代码如下：
library(igraph)

library(xlsx)
WindowPhone<-"/Users/whiskey/Desktop/data/WindowPhone.xlsx"
mydataframe<-read.xlsx(WindowPhone,1)
g <- graph.data.frame(mydataframe)

is.simple(g)
gg = simplify(g)
is.simple(gg)
dg = degree(gg)
gg = subgraph(gg, which(dg > 0) - 1)

com = walktrap.community(gg, steps = 5)
subgroup = split(gg$a, gg$r)
## subgroup
V(gg)$sg = com$membership + 1
V(gg)$color = rainbow(max(V(gg)$sg))[V(gg)$sg]
## png("net_walktrap.png", width = 500, height = 500)
par(mar = c(0, 0, 0, 0))
set.seed(14)
plot(gg, layout = layout.fruchterman.reingold, vertex.size = 5,
    vertex.color = V(gg)$color, vertex.label = NA, edge.color = grey(0.5),
    edge.arrow.mode = "-")
## dev.off()


然而出现了错误：

Error in split.default(gg$a, gg$r) : 第一引数必须是向量


请问大神如何修改？