使用数据表同时重命名多个因子

Question

我喜欢数据表并使用它来有条件地重命名或添加因子.但是,我似乎不能一次做多个因素.这是一个例子:

a <- rep(c("A", "B", "C", "D"), each=3)
b <- 1:12
df <- data.frame(a,b)
DT <- data.table(df)

现在添加新列"New",对于列"a"中的所有"A"等于"z"

DT[a=="A", New:="z"]

这很好用.现在,如果我想改变说"A"和"C"等于"z":

DT[a==c("A", "C"), New:="z"]

给我有趣的答案:

dput(DT)
structure(list(a = structure(c(1L, 1L, 1L, 2L, 2L, 2L, 3L, 3L, 
3L, 4L, 4L, 4L), .Label = c("A", "B", "C", "D"), class = "factor"), 
    b = 1:12, New = c("z", NA, "z", NA, NA, NA, NA, "z", NA, 
    NA, NA, NA)), .Names = c("a", "b", "New"), row.names = c(NA, 
-12L), class = c("data.table", "data.frame"), .internal.selfref = <pointer: 0x0000000000140788>, index = structure(integer(0), a = integer(0)))

我确信它很简单,我似乎无法在SO上找到它(排队!).谢谢

为了确认,我想要的输出是:

dput(DT)
structure(list(a = structure(c(1L, 1L, 1L, 2L, 2L, 2L, 3L, 3L, 
3L, 4L, 4L, 4L), .Label = c("A", "B", "C", "D"), class = "factor"), 
    b = 1:12, New = c("z", "z", "z", NA, NA, NA, "z", "z", "z", 
    NA, NA, NA)), .Names = c("a", "b", "New"), row.names = c(NA, 
-12L), class = c("data.table", "data.frame"), .internal.selfref = <pointer: 0x0000000000140788>, index = structure(integer(0), a = integer(0)))

Answer 1

您应该使用%in%而不是==,因此您将需要:

DT[a %in% c("A", "C"), New:="z"]

这使:

> DT
    a  b New
 1: A  1   z
 2: A  2   z
 3: A  3   z
 4: B  4  NA
 5: B  5  NA
 6: B  6  NA
 7: C  7   z
 8: C  8   z
 9: C  9   z
10: D 10  NA
11: D 11  NA
12: D 12  NA

使用数据:

a <- rep(c("A", "B", "C", "D"), each=3)
b <- 1:12
DT <- data.table(a,b)

在数据框中,您可以执行以下操作:

df <- data.frame(a,b)
df$New <- NA
df[df$a %in% c("A", "C"), "New"] <- "z"

达到同样的效果.