How to select certain geometries from a geometrycollection after st_intersect?

Question

I am running an intersect of two polygons or other sf objects using the fantastic new sf package. It's similar to this:

a <- st_polygon(list(cbind(c(0,0,7.5,7.5,0),c(0,-1,-1,0,0))))
b <- st_polygon(list(cbind(c(0,1,2,3,4,5,6,7,7,0),c(1,0,.5,0,0,0.5,-0.5,-0.5,1,1))))
i <- st_intersection(a,b)
## GEOMETRYCOLLECTION(POINT(1 0), LINESTRING(4 0, 3 0), POLYGON((5.5 0, 7 0, 7 -0.5, 6 -0.5, 5.5 0)))

how do I only keep the POLYGON of the GEOMETRYCOLLECTION? Selecting different types in a feature collection is easy enough, but I can't seem to find the equivalent of ST_CollectionExtract in the sf package.

Answer 1

Output is a list so that you can extract with i[[3]] here.
If you a more standard way to find which element is a polygon use:

w.pol <- purrr::map_lgl(i, ~st_is(.x, c("POLYGON", "MULTIPOLYGON")))
pol <- i[[which(w.pol)]]

##> pol
## POLYGON((5.5 0, 7 0, 7 -0.5, 6 -0.5, 5.5 0))

Edit: If you have a sfc, you can use st_cast to separate feature types, and then select the lines of interest:

# Simple feature data frame of spatial collection
a1 <- st_sf(a=1, geom = st_sfc(i))
a2 <- st_sf(a=2, geom = st_sfc(i))

ii <- rbind(a1, a2)

# Use st_cast to separate all features types
st_cast(ii)[which(st_is(st_cast(ii), c("POLYGON", "MULTIPOLYGON"))),]

Edit: 2017-12-23

You can directly use:

st_collection_extract(i, "POLYGON")