Randomly sample existing points from within polygons in SF | R

Question

I have a set of existing sf points and want to randomly sample 1 point from each of a 1km2 grid cell over a landscape.

I can create a grid of 1km2 polygons, but I can't work out in sf how to then sample points within these polygons.

E.g.

library("sf")
polys <- st_read(system.file("shape/nc.shp", package="sf"))
polys <- polys[1:3,]
polys <- st_geometry(polys) %>% st_cast("POLYGON")

pts <- st_sample(polys, 30)

plot(polys)
plot(pts, add = TRUE)

Now I want to extract one element of pts for each polygon in polys, with the result looking like:

Is there a function to do this within the sf framework?

Answer 1

Create a vector with the number of polygons:

spts <- numeric(length = length(polys))

Get a table of which points are in each polygon:

pwin <- t(st_within(pts, polys))
pwin
> Sparse geometry binary predicate list of length 3, where the predicate was `within'
   1: 2, 5, 9, 12, 14, 16, 21, 23, 25, 27
   2: 4, 7, 10, 11, 26, 28, 29
   3: 1, 3, 6, 8, 13, 15, 17, 18, 19, 20, ...

Randomly sample a single point in each polygon: (NB: the ifelse(is_empty(... call saves this from breaking where there are no points in a polygon, while second ifelse( save misleading failure where there is a single point in each polygon, see comment from @Spacedman)

library("purrr")

for(i in 1:length(polys)){

  spts[i] <- ifelse(is_empty(pwin[[i]]), 0, ifelse(length(pwin[[i]]) == 1, pwin[[i]][1], sample(pwin[[i]], 1)))
}

nspts <- pts[spts]

Viola

plot(polys)
plot(pts, add = TRUE)
plot(nspts, add = TRUE, col = "red", pch = 19)