我有一個data.table dt。此data.table首先按列date（我的分組變量）排序，然后按列排序age：library(data.table)setkeyv(dt, c("date", "age")) # Sorts table first by column "date" then by "age"> dt date age name1: 2000-01-01 3 Andrew2: 2000-01-01 4 Ben3: 2000-01-01 5 Charlie4: 2000-01-02 6 Adam5: 2000-01-02 7 Bob6: 2000-01-02 8 Campbell我的問題是：我想知道是否可以提取每個唯一日期的前兩行？或更籠統(tǒng)地說：如何提取每個組中的前n行？在此示例中，結(jié)果dt.f為：> dt.f = ???????? # function of dt to extract the first 2 rows per unique date> dt.f date age name1: 2000-01-01 3 Andrew2: 2000-01-01 4 Ben3: 2000-01-02 6 Adam4: 2000-01-02 7 Bobps這是創(chuàng)建上述data.table的代碼：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"
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TA貢獻(xiàn)1786條經(jīng)驗(yàn) 獲得超13個贊
是的，只需.SD根據(jù)需要使用它并為其編制索引。
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
根據(jù)@eddi的建議進(jìn)行編輯。
@eddi的建議是：
請改用此命令以提高速度：
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
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