[R] reexpand a matrix after subsetting

[Duncan Murdoch]

On 8/29/2005 12:11 PM, Ido M. Tamir wrote:
> Hi,
> 
> suppose I have a matrix (or dataframe) 
> as a result from subsetting.
> 
> mat <- matrix(1:20,ncol=2)
> mat[c(3,6,9),] <- NA
> cc <- complete.cases(mat)
> sub <- mat[cc,,drop=FALSE]
> sub <- sub * 2
> #some caluculations with sub.
> 
> now I would like to expand sub somehow
> so row 3,6, and 9 would be filled with 
> NAs but the rest should be in place again.
> Is there a simple function for this?
> 
> merge is not an option.
> 
> Thank you very much for your help.

You just need to calculate the original row numbers.  For example,

goodrows <- (1:nrow(mat))[cc]
mat[goodrows,,drop=FALSE] <- sub

Duncan Murdoch