[R] Subarrays

[Huntsinger, Reid]

You could write an R function to create an integer matrix whose rows are the
coordinates of the lattice points in the box with specified lower and upper
limits (code at end)

makeLattice <- function(lower,upper) {
# generate lattice points in box from from lower to upper
}

for example, then you can create the index matrix via

lower <- rep(1,length(dim(A)))
lower[v1] <- v2
upper <- dim(A)
upper[v1] <- v2

indx <- makeLattice(lower,upper)

and now A[indx] is the slice you want. You can dimension it as dim(A)[-v1].

Reid Huntsinger

Code:

makeLattice <- function(lower,upper) {
# generate lattice points in box from from lower to upper
n <- length(lower)
if (n != length(upper)) stop("vectors must have same length")
d <- upper - lower + 1
latticePts <- vector(length=n*prod(d), mode="integer")
dim(latticePts) <- c(prod(d), n)
 

# create each column 
D <- c(1,cumprod(d[-n]))

for (j  in 1:n) {
latticePts[,j] <- as.integer(rep(lower[j]:upper[j],rep(D[j],d[j])))

}
latticePts
}


-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Gunnar Hellmund
Sent: Friday, April 29, 2005 11:34 AM
To: r-help at stat.math.ethz.ch
Subject: [R] Subarrays


Define an array

> v<-1:256
> dim(v)<-rep(4,4)

Subarrays can be obtained as follows:

> v[3,2,,2]
[1]  71  87 103 119
> v[3,,,2]
     [,1] [,2] [,3] [,4]
[1,]   67   83   99  115
[2,]   71   87  103  119
[3,]   75   91  107  123
[4,]   79   95  111  127

In the general case this procedure is very tedious. 

Given an array 
A, dim(A)=(dim_1,dim_2,...,dim_d) 
and two vectors
v1=(n_i1,...n_ik), v2=(int_1,...,int_k) ('marginals' and relevant
'interval numbers')
is there a smart way to obtain 
A[,...,int_1,....,int_2,....,....,int_k,....]
?

Best wishes
Gunnar Hellmund

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