26:38 -0400 (EDT)*References*: <200905150822.EAA18773@smc.vnet.net>

Hi Valeri, RandomChoice does not guarantee that all picked items (numbers) will be different. In v.6,0+, use RandomSample: In[1] = RandomSample[Range[500],100]//Short Out[1] = {467,487,153,196,<<92>>,65,104,399,386} For earlier versions, you can load Combinatorica: In[2] = << DiscreteMath`Combinatorica`; and use RandomPermutation: In[3] = #[[Take[RandomPermutation[#], 100]]] &@Range[500]//Short Out[3] = {120,122,328,6,241,<<90>>,291,58,409,314,109} However, both methods will become very memory-hungry if your upper limit is large enough (more than say 10^7), due to the necessity to temporarily store an entire range of numbers. The used memory will be quickly released back to the OS, of course (Mathematica garbage collector is quite good at this), but the peak load may be significant. Here is the version from my book (slightly modified) ( http://www.mathprogramming-intro.org/book/node514.html ), which is free from this drawback. It works best when the third optional parameter (updatenum) is about a quarter of the total numbers asked (n) (this is a heuristics of course). In[4] = Clear[randomNumsOrdered]; randomNumsOrdered[numrange_List, n_Integer, updatenum_Integer: 100] := Take[NestWhile[ Union[Join[#, RandomInteger[numrange, updatenum]]] &, {}, Length[#] < n &], n]; For example, In[5] = randomNumsOrdered[{1, 50000000}, 100] // Timing // Short Out[5] = {0.,{292330,<<98>>,49669303}} Finally, you may do it more systematically by constructing a binary search tree (see Mathematica implementation, for example, in R.Maeder "Computer science with Mathematica") and then test each newly generated number against those in the tree and insert if it isn't present already. But in practice, I doubt that this solution will be faster in Mathematica. Hope this helps. Regards, Leonid On Fri, May 15, 2009 at 1:22 AM, Valeri Astanoff <astanoff at gmail.com> wrote: > Good day, > > Given this example : > > In[1]:= RandomChoice[Range[500], 100] // Union // Length > > Out[1]= 91 > > what is the best way to get *exactly* 100 different > random numbers ranging from 1 to 500 ? > > Any piece of advice would be appreciated. > > -- > > Valeri Astanoff > >

**Follow-Ups**:**Re: Re: Random choice***From:*Filippo Miatto <miatto@gmail.com>

**References**:**Random choice***From:*Valeri Astanoff <astanoff@gmail.com>