r - How to loop over multiple lists

Question

I am working with precipitation data for four river basins. The four rivers are Main, Danube, Isar and Inn. The precipitation data set is organised in an array (dimensions are 150 150 80). The third dimension respresents the single time steps. For every river basin I have a matrix called "mask_rivername", so "mask_Main", "mask_Danube" etc. Each matrix is of dimensions 150 150. The goal is to multiply the single matrices with every single time step of the precipitation data. To achieve this, I created a list for every river called "Main.list", "Danube.list", "Isar.list" and "Inn.list". Now I would like to fill the single lists, for example by using a loop. If I do that one by one it works well:

for (i in 1:ntime) {
  Main.list[[paste0("Main.rr",i)]] <- PRECIPITATION[,,i]*mask_Main
  Danube.list[[paste0("Danube.rr",i)]] <- PRECIPITATION[,,i]*mask_Danube
  Isar.list[[paste0("Isar.rr",i)]] <- PRECIPITATION[,,i]*mask_Isar
  Inn.list[[paste0("Inn.rr",i)]] <- PRECIPITATION[,,i]*mask_Inn
}

I am sure there must be a more elegant way, but I could not find it. My problem is that get(paste0()) does not work at the beginning of a loop.

Anybody here with a hint?

question from:https://stackoverflow.com/questions/65940328/how-to-loop-over-multiple-lists

Answer 1

The question can be answered the R way with a double lapply loop. As long as both arguments, the precipitation array and the masks are in list objects. That's what the preliminary code does, to create two lists.

mask_list <- mget(ls(pattern = "^mask_", envir = .GlobalEnv), envir = .GlobalEnv)
nmat <- dim(PRECIPITATION)[3]
precip_list <- lapply(seq.int(nmat), function(i) PRECIPITATION[,, i])

new_precip <- lapply(mask_list, function(x, y){
  lapply(y, function(.y) .y * x)
}, y = precip_list)

rivers <- sub("^mask_", "", names(mask_list))
new_precip <- lapply(seq_along(new_precip), function(i){
  names(new_precip[[i]]) <- paste(rivers[i], seq.int(nmat), sep = ".")
  new_precip[[i]]
})
names(new_precip) <- rivers

To have this result as lists in the .GlobalEnv, you can use list2env.

Test data

PRECIPITATION <- array(-199:200, dim = c(10, 10, 4))
mask_Main <- 
  mask_Danube <-
  mask_Isar <-
  mask_Inn <- matrix(-1:1, nrow = 10, ncol = 10)