r - 如何提取每组的前 n 行？

Question

我有一个数据表 dt .此数据表首先按列排序 date (我的分组变量)，然后按列 age :

library(data.table)
setkeyv(dt, c("date", "age")) # Sorts table first by column "date" then by "age"
> dt
         date age     name
1: 2000-01-01   3   Andrew
2: 2000-01-01   4      Ben
3: 2000-01-01   5  Charlie
4: 2000-01-02   6     Adam
5: 2000-01-02   7      Bob
6: 2000-01-02   8 Campbell

我的问题是:我想知道是否可以为每个唯一日期提取前 2 行？或者更笼统地说:

如何提取每组中的前 n 行？

在这个例子中，结果在 dt.f将是:

> dt.f = ???????? # function of dt to extract the first 2 rows per unique date
> dt.f
         date age   name
1: 2000-01-01   3 Andrew
2: 2000-01-01   4    Ben
3: 2000-01-02   6   Adam
4: 2000-01-02   7    Bob

附言这是创建上述 data.table 的代码:

install.packages("data.table")
library(data.table)
date <- c("2000-01-01","2000-01-01","2000-01-01",
    "2000-01-02","2000-01-02","2000-01-02")
age <- c(3,4,5,6,7,8)
name <- c("Andrew","Ben","Charlie","Adam","Bob","Campbell")
dt <- data.table(date, age, name)
setkeyv(dt,c("date","age")) # Sorts table first by column "date" then by "age"

Answer 1

是的，只需使用 .SD并根据需要对其进行索引。

  DT[, .SD[1:2], by=date]

           date age   name
  1: 2000-01-01   3 Andrew
  2: 2000-01-01   4    Ben
  3: 2000-01-02   6   Adam
  4: 2000-01-02   7    Bob

根据@eddi 的建议进行编辑。

@eddi 的建议是:

使用它来提高速度:

  DT[DT[, .I[1:2], by = date]$V1]

  # using a slightly larger data set
  > microbenchmark(SDstyle=DT[, .SD[1:2], by=date], IStyle=DT[DT[, .I[1:2], by = date]$V1], times=200L)
  Unit: milliseconds
      expr       min        lq    median        uq      max neval
   SDstyle 13.567070 16.224797 22.170302 24.239881 88.26719   200
    IStyle  1.675185  2.018773  2.168818  2.269292 11.31072   200