Sf find number of parts in multigeometry

I see perfectly good answers to this, but can't resist sharing my own
approach.

 I use  gibble::gibble for a summary of parts. See how the multi-part
object is number 4, and has 3 subobjects (subobject will repeat within
object for holes).

library(sf)
x <- read_sf(system.file("gpkg/nc.gpkg", package = "sf"))
gibble::gibble(x)
#    A tibble: 108 x 5
#     nrow  ncol type         subobject object
#    <int> <int> <chr>            <int>  <int>
#1    27     2 MULTIPOLYGON         1      1
#2    26     2 MULTIPOLYGON         1      2
#3    28     2 MULTIPOLYGON         1      3
#4    26     2 MULTIPOLYGON         1      4
#5     7     2 MULTIPOLYGON         2      4
#6     5     2 MULTIPOLYGON         3      4
#7    34     2 MULTIPOLYGON         1      5
#...

(I use that for mapping out set-based operations on geometry data, it
doesn't make a huge amount of sense on its own. I suppose a rapply scheme
could be constructed to pluck out things, but you'd also want path extents
and sizes and so forth for greater control).

But, if the biggest area of each multipolygon is what you want, give each a
unique ID,  use st_cast to POLYGON, group by parent and slice out the
largest.

library(dplyr)

x %>% mutate(ID = row_number()) %>% st_cast("POLYGON") %>%
group_by(ID) %>% arrange(desc(st_area(.))) %>% slice(1) %>% ungroup()

Note that holes within a part might reduce the area logic, but so will
details of the map projection in use and so on. It's helpful to learn the
structure of the underlying geometry lists to craft your own rogue
solutions.

HTH

Sf find number of parts in multigeometry

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