[R-sig-Geo] sample.Polygons fails for shapes with more than four Polygons

[Aman Verma]

Hi everybody,

I have discovered some strange behaviour in the function sample.Polygons in the sp package. Consider this code:

library(maptools)
library(sp)
# Download and unpack this shape file to your working directory:
# http://surveillance.mcgill.ca/countries_subdiv_Project_PP_project_region.zip
shapefile<-readShapePoly("countries_subdiv_Project_PP_project_region.shp")
	
# Try and sample some points from different shapes (with multiple polygons):
sample.Polygons(shapefile at polygons[[1930]],n=1,type='regular') # Has 2 polygons
# Works just fine.
sample.Polygons(shapefile at polygons[[1933]],n=1,type='regular') # Has 10 polygons
# Fails: 
# Error: subscript out of bounds

I have done some debugging, and located the source of the error. In sample.Polygons, if there is more than one Polygon in the 'shape', this for loop is run:

for (i in seq(along = pls)) {
	bbi <- .bbox2SPts(bbox(pls[[i]]), proj4string = proj4string) # Convert bbox to set of 4 points
	# Check which other shapes overlap the bbox of this shape.
	# For a two dimensional object, bb_in is a list of vectors with length 4:
	# one for each point in the bbox.
	bb_in <- lapply(pls[-i], function(x, pts) pointsInPolygon(pts, x), pts = bbi)
	# For a two dimensional object, zzz must be a matrix with exactly four columns.
	zzz <- do.call("rbind", bb_in) 
	# For a 2D object, zzz only has four columns, but i can exceed four!
	# But 'i' represents the index of the polygon in the shape, which can exceed four.
	# zzz[, i] will fail when i exceeds four.
	if (holes[i] || (any(unlist(bb_in) == 1) && !(sum(zzz[, i])%%2) == 0)) smple[i] <- FALSE
}

The comments are my own. Basically, for two-dimensional shapes, the last line will fail when the number of Polygons in the shape exceeds four, and at least one of those shapes doesn't have a "hole". (If they do have holes, then zzz[,i] won't be evaluated.)

I'm not sure exactly what this last line is supposed to be doing... which is why I can't make a suggestion for a correction. I suspect that zzz[,i] should be zzz[i,], but that is just a guess, really.

I have tested this on R version 2.12.2 with sp package 0.9-78 on both the Windows and UNIX (Ubuntu) platforms.

Thanks for any help.

aman