R Under development (unstable) (2023-07-09 r84667 ucrt) -- "Unsuffered Consequences"
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> library(sf)
Linking to GEOS 3.11.2, GDAL 3.6.2, PROJ 9.2.0; sf_use_s2() is TRUE
> # aggregate
> pl1 = st_polygon(list(rbind(c(0,0),c(1,0),c(1,1),c(0,0))))
> pl2 = st_polygon(list(rbind(c(0,0),c(1,1),c(0,1),c(0,0))))
> s = st_sf(a = 1:2, geom = st_sfc(pl1, pl2))
> (a = aggregate(s, list(c(1,1)), mean, do_union = FALSE))
Simple feature collection with 1 feature and 2 fields
Attribute-geometry relationships: aggregate (1), identity (1)
Geometry type: MULTIPOLYGON
Dimension:     XY
Bounding box:  xmin: 0 ymin: 0 xmax: 1 ymax: 1
CRS:           NA
  Group.1   a                       geometry
1       1 1.5 MULTIPOLYGON (((0 0, 1 0, 1...
> (a = aggregate(s, list(c(1,1)), mean, do_union = TRUE))
Simple feature collection with 1 feature and 2 fields
Attribute-geometry relationships: aggregate (1), identity (1)
Geometry type: POLYGON
Dimension:     XY
Bounding box:  xmin: 0 ymin: 0 xmax: 1 ymax: 1
CRS:           NA
  Group.1   a                       geometry
1       1 1.5 POLYGON ((1 0, 0 0, 0 1, 1 ...
> # expect_warning(st_cast(a, "POINT"))
> if (suppressPackageStartupMessages(require(sp, quietly = TRUE))) {
+  demo(meuse_sf, echo = FALSE, ask = FALSE)
+  a = aggregate(meuse_sf, list(meuse_sf$soil), mean)
+  print(attributes(a)$agr)
+  a = aggregate(meuse_sf, list(soil = meuse_sf$soil), mean)
+  print(attributes(a)$agr)
+  a = aggregate(meuse_sf, list(meuse_sf$soil, meuse_sf$ffreq), mean)
+  print(attributes(a)$agr)
+  a = aggregate(meuse_sf, list(soil = meuse_sf$soil, ff = meuse_sf$ffreq), mean)
+  print(attributes(a)$agr)
+ }
  Group.1   cadmium    copper      lead      zinc      elev      dist        om 
 identity aggregate aggregate aggregate aggregate aggregate aggregate aggregate 
    ffreq      soil      lime   landuse    dist.m 
aggregate aggregate aggregate aggregate aggregate 
Levels: constant aggregate identity
     soil   cadmium    copper      lead      zinc      elev      dist        om 
 identity aggregate aggregate aggregate aggregate aggregate aggregate aggregate 
    ffreq    soil.1      lime   landuse    dist.m 
aggregate aggregate aggregate aggregate aggregate 
Levels: constant aggregate identity
  Group.1   Group.2   cadmium    copper      lead      zinc      elev      dist 
 identity  identity aggregate aggregate aggregate aggregate aggregate aggregate 
       om     ffreq      soil      lime   landuse    dist.m 
aggregate aggregate aggregate aggregate aggregate aggregate 
Levels: constant aggregate identity
     soil        ff   cadmium    copper      lead      zinc      elev      dist 
 identity  identity aggregate aggregate aggregate aggregate aggregate aggregate 
       om     ffreq    soil.1      lime   landuse    dist.m 
aggregate aggregate aggregate aggregate aggregate aggregate 
Levels: constant aggregate identity
There were 50 or more warnings (use warnings() to see the first 50)
> 
> # aggregate by sf/sfc
> a = st_polygon(list(rbind(c(0,0), c(1,0), c(1,1), c(0,1), c(0,0)))) * 2
> b = a + 1
> p = st_sfc(st_point(c(0.1,0.1)), st_point(c(1.5,1.5)), st_point(c(2.9,2.9)))
> x = st_sf(count = 1:3, geom = p)
> aggregate(x, st_sfc(a,b), mean)
Simple feature collection with 2 features and 1 field
Geometry type: POLYGON
Dimension:     XY
Bounding box:  xmin: 0 ymin: 0 xmax: 3 ymax: 3
CRS:           NA
  count                       geometry
1   1.5 POLYGON ((0 0, 2 0, 2 2, 0 ...
2   2.5 POLYGON ((1 1, 3 1, 3 3, 1 ...
> aggregate(x, st_sf(st_sfc(a,b)), mean)
Simple feature collection with 2 features and 1 field
Geometry type: POLYGON
Dimension:     XY
Bounding box:  xmin: 0 ymin: 0 xmax: 3 ymax: 3
CRS:           NA
  count                       geometry
1   1.5 POLYGON ((0 0, 2 0, 2 2, 0 ...
2   2.5 POLYGON ((1 1, 3 1, 3 3, 1 ...
> aggregate(x, st_sf(st_sfc(a,b,b+10)), mean)
Simple feature collection with 3 features and 1 field
Geometry type: POLYGON
Dimension:     XY
Bounding box:  xmin: 0 ymin: 0 xmax: 13 ymax: 13
CRS:           NA
  count                       geometry
1   1.5 POLYGON ((0 0, 2 0, 2 2, 0 ...
2   2.5 POLYGON ((1 1, 3 1, 3 3, 1 ...
3    NA POLYGON ((11 11, 13 11, 13 ...
> 
> proc.time()
   user  system elapsed 
   1.03    0.15    1.17