[R] "which" command

[AgusSusanto]

I obtained error messages when I run these commands in UNIX, but I 
obtained correct result when I run these command in WINDOWS.  Can 
somebody point out the problem and give the solution. Thanks.

 > dt<-read.table(file="Fall.dat")
 > dim(dt)
[1] 1941    5
 > table(dt$V2)
  0   1   2   3
220 989 639  93
 >
 > Favg<-as.matrix(c(1:max(dt$V2)))
 >
 > for(i in 1:max(dt$V2)){Favg[i]<-mean(dt[which(dt[,2] == i),5])}
 > Favg
     [,1]
[1,]   NA
[2,]   NA
[3,]   NA
Warning messages:
argument is not numeric or logical: returning NA (etc)