Le Monde puzzle [#875]

July 11, 2014
By

(This article was first published on Xi'an's Og » R, and kindly contributed to R-bloggers)

I learned something in R today thanks to Le Monde mathematical puzzle:

A two-player game consists in A picking a number n between 1 and 10 and B and A successively choosing and applying one of three transforms to the current value of n

  • n=n+1,
  • n=3n,
  • n=4n,

starting with B, until n is larger than 999. Which value(s) of n should A pick if both players act optimally?

Indeed, I first tested the following R code

sole=function(n){
  if (n>999){ return(TRUE)
  }else{
     return((!sole(3*n))&(!sole(4*n))&(!sole(n+1)))
}}

which did not work because of too many calls to sole:

>sole(1)
Error: evaluation nested too deeply: infinite recursion
/ options(expressions=)?

So I included a memory in the calls to sole so that good and bad entries of n were saved for later calls:

visit=rep(-1,1000) #not yet visited
sole=function(n){
  if (n>999){ return(TRUE)
  }else{
     if (visit[n]>-1){ return(visit[n]==1)
     }else{
       visit[n]<<-((!sole(3*n))&(!sole(4*n))&
    (!sole(n+1)))
       return(visit[n]==1)
  }}}

Trying frontal attack

>sole(1)
Error: evaluation nested too deeply: infinite recursion
/ options(expressions=)?

did not work, but one single intermediary was sufficient:

> sole(500)
[1] FALSE
> for (i in 1:10)
+ print(sole(i))
[1] FALSE
[1] FALSE
[1] FALSE
[1] TRUE
[1] FALSE
[1] TRUE
[1] FALSE
[1] FALSE
[1] FALSE
[1] FALSE

which means that the only winning starters for A are n=4,6. If one wants the winning moves on top, a second counter can be introduced:

visit=best=rep(-1,1000)
sole=function(n){
  if (n>999){ return(TRUE)
  }else{
     if (visit[n]>-1){ return(visit[n]==1)
     }else{
       visit[n]<<-((!sole(3*n))&(!sole(4*n))&
       (!sole(n+1)))
       if (visit[n]==0) best[n]<<-max(
         3*n*(sole(3*n)),
         4*n*(sole(4*n)),
         (n+1)*(sole(n+1)))
       return(visit[n]==1)
  }}}

From which we can deduce the next values chosen by A or B as

> best[1:10]
 [1]  4  6  4 -1  6 -1 28 32 36 40

(where -1 means no winning choice is possible).

Now, what is the R trick I learned from this toy problem? Simply the use of the double allocation symbol that allows to change global variables within functions. As visit and best in the latest function. (The function assign would have worked too.)

Filed under: Kids, R, Statistics, University life Tagged: assign(), global variable, Le Monde, local variable, mathematical puzzle

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 on topics such as: Data science, Big Data, R jobs, visualization (ggplot2, Boxplots, maps, animation), programming (RStudio, Sweave, LaTeX, SQL, Eclipse, git, hadoop, Web Scraping) statistics (regression, PCA, time series, trading) and more...



If you got this far, why not subscribe for updates from the site? Choose your flavor: e-mail, twitter, RSS, or facebook...

Comments are closed.

Sponsors

Mango solutions



plotly webpage

dominolab webpage



Zero Inflated Models and Generalized Linear Mixed Models with R

Quantide: statistical consulting and training

datasociety

http://www.eoda.de





ODSC

ODSC

CRC R books series





Six Sigma Online Training









Contact us if you wish to help support R-bloggers, and place your banner here.

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)