[R] Avoiding for loop

[Thomas Lumley]

On Thu, 4 Aug 2005, Matt Crawford wrote:

> I understand that in R, for loops are not used as often as other
> languages, and am trying to learn how to avoid them.  I am wondering
> if there is a more efficient way to write a certain piece of code,
> which right now I can only envision as a for loop.  I have a data file
> that basically looks like:
> 1,55
> 1,23
> 2,12
> ...
> that defines a matrix.  Each row of the data file corresponds to a row
> of the matrix, where each number in the row tells me what column a "1"
> or "-1" should go into.  So the first row in the data snippet above
> means that the first row of my matrix needs to have a 1 in the 1st
> column, and a -1 in the 55nd column.  (And 0 elsewhere, which is
> already there as I've created the matrix filled with 0s beforehand.)
>

This may be a job for matrix indexes

    ii<-1:nrow(rawdata)
    X[cbind(ii,rawdata[,1])] <- 1
    X[cbind(ii,rawdata[,2])] <- -1


 	-thomas