识别R（空间）中栅格中的最近邻

我想创建一个正方形网格，并确定与一组二进制变量取1的其他网格单元相邻的网格单元。在下面的示例中，我想生成一个单元格id向量，该向量与id g13和g24相邻：

require(sp)         
grid <- GridTopology(c(0,0), c(1,1), c(5,5))    
polys <- as(grid, "SpatialPolygons")
centroids <- coordinates(polys)
id <- names(polys)
tr <- ifelse(id == "g13" | id == "g24", 1, 0)       
ex <- SpatialPolygonsDataFrame(polys, data = data.frame(id = id, tr = tr, row.names = row.names(polys)))

plot(ex)
text(coordinates(polys), labels = row.names(polys))

require（sp）
grid
rgeos:：gTouches
非常适合：

library(rgeos)
adj <- gTouches(polys, polys[which(ex$tr==1)], byid=TRUE)
apply(adj, 1, which)

# $g13
#  g7  g8  g9 g12 g14 g17 g18 g19 
#   7   8   9  12  14  17  18  19 
# 
# $g24
# g18 g19 g20 g23 g25 
#  18  19  20  23  25 

两个
poly
应该是
poly
（反之亦然）。
plot(ex, col=ifelse(seq_along(ex) %in% c(unlist(adj), which(ex$tr==1)), 'gray', NA))
text(coordinates(polys), labels=row.names(polys))