Avoiding for loop
On 8/4/05, Matt Crawford <mcrawford at gmail.com> 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.)
So my current code looks like:
if(nrow(rawdata) >= 1) for(i in 1:nrow(rawdata)) {
X[i, rawdata[i, 1]] <- 1
X[i, rawdata[i, 2]] <- -1
}
where rawdata is the original data file.
This sort of assignment happens many times in my program so any
improvement would be much appreciated. Thanks.
idx <- seq(length = max(rawdata, 0)) X <- outer(rawdata[,1], idx, "==") - outer(rawdata[,2], idx, "==") Note that we did not have to predefine X and it also works if rawdata has zero rows: rawdata <- matrix(0, nr = 0, nc = 2) in which case it gives a 0 by 0 matrix.