R data.table 包 - 使用 := 运算符在列中添加值

Question

问题

我有一个 data.frame，并希望根据其他列中的数据将数据放在一列中。

所以这是我的 data.frame 的一个例子（缩短版）：

Fertilization=c("N0","N0","N0","N0","N2","N2","N2","N2")
Sowing=c("S1","S1","S2","S2","S1","S1","S2","S2")
FoliarRank=c("F2","F3","F2","F3","F2","F3","F2","F3")
New_FoliarRank=rep(0,length(Fertilization))
DT=data.frame(Fertilization,Sowing,FoliarRank,New_FoliarRank)

我想根据施肥、播种、FoliarRank 列中的条件为 New_FoliarRank 赋值。 例如：

• 如果施肥=="N0"、播种=="S1" 和 FoliarRank=="F2"，则 New_FoliarRank=="F3*"
• 如果施肥=="N0"、播种=="S1" 和 FoliarRank=="F3"，则 New_FoliarRank=="F2*"

至于解决方案：

• 我可以让它与一堆 for/if 一起工作，但它会很慢而且不会 非常“R-ish”，也许即使我“应用”它
• 据我了解，我可以使用 := 运算符 {data.table} 包。它可能会好得多。其实已经 在 Stack Overflow "Replace a numerical value by NA based on conditions from other columns" 的其他地方讨论过，但我找不到让这篇文章的解决方案发挥作用的方法。而且我不明白为什么，即使看着 ?":=".我遗漏了一些东西，也许很明显，所以我想我可以问一下。抱歉重复了。

我尝试过的一些解决方案：

library(data.table)
DT[Fertilization=="N0" & Sowing=="S1" & FoliarRank=="F2", New_FoliarRank:="F3*"] # seems to be same script as other post
DT[ , New_FoliarRank:= {Fertilization=="N0" & Sowing=="S1" & FoliarRank=="F2"; "F3*"}] # adapted from another post; doesn't work either

它给了我回报：

Error in `:=`(New_FoliarRank, "F3*") : 
Check that is.data.table(DT) == TRUE. Otherwise, := and `:=`(...) are defined for use in j, once only and in particular ways. See help(":=").

建议的解决方案（下面发布了另一个解决方案）

# Initial vectors (no need for New_FoliarRank)
Fertilization=c("N0","N0","N0","N0","N2","N2","N2","N2")
Sowing=c("S1","S1","S2","S2","S1","S1","S2","S2")
FoliarRank=c("F2","F3","F2","F3","F2","F3","F2","F3")

# Actually I was missing the class of DT (data.table instead of data.frame)
DT=data.table(Fertilization,Sowing,FoliarRank)

library(data.table)
# And I shouldn't have created New_FoliarRank (esp. in with numerical values), as it is created "on the spot"
setDT(DT)[Fertilization=="N0" & Sowing=="S1" & FoliarRank=="F2", New_FoliarRank := "F3*"]
setDT(DT)[Fertilization=="N0" & Sowing=="S1" & FoliarRank=="F3", New_FoliarRank := "F2*"]

Answer 1

你可以使用因子：

library(data.table)
setDT(DT)
DT[, New_FoliarRank := interaction(Fertilization, Sowing, FoliarRank)]
#check levels
levels(DT[, New_FoliarRank])

#assign new labels
DT[, New_FoliarRank := factor(New_FoliarRank, 
                              levels = levels(New_FoliarRank),
                              labels = c("012", "212", "022", "222", "013", "213", "023", "223"))]

#   Fertilization Sowing FoliarRank New_FoliarRank
#1:            N0     S1         F2            012
#2:            N0     S1         F3            013
#3:            N0     S2         F2            022
#4:            N0     S2         F3            023
#5:            N2     S1         F2            212
#6:            N2     S1         F3            213
#7:            N2     S2         F2            222
#8:            N2     S2         F3            223