Le Monde puzzle [#810]

[This article was first published on Xi'an's Og » R, and kindly contributed to R-bloggers]. (You can report issue about the content on this page here)
Want to share your content on R-bloggers? click here if you have a blog, or here if you don't.

The current puzzle is as follows:

Take a board with seven holes and seeds. The game starts with one player putting the seeds on the holes as he or she wishes. The other player picks a seed wherever. Then, alternatively, each player picks a seed in a hole contiguous to the previous one. The loser is the one finding only empty holes to pick from. Who is the winner with 28? 29? 30 seeds?

This is a simplified version of the awalé or oware we used to play with my kids.

I first defined a recursive function on the win/loose value of a particular location, based on the assumption that each player was picking the best location at each step:

f=function(x,i){
 if (x[i]==0){# losing location
    v=0;return(v)}else{

   if ((i>1)&&(i<7)){
     x[i]=x[i]-1;return(1-max(f(x,i-1),f(x,i+1)))
     }else{

       if (i==1){ x[i]=x[i]-1;return(1-f(x,2))}
       if (i==7){ x[i]=x[i]-1;return(1-f(x,6))}
       }
}}

and then checked whether or not winning solutions were available for 28, 29, and 30 seeds dropped at random:

N=28 #number of seeds
glosol=1  #boolean
for (t in 1:10^3){#random starts

  seeds=sample(1:7,N,rep=TRUE)
  x=rep(0,7)
  for (i in 1:7) x[i]=sum(seeds==i)

  sol=i=0 #second player result
  while ((i<7)&&(sol==0)){
        i=i+1;sol=f(x,i)}
  if (sol==0){ #winning configuration for first player
    glosol=0;print(x);break()}
  }

getting solutions for 28 (5 6 2 3 5 5 2) and 30 (6 6 2 4 4 5 3), but none for 29.

Actually, the rule seems to be that odd numbers get no solutions and even numbers get solutions (e.g., 1 1 1 1 1 2 1 for 8 seeds). (This means further that to build a winning allocation for 2N seeds, we only need to take a configuration at random with 2N+1 seeds and check which seed we need to remove to get a winning (for the “other” player) configuration.)


Filed under: Books, Kids, R Tagged: awalé, Le Monde, mathematical puzzle, oware, R, seeds

To leave a comment for the author, please follow the link and comment on their blog: Xi'an's Og » R.

R-bloggers.com offers daily e-mail updates about R news and tutorials about learning R and many other topics. Click here if you're looking to post or find an R/data-science job.
Want to share your content on R-bloggers? click here if you have a blog, or here if you don't.

Never miss an update!
Subscribe to R-bloggers to receive
e-mails with the latest R posts.
(You will not see this message again.)

Click here to close (This popup will not appear again)