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programming - How to correct my code for solving the Josephus problem?


Problem Description


Recently, I have been reading the book Schaum's Outline of Mathematica (2nd Edition), where I encountered the problem:




Flavius Josephus was a Jewish historian of the first century. He wrote about a group of ten Jews in a cave who, rather than surrender to the Romans, chose to commit suicide, one by one. They formed a circle and every other one was killed. Who was the lone survivor?



The author's solution:


list = Range[10];
While[Length[list] > 1, list = Rest[RotateLeft[list]]];
list


{5}




However, I know it is not efficient to use the procedural methods such as Do, While, etc. Rather, I want to use a functional method like NestWhile, Nest, or FixedPoint to solve the problem.


My solutions:


Method 1:


list = Range @ 10; 
NestList[Rest @ RotateLeft[#] &, list, 9]


 {{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, 
{3, 4, 5, 6, 7, 8, 9, 10, 1},

{5, 6, 7, 8, 9, 10, 1, 3},
{7, 8, 9, 10, 1, 3, 5},
{9, 10, 1, 3, 5, 7},
{1, 3, 5, 7, 9},
{5, 7, 9, 1},
{9, 1, 5},
{5, 9},
{5}}

Furthermore, this method has the flaw that I must give the number of iterations. In fact, sometimes that is unknown.



Method 2:


list = Range @10;
FixedPoint[If[Length@# != 1 &, Rest @ RotateLeft[#] &], list]

Unfortunately, method 2 doesn't work.


Method 3:


 list = Range @ 10;
NestWhileList[Rest @ RotateLeft[#] &, list, Length@list != 1]



{{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}}

So my question is: what is a good way to do it?



Answer



NestWhile[Rest @ RotateLeft @ # &, Range @ 10, Length @ # > 1 &]


{5}



FixedPoint[If[Length @ # > 1, Rest @ RotateLeft[#], #] &, Range @ 10]


Edit


Historical note: As far as I can remember, Josephus roulette (a plain treason to his companions) consisted of killing every third person.


FixedPoint[If[Length@# != 1, Rest@RotateLeft[#, 2], #] &, Range@10]


{4}



Note: The direction is important. RotateRight[] will select another victim.


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