Subsetting SpatialPolygonsDataFrame and empty ID slots

On Mon, 29 Oct 2007, Agustin Lobo wrote:

Dear list,

I have imported a shp file with:

pols <- readOGR("../AllTransectPolygons02",

layer="AllTransectPolygons02")

class(pols)

[1] "SpatialPolygonsDataFrame"
attr(,"package")
[1] "sp"

pols at data

    AREA PERIMETER               Site
0   2500      1010           Lupuna 1
1   2500      1010           Lupuna 3
2   2500      1010           Lupuna 2
3   2500      1010             Aleman
4   2500      1010        Zancudoyacu
etc

and now I want to select few polygons according
to a vector of Site names.

Thinking on how doing this, I thought I could select
the corresponding IDs in the data table and then use
the ID slot in the polygons, but
I've found that the polygons themselves actually
lack the ID information:

Please treat all the long named access thingies in sp as deprecated - 
they will be flagged as such in the next release, and removed one 
release after that. S4 is not like that.

library(sp)
library(rgdal)
example(readOGR)
class(Up)
getSlots(class(Up))
class(slot(Up, "polygons")[[1]])
getSlots(class(slot(Up, "polygons")[1]))

So:

sapply(slot(Up, "polygons"), function(x) slot(x, "ID"))

getPolygonsIDSlot(pols)

Error in getPolygonsIDSlot(pols) :
  no slot of name "ID" for this object of class 
"SpatialPolygonsDataFrame"

How does sp relate the data table to the polygons if there is no ID?

Of course there is an ID, but you can get at either by:

sapply(slot(pols, "polygons"), function(x) slot(x, "ID"))

or

rownames(as(pols, "data.frame"))

In any case, which would the most efficient way of subseting
the SpatialPolygonsDataFrame into another SpatialPolygonsDataFrame
with the selected polygons?

Returning to the original question, subsetting is simply by the "[" 
operator, just like any other data.frame:

sites_Lupuna <- length(grep("Lupuna", as.character(pols$Site))) > 0
summary(sites_Lupuna)
pols_Lupuna <- pols[sites_Lupuna,]

Just think of Spatial*DataFrame objects as data.frame objects and things 
will be much clearer.

Roger

Should I convert to a maptools object
and use subset.polylist()? If so, how do I convert? I've tried:

pols.maptools <- SpatialPolygons2PolySet(pols)

PS. PolySet objects are really for drawing coastlines in this context.

but the data table is lost.

Thanks!