[R-sig-Geo] create a raster with values from SpatialPolygonsDataFrame

[Benjamin Mack]

Hello list,
I would like to create a new raster with values from a 
SpatialPolygonsDataFrame. It works with the code below, however it is 
really slow. I would appreciate any suggestions which could speed up the 
code!
Thanks in advance!
Ben

Code:
"r" is the raster file
"ref" is the SpatialPolygonsDataFrame with one column named "lc" with 
integer values.
to write the values from the polygons to the raster I do the following:

   u.IDs <- unique(ref at data$lc) # unique lc(land cover) values/IDs
   for (ii in 1:length(u.IDs)) {
       temp <- ref[ref at data$lc==u.IDs[ii],] # create a new 
SpatialPolygonsDataFrame for a specific lc-ID
       ra[temp] <- u.IDs[ii] # write the values to the raster  }