R function to count coordinates

Question

Trying to get it done via mapply or something like this without iterations - I have a spatial dataframe in R and would like to subset all more complicated shapes - ie shapes with 10 or more coordinates. The shapefile is substantial (10k shapes) and the method that is fine for a small sample is very slow for a big one. The iterative method is

Street$cc <-0
i <- 1
while(i <= nrow(Street)){
  Street$cc[i] <-length(coordinates(Street)[[i]][[1]])/2
  i<-i+1
}

How can i get the same effect in any array way? I have a problem with accessing few levels down from the top (Shapefile/lines/Lines/coords)

I tried:

    Street$cc <- lapply(slot(Street, "lines"), 
     function(x) lapply(slot(x, "Lines"),   
     function(y) length(slot(y, "coords"))/2))

/division by 2 as each coordinate is a pair of 2 values/ but is still returns a list with number of items per row, not the integer telling me how many items are there. How can i get the number of coordinates per each shape in a spatial dataframe? Sorry I do not have a reproducible example but you can check on any spatial file - it is more about accessing low level property rather than a very specific issue.

EDIT: I resolved the issue - using function

tail()

dww dww · Accepted Answer · 2016-09-12T12:46:36

Here is a reproducible example. Slightly different to yours, because you did not provide data, but the principle is the same. The 'principle' when drilling down into complex S4 structures is to pay attention to whether each level is a list or a slot, using [[]] to access lists, and @ for slots.

First lets get a spatial ploygon. I'll use the US state boundaries;

library(maps)  
local.map = map(database = "state", fill = TRUE, plot = FALSE) 
IDs = sapply(strsplit(local.map$names, ":"), function(x) x[1])
states = map2SpatialPolygons(map = local.map, ID = IDs)

Now we can subset the polygons with fewer than 200 vertices like this:

# Note: next line assumes that only interested in one Polygon per top level polygon.
# I.e. assumes that we have only single part polygons
# If you need to extend this to work with multipart polygons, it will be
# necessary to also loop over values of lower level Polygons
lengths = sapply(1:length(states), function(i) 
            NROW(states@polygons[[i]]@Polygons[[1]]@coords))

simple.states = states[which(lengths < 200)]
plot(simple.states)

R function to count coordinates

1 Answers