为了用一个例子来说明上面的 comments ,让我们举一个例子
set.seed(10238)
# A and B are the "id" variables within which the
# "data" variables C and D vary meaningfully
DT = data.table(
A = rep(1:3, each = 5L),
B = rep(1:5, 3L),
C = sample(15L),
D = sample(15L)
)
DT
# A B C D
# 1: 1 1 14 11
# 2: 1 2 3 8
# 3: 1 3 15 1
# 4: 1 4 1 14
# 5: 1 5 5 9
# 6: 2 1 7 13
# 7: 2 2 2 12
# 8: 2 3 8 6
# 9: 2 4 9 15
# 10: 2 5 4 3
# 11: 3 1 6 5
# 12: 3 2 12 10
# 13: 3 3 10 4
# 14: 3 4 13 7
# 15: 3 5 11 2
比较以下各项:
#Sum all columns
DT[ , lapply(.SD, sum)]
# A B C D
# 1: 30 45 120 120
#Sum all columns EXCEPT A, grouping BY A
DT[ , lapply(.SD, sum), by = A]
# A B C D
# 1: 1 15 38 43
# 2: 2 15 30 49
# 3: 3 15 52 28
#Sum all columns EXCEPT A
DT[ , lapply(.SD, sum), .SDcols = !"A"]
# B C D
# 1: 45 120 120
#Sum all columns EXCEPT A, grouping BY B
DT[ , lapply(.SD, sum), by = B, .SDcols = !"A"]
# B C D
# 1: 1 27 29
# 2: 2 17 30
# 3: 3 33 11
# 4: 4 23 36
# 5: 5 20 14
几点注意事项
答案是no,这对data.table
人来说非常重要.返回的对象是new data.table
,DT
中的所有列与运行代码之前完全相同.
再次提到上面这一点,请注意,您的代码(DT[ , lapply(.SD, as.factor)]
)返回一个new data.table
,并且根本不会更改DT
.一种(incorrect)方法是用base
中的data.frame
s来完成,即用返回的新data.table
覆盖旧data.table
,即DT = DT[ , lapply(.SD, as.factor)]
.
这是浪费,因为它涉及到创建DT
的副本,而当DT
很大时,这可能是效率杀手.解决此问题的正确方法是使用`:=`
(例如DT[ , names(DT) := lapply(.SD, as.factor)]
)通过引用更新列,这不会创建数据的副本.更多信息请参见data.table
's reference semantics vignette.
- 你提到比较
lapply(.SD, sum)
和colSums
的效率.sum
在data.table
中进行了内部优化(您可以从[]
中添加verbose = TRUE
参数的输出中注意到这一点);为了看到这一点的实际效果,让我们加强一下你的DT
,并运行一个基准测试:
结果:
library(data.table)
set.seed(12039)
nn = 1e7; kk = seq(100L)
DT = setDT(replicate(26L, sample(kk, nn, TRUE), simplify=FALSE))
DT[ , LETTERS[1:2] := .(sample(100L, nn, TRUE), sample(100L, nn, TRUE))]
library(microbenchmark)
microbenchmark(
times = 100L,
colsums = colSums(DT[ , !c("A", "B")]),
lapplys = DT[ , lapply(.SD, sum), .SDcols = !c("A", "B")]
)
# Unit: milliseconds
# expr min lq mean median uq max neval
# colsums 1624.2622 2020.9064 2028.9546 2034.3191 2049.9902 2140.8962 100
# lapplys 246.5824 250.3753 252.9603 252.1586 254.8297 266.1771 100