Is there any way to force a loop to go until the last list object in R?

Question

I am running a loop in R to rasterize a list of data frames and for comparison purposes I have to keep the same list length in the output.

I am facing some problems with NULL objects on the list. I managed to handle NULL objects inside the list with tryCatch, however when I have null objects at the end of the list (e.g. last 3 objects) they are not included in the output of the loop, so the output list (fry) get shorter than the original list (fr.fr). See the output below.

I would like to know if there is an efficient way to handle this problem in R and to ensure the same list length. I also looking to more straightforward approaches.

Here is my code:

# Raster 
st <- raster(xxx)

# List of data frame to Raster
fry <- list()

for (i in 1:length(fry.fr)){
  tryCatch({
    fry[[i]] <- rasterize(fry.fr[[i]], st, fry.fr[[i]]$AREA, fun = sum, na.rm=FALSE)        
  }
  ,error= function(e) {cat("Error", "\n")})
}

Here is my output:

length(fry.fr) 336

length(fry) 333

Answer 1

initialise the list element to NA or some other flag value before you do the possibly failing computation:

for (i in 1:length(fry.fr)){
  fry[[i]] = NA # flag failure
  tryCatch({
    fry[[i]] <- rasterize(fry.fr[[i]], st, fry.fr[[i]]$AREA, fun = sum, na.rm=FALSE)        
  }
  ,error= function(e) {cat("Error", "\n")})
}

On success that NA gets overwritten, on fail it remains.

Answer 2

You can initialize the list so it has the correct length in the beginning. In your example this would look something like this:

fry <- vector (mode = "list", length (fry.fr))