各位大神，R语言做空间权重阵出现以下情况，什么情况？怎么解决？
> library(foreign)
> swm <- read.table("C:/Users/ggg/Desktop/pswmEx.txt",
+ header=T, sep=",")
> swm[[3]] <- swm[[3]]+1;
> swm[[4]] <- swm[[4]]+1
> n = length(unique(swm[[3]]))
> swmmat <- matrix(0, n, n)
> apply(swm, 1, function(x) swmmat[x[3], x[4]]<<-x[5])
Error in `[<-`(`*tmp*`, x[3], x[4], value = 0.5) : 下标出界