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Groups > comp.games.development.programming.algorithms > #37

Re: Shuffling

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Am 30.07.2012 12:52, schrieb Mok-Kong Shen:
[snip]
> I have done some small amount of experiments but I don't think
> to have yet found a really optimal permutation operation.

A presumably not too bad, though I think yet sub-optimal, scheme
I found sofar is as follow:

Let n be the length of the list to be permuted and kn be a point of
division of the list into two sections. One applies riffle shuffle
to each section and then reverse the ordering of the whole.

It seems to me that, if kn is (randomly chosen and) located in the
middle half of the list, then the permutation effected is a sufficiently
"random" one in some sense.

Perhaps someone could give hints on improvements or provide a similar
but better scheme. For those who like to try out mine, I am attaching
a Python code below.

M. K. Shen

-----------------------------------------------------------

def makecards(n):
   cards=[]
   for i in range(n):
     cards+=[i]
   return(cards)

def riffle(cards,n):
   newcards=[]
   nh=n//2
   for i in range(nh):
     newcards=newcards+[cards[nh+i]]+[cards[i]]
   if nh*2<n:
     newcards+=[cards[n-1]]
   return(newcards)

def newshuffle(cards,kn,n):
   newcards=riffle(cards[0:kn],kn)+riffle(cards[kn:n],n-kn)
   newcards.reverse()
   return(newcards)

# example:

n=52
cards=makecards(n)
kn=18
newcards=newshuffle(cards,kn,n)
print(newcards)

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Shuffling Mok-Kong Shen <mok-kong.shen@t-online.de> - 2012-07-30 12:52 +0200
  Re: Shuffling Mok-Kong Shen <mok-kong.shen@t-online.de> - 2012-08-01 20:14 +0200
  Re: Shuffling hexxial <hexxial@gmail.com> - 2013-01-20 22:58 +0000

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