假设我有一个data.table，里面有一些棒球运动员:
library(plyr)
library(data.table)
bdt <- as.data.table(baseball)
对于每个组(由玩家"id"给出)，我想 Select 与最大游戏数"g"对应的行.这在plyr中很简单:
ddply(baseball, "id", subset, g == max(g))
data.table的等效代码是多少？
我试过:
setkey(bdt, "id")
bdt[g == max(g)] # only one row
bdt[g == max(g), by = id] # Error: 'by' or 'keyby' is supplied but not j
bdt[, .SD[g == max(g)]] # only one row
这是有效的:
bdt[, .SD[g == max(g)], by = id]
但它只比plyr快30%，这表明它可能不是惯用语.