r - data.table 给出错误 Error : k <= n is not TRUE

Question

我正在尝试将以下 dplyr 代码转换为等效的 data.table 代码。 但是，是 data.table 而不是 dplyr 给了我错误。
Error: k <= n is not TRUE 。

#using dplyr
library(dplyr)
library(zoo) #rollmean function
DF<-mtcars
DF %>%
  filter(cyl==6)%>%
  group_by(am,vs) %>%
  mutate(cumsum_mpg=cumsum(mpg),cummin_disp=cummin(disp),rollmean_wt=rollmean(wt,k=2,fill=0,align="right")) 

#using data.table 
library(data.table)
library(zoo) #rollmean function
DT<-data.table(mtcars)
setkey(DT,am,vs)
mynames<-c("cumsum_mpg","cummin_disp","rollmean_wt")
DT[,.SD[cyl==6] [,eval(mynames):=list(cumsum(mpg),cummin(disp),rollmean(wt,k=2,fill=0,align="right"))],by=.(am,vs)]

Answer 1

你可以试试

DT[cyl==6][,(mynames):= list(cumsum(mpg), cummin(disp), 
         rollmean(wt,k=2,fill=0,align="right")), by=.(am, vs)][]
#            mpg cyl  disp  hp drat    wt  qsec vs am gear carb cumsum_mpg cummin_disp
#1: 21.4   6 258.0 110 3.08 3.215 19.44  1  0    3    1       21.4       258.0
#2: 18.1   6 225.0 105 2.76 3.460 20.22  1  0    3    1       39.5       225.0
#3: 19.2   6 167.6 123 3.92 3.440 18.30  1  0    4    4       58.7       167.6
#4: 17.8   6 167.6 123 3.92 3.440 18.90  1  0    4    4       76.5       167.6
#5: 21.0   6 160.0 110 3.90 2.620 16.46  0  1    4    4       21.0       160.0
#6: 21.0   6 160.0 110 3.90 2.875 17.02  0  1    4    4       42.0       160.0
#7: 19.7   6 145.0 175 3.62 2.770 15.50  0  1    5    6       61.7       145.0
#   rollmean_wt
#1:      0.0000
#2:      3.3375
#3:      3.4500
#4:      3.4400
#5:      0.0000
#6:      2.7475
#7:      2.8225

或者，如果您需要按照与 dplyr 中相同的行顺序获取结果

 DT <- as.data.table(mtcars)
 setkey(DT, cyl)
 DT[J(6)][, (mynames) := list(cumsum(mpg), cummin(disp), 
       rollmean(wt,k=2,fill=0,align="right")), by=.(am, vs)][]