[R] Create Arrays

[Barry Rowlingson]

On Fri, Oct 15, 2010 at 9:55 AM, dpender <d.pender at civil.gla.ac.uk> wrote:
>
> Hi,
>
> For this example:
>
> O <- c(0 0 0 2 0 0 2 0)
>
> I want to create an array every time O[i] > 0.  The array should be in the
> form;
>
> R[j] <- array(-1, dim=c(2,O[i]))
>
> i.e. if O[i] > 0 4 times I want 4 R arrays.
>
> Does anyone have any suggestions?
>

 Suggestion number l is don't use O for objects! Far too confusing!

 Serious suggestion is that a concrete example will help. Let me try:

If:

 X = c(0,0,0,2,0,0,3,0) # I'm not using O here!

 Then

R will be a list of length 2 because there are 2 values in X bigger than 0.

R[1] will be array(-1,dim=c(2,2))   # because X[4] is 2
and
R[2] will be array(-1,dim=c(2,3))  # because X[7] is 3

 Yup?

 Okay, first get rid of the zeroes:

Xnz = X[X!=0]

 That simplifies the problem. Then use lapply to iterate over Xnz with
a function that returns the array given the value:

> lapply(Xnz,function(x){array(-1,dim=c(2,x))})
[[1]]
     [,1] [,2]
[1,]   -1   -1
[2,]   -1   -1

[[2]]
     [,1] [,2] [,3]
[1,]   -1   -1   -1
[2,]   -1   -1   -1

 2-d arrays are just matrices, so you can do it all in one line with:

lapply(X[X!=0],function(x){matrix(-1,2,x)})

Barry