foreach {parallel} nested with for loop to update data.frame column

fac<-interaction(xyz[,1], xyz[,2], drop=TRUE) 
xyz[,4]<-ave(xyz$z, fac, FUN= min) 
head(xyz) 

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Subject: [R] foreach {parallel} nested with for loop to update data.frame 
column 

Hi List, 

Trying to update a data.frame column within a foreach nested for loop 

### trial data 
set.seed(666) 
xyz<-as.data.frame(cbind(x=rep(rpois(5000,10),2)+1, 
y=rep(rpois(5000,10),2)+1,z=round(runif(10000, min=-3, max=40),2))) 
xyz$mins<-rep(NA, nrow(xyz)) 

cl<-makeCluster(16) ?#adjust to your cluster number 
registerDoParallel(cl) 

counter=0 
foreach(i=unique(xyz[,1]), .combine=data.frame, .verbose=T) %dopar% { 
? ? ? ? for( j in unique(xyz[,2])) { 
? ? ? ? ? ? ? ? xyz[xyz[,2] == j ,4]<-min(xyz[xyz[,2] == j ,2]) 
? ? ? ? } 

} 

stopCluster(cl) 

This is obviously not working. Any hints? 

Thanx 
Herry