rgeos 'union' causes R crash

Hi all-

I'm having an odd issue with the gUnion family of commands from the rgeos
package. I'm importing a series of polygons (species) and trying to find the
centroid. Each species is only allowed one centroids, so i need to union the
multiple polygons. this code works great when run line by line. But when run
as an actual loop, but adding the {} at beginning and end, it gives me a
runtime error. I have debugged it to the level that i know it is the union
polygon step. I am running R 2.13, and it was designed for 2.13.1, is that
really making the difference? When run, R completely freezes and windows
shuts it down.

library(rgdal)
library(PBSmapping)
library(maptools)
library(raster)
library(rgeos)
#Enter File Directory Below
setwd("D:/Ben/R/Trochilidae")
#Pull in all shapefiles in that folder that are polygons
a<-list.files(pattern='pl.shp$', full.names=FALSE)
output<-matrix(nrow=length(a),ncol=3)
colnames(output)<-c("Sci_Name","X_centroid","Y_centroid")
for(x in 1:length(a)){
output[(x),1]<-paste(gsub(".shp","",a[x]))
range<-readShapePoly(a[x], delete_null_obj=TRUE)
subsetx<-range[range$PRESENCE == 1,]
subsetx2<-subsetx[subsetx$ORIGIN ==1|subsetx$ORIGIN==2,]
z<-matrix(length(subsetx2),1, data=1)
Union<-unionSpatialPolygons(subsetx2,z)
Breeding.ps<- SpatialPolygons2PolySet(Union)
Breeding.centroids<- calcCentroid(Breeding.ps, rollup=1)
output[x,2:3]<-as.matrix(Breeding.centroids[2:3])}

thanks

ben weinstein




-- 
Ben Weinstein
Graduate Student
Ecology and Evolution
Stony Brook University

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