Le Monde puzzle [#887bis]

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As mentioned in the previous post, an alternative consists in filling the permutation of {1,…,N} by adding squares left and right until the permutation is complete or no solution is available. While this sounds like the dual of the initial solution, it brings a considerable increase in computing time, as shown below. I thus redefined the construction of the solution by initialising the permutation at random (it could be 1 as well)


and then completing only with possible neighbours, left

if (max(out)==1){

or right

if (max(out)==1){

(If you wonder about the rep in the sample step, it is a trick I just found to avoid the insufferable feature that sample(n,1) is equivalent to sample(1:n,1)! It costs basically nothing but bypasses reprogramming sample() each time I used it…) The gain in computing time is amazing:

> system.time(for (i in 1:50) pick(15))
utilisateur     système      écoulé
      5.397       0.000       5.395
> system.time(for (i in 1:50) puck(15))
utilisateur     système      écoulé
      0.285       0.000       0.287

An unrelated point is that an interesting (?) alternative problem consists in adding a toroidal constraint, namely to have the first and the last entries in the permutation to also sum up to a perfect square. Is it at all possible?

Filed under: Kids, R, Statistics, University life Tagged: Le Monde, mathematical puzzle, sample

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