# Le Monde puzzle [#7]

March 26, 2011
By

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

The mathematical puzzle from the weekend edition of Le Monde from a few weeks ago was not too hard to solve by induction but my R code failed miserably! The puzzle was as follows:

A calculator is broken in such a way that it starts by exhibiting 0, then pressing 4, 6 or 0 keeps adding this figure to the right of the current number (if not zero), while pressing 2 divides the current number by 2. For instance, a possible sequence is

0
4 [press 4]
46 [press 6]
23 [press 2]
230 [press 0]

Is it possible to exhibit any integer?

I thus wrote the simple R code

library(gsubfn)
digitBase=function(n){
strapply(as.character(n), ".", as.numeric)[[1]]
}
check=function(x){
step=0
xdigits=digitBase(x)
ok=sum(xdigits==0)+sum(xdigits==4)+sum(xdigits==6)
if (ok<length(xdigits)){
y=2*x
step=1+check(y)
}
step
}


which unfortunately runs very quickly into problems as can be seen by calling

> check(7)
Error in if (ok < length(xdigits)) { :
missing value where TRUE/FALSE needed
1: In do.call(FUN, as.list(s[, j])) : NAs introduced by coercion
2: In do.call(FUN, as.list(s[, j])) : NAs introduced by coercion
3: In do.call(FUN, as.list(s[, j])) : NAs introduced by coercion


because the 2x…x2x7 value gets out of bounds…

Filed under: R, Statistics Tagged: Le Monde, mathematical puzzle

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