Nested loop and terrain analysis R

Question

I have a raster stack with 11 layers that I am running through a triply nested lapply(). I expect 44 layers to be returned, but I'm only seeing 4!

Reproducible example:

# Load packages
library(raster)
library(pbapply)
library(magrittr)

# Reading in the data
dhm <- readRDS(gzcon(url('http://jacksonvoelkel.com/geog597/labs/DHM.RDS'))) 

# Looping through some terrain functions
terrain_opts <- c('slope'
                  ,'TPI'
                  ,'TRI'
                  ,'roughness'
                  ,'aspect'
                  #,'flowdir'
)

# Create terrain stack
dhm_terrain <- pblapply(terrain_opts, function(t) {
  r_terrain <- pblapply(c(4, 8), function(n) {
    r_neigh <- raster::terrain(x = dhm, opt = t, unit = 'radians', neighbors = n)
    names(r_neigh) <- paste0(names(dhm), '_terrain_', t, '_nb_', n)
    return(r_neigh)
  }) %>% stack()
  return(r_terrain)
}) %>% stack()

# Attach the original dhm to terrain stack
dhm_terrain %<>% stack(dhm)

Here's where I'm running into issues:

# Apply boundaries filter to terrain stack
terrain_bound <- pblapply(nlayers(dhm_terrain), function(i) {
  t_stack <- pblapply(c("inner", "outer"), function(t) {
    d_stack <-  pblapply(c(4, 8), function(d) {
      r_bound <- raster::boundaries(x = dhm_terrain[[i]], directions = d, type = t, asNA=F, classes=T)
      names(r_bound) <- paste0(names(dhm_terrain[[i]]), '_', d, '_', t)
      return(r_bound)
      }) %>% stack() 
    return(d_stack)
    }) %>% stack()
  return(t_stack)    
  }) %>% stack()

Answer 1

This:

terrain_bound <- lapply(nlayers(dhm_terrain), function(i) {

should be:

terrain_bound <- lapply(1:nlayers(dhm_terrain), function(i) {

(or use pblapply - I took it out in order to see what i was inside the function and the printing was easier without the progress bar updating).