Stop WriteRaster in a for loop in R

Question

I just picked up R this summer.

I created a nested for loop so that nls() will be performed in 24 raster layers and a new file will be created to save the data.

In this example, I have a raster stack of 48 layers worth 2 years of data. My for loop will perform the calculation in each year which consists of 24 layers, write a new ENVI file and proceed to the next year.

I managed to create a files for i = 1 and i = 2. However, R does not stop after creating new ENVI files. It is loading for forever and the temporary files in my C drive is updating from time to time (C:\Users\CandiceYingWoei\AppData\Local\Temp\Rtmp is empty, but the date modified keeps updating).

I tried to use break to stop the for loop. I think it is not necessary because i will never go over nyears since I already set i in (1:nyears). if (i > nyears) { break }

How do I stop R after creating the files?

library(sp)
library(raster)
library(ncdf4)
library(rgdal)

setwd("C:/")
new <- stack("82_83_test.envi")
nlayers <- nlayers(new)
nyears <- nlayers(new)/24

for (i in 1:nyears) {
  for (j in (1+(24*(i-1))):(24*i)) {
     fun<- function(x) { 
        # a function that perform nls through each pixel in all raster 
        # layers in each year and return several parameters (SOS, EOS, etc).
        return (c(SOS,EOS,LOS,SPUDOY,P_TAmplitude))
        }
     equation<-calc(new[[(1+(24*(i-1))):(24*i)]],fun,forceapply=TRUE)
     date<- 1981+i
     name<-paste("Output_",date,".envi",sep="")
     writeRaster(equation,filename=name,format="ENVI",overwrite=T)  
     # this is where the break statement is located
     if (i > nyears) { break }
  }
}

Answer 1

If you have to break a loop under this kind of circumstance, it should be a signal that you need to try something else. In this case, using lists and lapply would be a better idea. Try this approach instead, which will break your raster stack into a list of sub-stacks, one for each year, which you can then process in turn:

# just playing with some random files on this machine
setwd('C:/DATA')
x <- list.files(getwd(), pattern = '^MV_NSG.*\\.tif$', full.names = TRUE)

mvs <- stack(x)
mvlist <- unstack(mvs) # now a list of rasters (I have 6)
grpsize <- 3 # desired layers per stack - 12 for you

# use names to group - here, names(mvlist) becomes c('1','1','1','2','2','2')
# length.out will help if you have, say an extra 3 months of data in the original stack
names(mvlist) <- rep(1:(ceiling(length(mvlist) / grpsize )), 
                     each = grpsize, length.out = length(mvlist))

# make list of rasters into a list of stacks
# this gives me a list of 2 stacks with three rasters each 
stacks <- lapply(unique(names(mvlist)), function(y) {
  b <- mvlist[names(mvlist) == y]
  stack(b)
})

# now process the list - w is each stack in turn
stacksout <- lapply(stacks, function(w) {

  g <- calc(w, fun, forceapply = TRUE, etc)

})

consider using clusterR() to speed up your calc function too.

Answer 2

You are start by calling year 1, then the j loop runs 24 times. Then repeats for year 2.

you could put the required layer numbers in a list, then loop through list elements:

nlayers <- nlayers(st)

l <- list(1:nlayers(new)/2,(nlayers(new)+1):14)

for (i in 1:length(l)){

    fun<- function(x) { 
        # a function that perform nls through each pixel in all raster 
        # layers in each year and return several parameters (SOS, EOS, etc).
        return (c(SOS,EOS,LOS,SPUDOY,P_TAmplitude))
        }
     equation<-calc(new[[l[[i]]]],fun,forceapply=TRUE)
     date<- 1981+i
     name<-paste("Output_",date,".envi",sep="")
     writeRaster(equation,filename=name,format="ENVI",overwrite=T)  

}