dat1=data.frame(host=c("A","A","A","E","E","G"),
                guest=c("C","B","D","Q","F","W"),
                num=c(2,1,2,3,0,2))

library(reshape)
dat2=cast(dat1,host~guest,value="num")
dat2

Thanks，这个自己写函数也能解决，但我相信应该有包含有该函数