R Under development (unstable) (2023-07-09 r84667 ucrt) -- "Unsuffered Consequences"
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Platform: x86_64-w64-mingw32/x64

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> suppressPackageStartupMessages(library(sf))
> library(testthat)
> # "vertical" conversions:
> # column 1:
> mp = st_sfc(st_multipoint(matrix(0:3,,2)), st_multipoint(matrix(10:15,,2)))
> (ls = st_cast(mp, "LINESTRING"))
Geometry set for 2 features 
Geometry type: LINESTRING
Dimension:     XY
Bounding box:  xmin: 0 ymin: 2 xmax: 12 ymax: 15
CRS:           NA
LINESTRING (0 2, 1 3)
LINESTRING (10 13, 11 14, 12 15)
> st_cast(ls, "MULTIPOINT")
Geometry set for 2 features 
Geometry type: MULTIPOINT
Dimension:     XY
Bounding box:  xmin: 0 ymin: 2 xmax: 12 ymax: 15
CRS:           NA
MULTIPOINT ((0 2), (1 3))
MULTIPOINT ((10 13), (11 14), (12 15))
> 
> # column 2:
> mls = st_sfc(st_multilinestring(list(rbind(c(0,0), c(10,0), c(10,10), c(0,10)), 
+ 	rbind(c(5,5),c(5,6), c(6,6), c(6,5)))), 
+ 	st_multilinestring(list(rbind(c(0,0), c(1,0), c(1,1), c(0,1)))))
> (pol = st_cast(mls, "POLYGON"))
Geometry set for 2 features 
Geometry type: POLYGON
Dimension:     XY
Bounding box:  xmin: 0 ymin: 0 xmax: 10 ymax: 10
CRS:           NA
POLYGON ((0 0, 10 0, 10 10, 0 10, 0 0), (5 5, 5...
POLYGON ((0 0, 1 0, 1 1, 0 1, 0 0))
> st_cast(pol, "MULTILINESTRING")
Geometry set for 2 features 
Geometry type: MULTILINESTRING
Dimension:     XY
Bounding box:  xmin: 0 ymin: 0 xmax: 10 ymax: 10
CRS:           NA
MULTILINESTRING ((0 0, 10 0, 10 10, 0 10, 0 0),...
MULTILINESTRING ((0 0, 1 0, 1 1, 0 1, 0 0))
> 
> # "horizontal" conversions:
> 
> (pt = st_cast(mp, "POINT"))
Geometry set for 5 features 
Geometry type: POINT
Dimension:     XY
Bounding box:  xmin: 0 ymin: 2 xmax: 12 ymax: 15
CRS:           NA
POINT (0 2)
POINT (1 3)
POINT (10 13)
POINT (11 14)
POINT (12 15)
> (i = attr(pt, "ids"))
[1] 2 3
> (xx = st_cast(pt, "MULTIPOINT", rep(seq_along(i), i)))
Geometry set for 5 features 
Geometry type: MULTIPOINT
Dimension:     XY
Bounding box:  xmin: 0 ymin: 2 xmax: 12 ymax: 15
CRS:           NA
MULTIPOINT ((0 2))
MULTIPOINT ((1 3))
MULTIPOINT ((10 13))
MULTIPOINT ((11 14))
MULTIPOINT ((12 15))
> try(yy <- st_cast(pt, "LINESTRING", rep(seq_along(i), i)))
> 
> #(zz = st_cast(yy, "MULTILINESTRING"))
> #(zz = st_cast(yy, "POLYGON"))
> 
> st_cast(mls, "LINESTRING")
Geometry set for 3 features 
Geometry type: LINESTRING
Dimension:     XY
Bounding box:  xmin: 0 ymin: 0 xmax: 10 ymax: 10
CRS:           NA
LINESTRING (0 0, 10 0, 10 10, 0 10)
LINESTRING (5 5, 5 6, 6 6, 6 5)
LINESTRING (0 0, 1 0, 1 1, 0 1)
> 
> (g = st_sfc(c(mls, ls)))
Geometry set for 4 features 
Geometry type: GEOMETRY
Dimension:     XY
Bounding box:  xmin: 0 ymin: 0 xmax: 12 ymax: 15
CRS:           NA
MULTILINESTRING ((0 0, 10 0, 10 10, 0 10), (5 5...
MULTILINESTRING ((0 0, 1 0, 1 1, 0 1))
LINESTRING (0 2, 1 3)
LINESTRING (10 13, 11 14, 12 15)
> st_cast(g, "MULTILINESTRING")
Geometry set for 4 features 
Geometry type: MULTILINESTRING
Dimension:     XY
Bounding box:  xmin: 0 ymin: 0 xmax: 12 ymax: 15
CRS:           NA
MULTILINESTRING ((0 0, 10 0, 10 10, 0 10), (5 5...
MULTILINESTRING ((0 0, 1 0, 1 1, 0 1))
MULTILINESTRING ((0 2, 1 3))
MULTILINESTRING ((10 13, 11 14, 12 15))
> expect_warning(st_cast(g, "LINESTRING"))
> st_cast(st_cast(g, "MULTILINESTRING"), "LINESTRING") # will not loose
Geometry set for 5 features 
Geometry type: LINESTRING
Dimension:     XY
Bounding box:  xmin: 0 ymin: 0 xmax: 12 ymax: 15
CRS:           NA
LINESTRING (0 0, 10 0, 10 10, 0 10)
LINESTRING (5 5, 5 6, 6 6, 6 5)
LINESTRING (0 0, 1 0, 1 1, 0 1)
LINESTRING (0 2, 1 3)
LINESTRING (10 13, 11 14, 12 15)
> 
> gc = st_sfc(st_geometrycollection(
+   list(
+     st_multilinestring(list(rbind(c(0,0), c(10,0), c(10,10), c(0,10)), 
+ 	rbind(c(5,5),c(5,6), c(6,6), c(6,5)))), 
+ 	st_multilinestring(list(rbind(c(0,0), c(1,0), c(1,1), c(0,1)))),
+ 	st_point(0:1)
+   )))
> try(st_cast(mls, "POINT"))
Geometry set for 12 features 
Geometry type: POINT
Dimension:     XY
Bounding box:  xmin: 0 ymin: 0 xmax: 10 ymax: 10
CRS:           NA
First 5 geometries:
POINT (0 0)
POINT (10 0)
POINT (10 10)
POINT (0 10)
POINT (5 5)
> try(st_cast(mls, "MULTIPOINT"))
Geometry set for 3 features 
Geometry type: MULTIPOINT
Dimension:     XY
Bounding box:  xmin: 0 ymin: 0 xmax: 10 ymax: 10
CRS:           NA
MULTIPOINT ((0 0), (10 0), (10 10), (0 10))
MULTIPOINT ((5 5), (5 6), (6 6), (6 5))
MULTIPOINT ((0 0), (1 0), (1 1), (0 1))
> 
> outer = matrix(c(0,0,10,0,10,10,0,10,0,0),ncol=2, byrow=TRUE)
> hole1 = matrix(c(1,1,1,2,2,2,2,1,1,1),ncol=2, byrow=TRUE)
> hole2 = matrix(c(5,5,5,6,6,6,6,5,5,5),ncol=2, byrow=TRUE)
> pol1 = list(outer, hole1, hole2)
> pol2 = list(outer + 12, hole1 + 12)
> pol3 = list(outer + 24)
> mp = list(pol1,pol2,pol3)
> mp1 = st_multipolygon(mp)
> s = st_sfc(mp1)
> x = st_cast(s, "MULTIPOINT")
> x = st_cast(s, "POINT")
> expect_warning(st_cast(mp1, "LINESTRING"))
> expect_warning(st_cast(mp1, "POINT"))
> mls = mls[[1]]
> class(mls)
[1] "XY"              "MULTILINESTRING" "sfg"            
> #expect_error(st_cast(mls, "POLYGON"))
> st_cast(mls, "POLYGON")
POLYGON ((0 0, 10 0, 10 10, 0 10, 0 0), (5 5, 5 6, 6 6, 6 5, 5 5))
> 
> expect_warning(st_cast(mls, "POINT"))
> p1 = st_polygon(pol1)
> expect_warning(st_cast(p1, "POINT"))
> ls = ls[[1]]
> class(ls)
[1] "XY"         "LINESTRING" "sfg"       
> expect_warning(st_cast(ls, "POINT"))
> 
> mls = st_cast(p1, "MULTILINESTRING")
> p2 = st_cast(mls, "POLYGON")
> 
> # st_is:
> st_is(st_point(0:1), "POINT")
[1] TRUE
> sfc = st_sfc(st_point(0:1), st_linestring(matrix(1:6,,2)))
> st_is(sfc, "POINT")
[1]  TRUE FALSE
> st_is(sfc, "POLYGON")
[1] FALSE FALSE
> st_is(sfc, "LINESTRING")
[1] FALSE  TRUE
> st_is(st_sf(a = 1:2, sfc), "LINESTRING")
[1] FALSE  TRUE
> st_is(sfc, c("POINT", "LINESTRING"))
[1] TRUE TRUE
> 
> #1194:
> wkt <- "MULTICURVE (COMPOUNDCURVE (LINESTRING (-83.62333 35.55244, -83.62328 35.55232, -83.62323 35.55223, -83.62319 35.55216, -83.62312 35.55209, -83.6231 35.55207), CIRCULARSTRING (-83.6231 35.55207, -83.62307 35.55205, -83.62302 35.55204), LINESTRING (-83.62302 35.55204, -83.62299 35.55203, -83.62289 35.55198, -83.62281 35.55189, -83.62271 35.55182)))"
> g <- st_as_sfc(wkt)
> g <- st_sf(demo = "test", geom = g, crs = 4326)
> m = st_cast(g, "MULTILINESTRING")
> identical(m$geom[[1]], st_cast(g$geom[[1]], "MULTILINESTRING"))
[1] TRUE
> 
> st_cast(st_sfc(st_geometrycollection(), st_multipolygon()), 'MULTIPOLYGON') #1961
Geometry set for 2 features  (with 2 geometries empty)
Geometry type: MULTIPOLYGON
Dimension:     XY
Bounding box:  xmin: NA ymin: NA xmax: NA ymax: NA
CRS:           NA
MULTIPOLYGON EMPTY
MULTIPOLYGON EMPTY
> st_cast(st_sfc(st_geometrycollection(), st_multipolygon(), st_point(0:1)), 'POINT') #1961
Geometry set for 3 features  (with 2 geometries empty)
Geometry type: POINT
Dimension:     XY
Bounding box:  xmin: 0 ymin: 1 xmax: 0 ymax: 1
CRS:           NA
POINT EMPTY
POINT EMPTY
POINT (0 1)
> 
> proc.time()
   user  system elapsed 
   1.64    0.18    1.82