(year), colwise(nmissing))
ddply(baseball, ....(year), numcolwise(nmissing))
ddply(baseball, ....")
count(baseball[1:100,], vars = "id", wt_var = "g")
count(baseball[1:100,], c("id", "year"))
# match_df..., longterm, on="id")
# join 类似sql中的join,比merge速度更快
first ddply(baseball, "id", summarise, first =...(baseball,"id",summarize,length(year)))
system.time(tapply(baseball$year,baseball$id,function(x) length