我想按“会话”组扩展data.table中变量的选择:
session score1 score2
1: 1 0.11111111 0.6000000
2: 1 0.00000000 0.5333333
3: 1 0.27777778 0.6666667
4: 1 0.66666667 0.8666667
5: 1 0.83333333 1.0000000
6: 2 0.07692308 0.5757576
7: 2 0.25641026 0.6363636
8: 2 0.00000000 0.5303030
9: 2 0.64102564 0.7878788
10: 2 0.84615385 1.0000000
我试过了:
dt[,(2:3):=lapply(.SD,scale),by="session",.SDcols=2:3]
但我得到一个错误:
Error in `[.data.table`(dt, , `:=`((2:3), lapply(.SD, scale)), by = "session", :
All items in j=list(...) should be atomic vectors or lists. If you are trying something like j=list(.SD,newcol=mean(colA)) then use := by group instead (much quicker), or cbind or merge afterwards.
该代码有效,但仅在没有分组变量(会话)的情况下有效。我究竟做错了什么?
该scale
函数的输出是一个matrix
,所以它转换成vector
dt[, c("score1", "score2") := lapply(.SD, function(x) as.vector(scale(x))), by = session]
dt
# session score1 score2
# 1: 1 -0.7433155 -0.6859943
# 2: 1 -1.0530303 -1.0289917
# 3: 1 -0.2787433 -0.3429970
# 4: 1 0.8052585 0.6859944
# 5: 1 1.2698307 1.3719886
# 6: 2 -0.7847341 -0.6824535
# 7: 2 -0.2942753 -0.3650335
# 8: 2 -0.9949307 -0.9205191
# 9: 2 0.7567078 0.4285175
#10: 2 1.3172322 1.5394886
为了更好地理解它,请在简单的矢量上进行尝试
scale(1:10)
# [,1]
# [1,] -1.4863011
# [2,] -1.1560120
# [3,] -0.8257228
# [4,] -0.4954337
# [5,] -0.1651446
# [6,] 0.1651446
# [7,] 0.4954337
# [8,] 0.8257228
# [9,] 1.1560120
#[10,] 1.4863011
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我来说两句