Construct dynamic-sized array in R

Question

I was wondering about what are the ways to construct dynamic-size array in R.

For one example, I want to construct a n-vector but its dimension n is dynamically determined. The following code will work:

> x=NULL  
> n=2;   
> for (i in 1:n) x[i]=i;  
> x  
[1] 1 2  

For another example, I want to construct a n by 2 matrix where the number of rows n is dynamically determined. But I fail even at assigning the first row:

> tmp=c(1,2)  
> x=NULL  
> x[1,]=tmp  
Error in x[1, ] = tmp : incorrect number of subscripts on matrix  
> x[1,:]=tmp   
Error: unexpected ':' in "x[1,:"  

Thanks and regards!

Answer 1

I think the answers you are looking for are rbind() and cbind():

> x=NULL  #  could also use x <- c()

> rbind(x, c(1,2))
     [,1] [,2]
[1,]    1    2
> x <- rbind(x, c(1,2))
> x <- rbind(x, c(1,2))  # now extend row-wise
> x
     [,1] [,2]
[1,]    1    2
[2,]    1    2
> x <- cbind(x, c(1,2))  # or column-wise
> x
     [,1] [,2] [,3]
[1,]    1    2    1
[2,]    1    2    2

The strategy of trying to assign to "new indices" on the fly as you attempted can be done in some languages but cannot be done that way in R.

You can also use sparse matrices provided in the Matrix package. They would allow assignments of the form M <- sparseMatrix(i=200, j=50, x=234) resulting in a single value at row 200, column 50 and 0's everywhere else.

 require(Matrix)
 M <- sparseMatrix(i=200, j=50, x=234)
 M[1,1]
#   [1] 0
 M[200, 50]
#   [1] 234

But I think the use of sparse matrices is best reserved for later use after mastering regular matrices.