Use lapply and writeOGR to write multiple shapefiles within a list

Question

I have a nested list with multiple shapefiles and I want to use lapply with writeOGR ('rgdal') in order to export all the shapefiles. Sorry for not giving a reproducible example.

# My list 
shp.list
$e_ang
$e_ang$e_ang_g
class       : SpatialPolygonsDataFrame 
features    : 1291 
extent      : -1265736, 869263.7, -3051349, -1022949  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=eqdc +lat_0=0 +lon_0=25 +lat_1=20 +lat_2=-23 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs +ellps=WGS84 +towgs84=0,0,0 
variables   : 1
names       : e_ang_g 
min values  :       1 
max values  :       1 

$e_ang$e_ang_l
class       : SpatialPolygonsDataFrame 
features    : 1291 
extent      : -1265736, 869263.7, -3051349, -1022949  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=eqdc +lat_0=0 +lon_0=25 +lat_1=20 +lat_2=-23 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs +ellps=WGS84 +towgs84=0,0,0 
variables   : 1
names       : Value 
min values  :     1 
max values  :     1 

Here is the code I've used

# Using lapply to write shapefiles within list
lapply(unlist(shp.list), function(x) writeOGR(object = x,
                                         dsn = paste(fruit.bats, "output", sep = ''), # folder to store shapefiles 
                                         layer = names(x), # shapefile names as in shp.list
                                         driver = 'ESRI Shapefile',
                                         layer_options = 'RESIZE=YES',
                                         overwrite_layer = T))
# this returns 

$e_ang.e_ang_g
NULL

$e_ang.e_ang_l
NULL

I believe the problem is in the argument 'layer' in writeOGR. Probably I can create a vector to pass the names to the function, but I was wondering if there is a more direct way to do it. I would like the shapefiles to have the name of the nested elements, 'e_ang_g' and 'e_ang_l' in the example.

---

I was able to do the same with a loop, but still seek a solution using lapply.

Here is the code for the loop.

n <- substring(names(unlist(shp.list)), 7,13) # vector with shapefile names

for (j in 1:length(n)){
  writeOGR(unlist(shp.list)[[j]], 
           dsn = paste(fruit.bats, "output", sep = '/'),
           layer = n[j],
           driver = 'ESRI Shapefile',
           layer_options = 'RESIZE=YES',
           overwrite_layer = T) }

Answer 1

This seems to replicate your list structure, which appears to be a list of lists of spatial data. I'll just put some character data in place of the spatial data:

shp.list = list(
   e_ang = list(
     e_ang_g="ggg",
     e_ang_l="lll"),
   z_foo=list(
     a_foo="aaa",
     b_foo="bbb"))

To loop over a list of lists you need a nested lapply:

> tmp = lapply(shp.list, function(e){lapply(e,function(s){message(s)})})
ggg
lll
aaa
bbb

the outer lapply is over the top-level elements, and the inner lapply goes over the elements of each top-level element.

What you probably want to do is loop over the names of the elements within each top-level element, and use those names to construct the shapefile name. I recommend using raster::shapefile since it works out the layer name for you from the file name. So this should work:

library(raster)
tmp = lapply(shp.list, function(L){
          lapply(names(L),function(n){
             shapefile(L[[n]],file=paste0(n,".shp"))
              })
          })

seems to have created things:

> list.files(pattern=".*shp")
[1] "a_foo.shp"   "b_foo.shp"   "e_ang_g.shp" "e_ang_l.shp"