使用分配給多個列的最佳方法是什么data.table？例如：f <- function(x) {c("hi", "hello")}x <- data.table(id = 1:10)我想做這樣的事情（當然這種語法不正確）：x[ , (col1, col2) := f(), by = "id"]為了擴展它，我可能有很多列的名稱存儲在一個變量（比如說col_names）中，我想這樣做：x[ , col_names := another_f(), by = "id", with = FALSE]做這樣的事的正確方法是什么？
3 回答
慕尼黑的夜晚無繁華
TA貢獻1864條經(jīng)驗 獲得超6個贊
這現(xiàn)在適用于R-Forge的v1.8.3。謝謝你突出它！
x <- data.table(a = 1:3, b = 1:6)
f <- function(x) {list("hi", "hello")}
x[ , c("col1", "col2") := f(), by = a][]
# a b col1 col2
# 1: 1 1 hi hello
# 2: 2 2 hi hello
# 3: 3 3 hi hello
# 4: 1 4 hi hello
# 5: 2 5 hi hello
# 6: 3 6 hi hello
x[ , c("mean", "sum") := list(mean(b), sum(b)), by = a][]
# a b col1 col2 mean sum
# 1: 1 1 hi hello 2.5 5
# 2: 2 2 hi hello 3.5 7
# 3: 3 3 hi hello 4.5 9
# 4: 1 4 hi hello 2.5 5
# 5: 2 5 hi hello 3.5 7
# 6: 3 6 hi hello 4.5 9
mynames = c("Name1", "Longer%")
x[ , (mynames) := list(mean(b) * 4, sum(b) * 3), by = a]
# a b col1 col2 mean sum Name1 Longer%
# 1: 1 1 hi hello 2.5 5 10 15
# 2: 2 2 hi hello 3.5 7 14 21
# 3: 3 3 hi hello 4.5 9 18 27
# 4: 1 4 hi hello 2.5 5 10 15
# 5: 2 5 hi hello 3.5 7 14 21
# 6: 3 6 hi hello 4.5 9 18 27
x[ , mynames := list(mean(b) * 4, sum(b) * 3), by = a, with = FALSE][] # same
# a b col1 col2 mean sum Name1 Longer%
# 1: 1 1 hi hello 2.5 5 10 15
# 2: 2 2 hi hello 3.5 7 14 21
# 3: 3 3 hi hello 4.5 9 18 27
# 4: 1 4 hi hello 2.5 5 10 15
# 5: 2 5 hi hello 3.5 7 14 21
# 6: 3 6 hi hello 4.5 9 18 27
x[ , get("mynames") := list(mean(b) * 4, sum(b) * 3), by = a][] # same
# a b col1 col2 mean sum Name1 Longer%
# 1: 1 1 hi hello 2.5 5 10 15
# 2: 2 2 hi hello 3.5 7 14 21
# 3: 3 3 hi hello 4.5 9 18 27
# 4: 1 4 hi hello 2.5 5 10 15
# 5: 2 5 hi hello 3.5 7 14 21
# 6: 3 6 hi hello 4.5 9 18 27
x[ , eval(mynames) := list(mean(b) * 4, sum(b) * 3), by = a][] # same
# a b col1 col2 mean sum Name1 Longer%
# 1: 1 1 hi hello 2.5 5 10 15
# 2: 2 2 hi hello 3.5 7 14 21
# 3: 3 3 hi hello 4.5 9 18 27
# 4: 1 4 hi hello 2.5 5 10 15
# 5: 2 5 hi hello 3.5 7 14 21
# 6: 3 6 hi hello 4.5 9 18 27
慕絲7291255
TA貢獻1859條經(jīng)驗 獲得超6個贊
以下簡寫符號可能有用。所有的功勞都歸功于Andrew Brooks，特別是這篇文章。
dt[,`:=`(avg=mean(mpg), med=median(mpg), min=min(mpg)), by=cyl]
溫溫醬
TA貢獻1752條經(jīng)驗 獲得超4個贊
我應該如何修改以下行，以便從dim輸出中為每個objectName獲取兩列，而不是一列包含兩行？data.table(objectName=ls())[,c("rows","cols"):=dim(get(objectName)),by=objectName]（我正在使用data.table1.8.11）
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