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Re: Challenge!

  • To: mathgroup at
  • Subject: [mg917] Re: Challenge!
  • From: Lou D'Andria <lou>
  • Date: Sun, 30 Apr 1995 03:51:40 -0400
  • Organization: Wolfram Research, Inc.

In article <3nkc03$mq8 at> Paul E Howland,
PEHOWLAND at writes:
>Dear Mathgroup,
>I have two lists of equal length M.
>I wish to generate a third list also of length M, where the i
th element
>of this list is either the i th element of the first list, or
the i th
>element of the second list. 
>It should be equally probable that the new element be chosen
from the
>first or second list. 
>eg.  I have a list {a,b,c,d,e,f,g}
>     and another   {A,B,C,D,E,F,G}
>valid answers would be:
>                   {a,B,C,d,e,F,G}
>                   {a,b,c,d,E,f,g}
>                   {A,B,C,D,e,f,G}
>                   etc.


>What is the most compact function that could achieve my goal?

Well, using Listable Plus[] and Times[], how about something

swap2[list1_List,list2_List] :=
   list1 + (list2 - list1)

The use of Length[list1] can also be removed, to make it a bit
more compact.

swap3[list1_List, list2_List] := 
   list1 + (list2-list1) (Random[Integer]& /@ list1)

One drawback to this method is that if you wanted to choose
elements from 3 or more lists, the formulas underlying the
method become increasingly complex, unlike the solution posted

swap4[list1_List, list2_List] := 
   #[[1+Random[Integer]]]& /@ Transpose[{list1,list2}]

which is easily generalized to more than two lists.

swap4a[lists__] := With[{len = Length[First @ {lists}]},
   #[[ Random[Integer,{1,len}] ]]& /@ Transpose[{lists}] ]


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