R equivalent to diag(x,k) in matlab - r

Question

I guess, I have a two leveled question referring to diag in R and matlab.
1) I was wondering if there was a way already developed to access different diagonals of matrices in R similar to the way it is done in Matlab (see http://www.mathworks.com/help/techdoc/ref/diag.html).
2) If there is not already a current function how can my code be improved such that it is similar to the R diag where
diag(x = 1, nrow, ncol) # returns the values of the diagonal
diag(x) <- value # inserts values on the diagonal
Presently my code returns the elements on the diagonal given k but how can it be written so that if it is specified like the second way (above) that it allows me to insert the values on the diagonal. Presently to do this, I use diag.ind to give me the indices and then using those indices to insert the values on the k diagonal.
Here is the code:
'diag.ind'<-function(x,k=0){
if(k=='') k=0
x<-as.matrix(x)
if(dim(x)[2]==dim(x)[1]){
stp_pt_r<-dim(x)[1]
stp_pt_c<-dim(x)[2]
}
if(ncol(x)> dim(x)[1]){
stp_pt_r<-dim(x)[1]
stp_pt_c<-stp_pt_r + 1
}
if(ncol(x)< dim(x)[1]){
stp_pt_c<-dim(x)[2]
stp_pt_r<-stp_pt_c+1
}
if(k==0){
r<-as.matrix(seq(1,stp_pt_r,by=1))
c<-as.matrix(seq(1,stp_pt_c,by=1))
ind.r<- cbind(r,c)
}
if(k>0){
r<-t(as.matrix(seq(1,stp_pt_r,by=1)))
c<-t(as.matrix(seq((1+k),stp_pt_c,by=1)))
ind<-t(rbind.fill.matrix(r,c))
ind.r<-ind[!is.na(ind[,2]),]
}
if(k<0){
k<-abs(k)
r<-t(as.matrix(seq((1+k),stp_pt_r,by=1)))
c<-t(as.matrix(seq(1,stp_pt_c,by=1)))
ind<-t(rbind.fill.matrix(r,c))
ind.r<-ind[!is.na(ind[,1]),]
}
diag.x<-x[ind.r]
output<-list(diag.x=diag.x, diag.ind=ind.r)
return(output)
}
This is kind of clunky and I feel like I must be reinventing the wheel. Thanks in advance for any insight!

Answer 1

After your reply to Andrie this may satisfy:
exdiag <- function(mat, off) {mat[row(mat)+off == col(mat)]}
x <- matrix(1:16, ncol=4)
exdiag(x,1)
#[1] 5 10 15
I was thinking you wanted a function that can assign or return one of a diagonal or a sub- or super- diagonal matrix, This is the constructor function:
subdiag <- function(vec, size, offset=0){
M <- matrix(0, size, size)
M[row(M)-offset == col(M)] <- vec
return(M)}
> subdiag(1, 5, 1)
[,1] [,2] [,3] [,4] [,5]
[1,] 0 0 0 0 0
[2,] 1 0 0 0 0
[3,] 0 1 0 0 0
[4,] 0 0 1 0 0
[5,] 0 0 0 1 0
Called with only two arguments you would get a diagonal matrix. You can construct super-diagonal matrices with negative offsets. If this is what you wanted for the constructor, then it should not be too hard to construct a similar subdiag<- function to go along with it.

Answer 2

In MATLAB, to assign the values x to the diagonal of A:
n = size(A,1);
A(1:n+1:end) = x
Look up linear indexing.
Although, that might not be what you asked.