Le Monde puzzle [#738]
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The Friday puzzle in Le Monde this week is about “friendly perfect squares”, namely perfect squares x2>10 and y2>10 with the same number of digits and such that, when drifting all digits of x2 by the same value a (modulo 10), one recovers y2. For instance, 121 is “friend” with 676. Here is my R code:
xtrct=function(x){ x=as.integer(x) digs=NULL for (i in 0:trunc(log(x,10))){ digs[i+1]=trunc((x-sum(digs[1:i]*10^(trunc(log(x,10)):(trunc(log(x,10))- i+1))))/10^(trunc(log(x,10))-i))} return(digs) } pdfct=(4:999)^2 for (t in 2:6){ pfctsq=pdfct[(pdfct>=10^t)&(pdfct0) print(c(pfctsq[i],pfctsq[ ((i+1):dim(rstrct)[2])[(dive==1)]])) } }
which returns
[1] 121 676 [1] 1156 4489 [1] 2025 3136 [1] 13225 24336 [1] 111556 444889
namely the pairs (121,676), (1156,4489), (2025,3136), (13225,24336), and (111556,444889) as the solutions. The strange line of R code
if (is.matrix(dive)) dive=lapply(seq_len(ncol(dive)), function(i) dive[,i])
is due to the fact that, when the above result is a matrix, turning it into a list means each entry of the matrix is an entry of the list. After trying to solve the problem on my own for a long while (!), I found the above trick on stackoverflow. (As usual, the puzzle is used as an exercise in [basic] R programming. There always exists a neat mathematical solution!)
Filed under: R Tagged: arithmetics, Le Monde, list, mathematical puzzle, matrix, R
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