Read large number of .asc-files, delete rows and save as raster in R

Question

I need to read a large number of .asc-files, delete rows and transform them to a raster-stack by stack(). The source of the zip-packed data is here: ftp://ftp.dwd.de/pub/CDC/grids_germany/monthly/radiation_global/ I've already unziped the files. But now I wrote this code that's really slow, my computer can't accomplished it:

files <- list.files("mydirectory", pattern="\\.asc$", 
                    full.names=TRUE, recursive=TRUE)
i <- lapply(files, readLines) #read data
i <- lapply(i, function(x){x[-(1:28)]}) #delete rows
i <- lapply(i, function(x){gsub('-999', ' NA ', x, fixed = TRUE)}) #convert '-999' to NA
i <- lapply(i, function(x){scan(textConnection(x), what = double(), n = 866*654)}) #convert to numeric
i <- lapply(i, function(x){matrix(x, nrow = 866, ncol = 654, byrow = TRUE)}) #convert to matrix
r <- lapply(i, function(x){raster(x)}) #rasterize data
st <- stack(r) #convert to stack-raster

I wonder if there is a better way to convert this data to raster-files. Other .asc files have got only 6 lines as an header like here: ftp://ftp.dwd.de/pub/CDC/grids_germany/monthly/precipitation/01_Jan/ . I read that data by a much simpler function which use only the stack()-function:

loadRaster <- function(directory, output, clipping){
  files <- list.files(path=directory, pattern="\\.asc$", 
                     full.names=TRUE, recursive=TRUE)
  stc <- stack(files)
  crs(stc) <- gk3
  stcC <- crop(stc, extent(clipping))
  writeRaster(stcC, filename=output)
} 
#You can ignore the code below "stc <-stack(files)"

Answer 1

Finally I got it by using textConnection() stepwise (opening and closing again), because – and there was maybe the cause for slowness – there is a limit for opened connections.

files <- list.files(path="mydirectory", pattern="\\.asc$", 
                    full.names=TRUE, recursive=TRUE)
i <- lapply(files, readLines)
i <- lapply(i, function(x){x[-(1:28)]})
i <- lapply(i, function(x){gsub('-999', ' NA ', x, fixed = TRUE)})
names(i) <- substr(files, 92,97)
i1 <- lapply(i[1:100], function(x){scan(textConnection(x), what = double(), n = 866*654)})
closeAllConnections()
i2 <- lapply(i[101:200], function(x){scan(textConnection(x), what = double(), n = 866*654)})
closeAllConnections()
i3 <- lapply(i[201:300], function(x){scan(textConnection(x), what = double(), n = 866*654)})
closeAllConnections()
i4 <- lapply(i[301:length(i)], function(x){scan(textConnection(x), what = double(), n = 866*654)})
closeAllConnections()
i <- c(i1, i2, i3, i4)
m <- lapply(i, function(x){matrix(x, nrow = 866, ncol = 654, byrow = TRUE)})
r <- lapply(m, function(x){raster(x)})
stc <- stack(r)