Question

There seems to be variations to this question, but none seem to address the situation of being in a loop AND naming and output file. How I thought this might work:

for(j in 1:3) {
    for(k in 1:17){
       extract_[j]km <- extract(RasterStack, SpatialPolygonsDataFrame_[j]km, layer=[k], nl=1, df=TRUE)
    }
}

The extract function is from the raster package. I have already created a series of RasterStacks and SpatialPolygons and I want to pass these to a function ("extract") that has several parameters, some of which I wish to manipulate through the loop, and label the output accordingly. This is a breeze in BASH, but I can’t figure this out in R.

Ultimately, I’d like to pass strings as well, but another post seems to show the way there.

EDIT: I originally posted the above function as being a single dataframe, when in fact, they are specified objects from the raster package (which are ultimately dataframes).

Answer 1

As Justin points out, working with a list is more inline with R’s structure than messing up the workspace with lots of named variables. It quickly becomes challenging to work when you have a lot of objects in the workspace to “know” what’s next.

Your way:

for(j in 1:3) { 
assign(
   paste("extract",j,"km",sep=""), # or paste0 to avoid need for sep=""
   function(
            get(
                paste("data",j,"km",sep="")
               )
           )
      )
}

Personally, I prefer working with lists, so below, I convert your data objects to a list and show you how to run a function on all elements of that list. Working in this way usually relegates the need to use strings in the “get” and “assign” fashion.

# just converting your variables to a list    
data.list <- mget(grep("data",ls(),value=TRUE),envir=.GlobalEnv) 

# then output results
result.list <- lapply(data.list,your_function)