# [R] "(de)linearizing" array indices

Tamas Papp tpapp at axelero.hu
Fri Apr 2 14:15:24 CEST 2004

```Hi,

I have a dynamic programming problem with many state variables, let's
call them l, n, m, etc.  The transition probabilities are originally
given in an array form, eg

transtition[l,m,n,ll,mm,nn]

give the probability of going from l,m,n to ll,mm,nn.

However, the numerical solution is best handled when I "flatten" the L
x M x N state space into a single one (call it S), ie a linear index,
so I can deal with the problem using simple matrix algebra.  After I
get the solution, I need to get back the original state variables.

At the moment I am using two functions like this:

pack <- function(l, m, n) {
(((((l - 1) * M) + m - 1) * N) + n
}

unpack <- function(s) {
s <- s - 1
n <- s %% N + 1
s <- s %/% N
m <- s %% M + 1
l <- s %/% M + 1
list(l=l, m=m, n=n)
}

to convert between S and L x N x M.

Sure, it works, but looks ugly as hell.  And I am positive that I am
abusing the R language with the above code.  So could somebody give me
a nicer solution?

Thanks,

Tamas

--
Tamás K. Papp
E-mail: tpapp at axelero.hu
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```