r-为什么data.table不能从表中生成data.table，而data.frame可以

Question

我有以下数据和代码：

mydf
  grp categ condition value
1   A     X         P     2
2   B     X         P     5
3   A     Y         P     9
4   B     Y         P     6
5   A     X         Q     4
6   B     X         Q     5
7   A     Y         Q     8
8   B     Y         Q     2
> 
> 
mydf = structure(list(grp = structure(c(1L, 2L, 1L, 2L, 1L, 2L, 1L, 
2L), .Label = c("A", "B"), class = "factor"), categ = structure(c(1L, 
1L, 2L, 2L, 1L, 1L, 2L, 2L), .Label = c("X", "Y"), class = "factor"), 
    condition = structure(c(1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L), .Label = c("P", 
    "Q"), class = "factor"), value = c(2, 5, 9, 6, 4, 5, 8, 2
    )), .Names = c("grp", "categ", "condition", "value"), out.attrs = structure(list(
    dim = structure(c(2L, 2L, 2L), .Names = c("grp", "categ", 
    "condition")), dimnames = structure(list(grp = c("grp=A", 
    "grp=B"), categ = c("categ=X", "categ=Y"), condition = c("condition=P", 
    "condition=Q")), .Names = c("grp", "categ", "condition"))), .Names = c("dim", 
"dimnames")), row.names = c(NA, -8L), class = "data.frame")

然而，以下适用于data.frame，但不适用于data.table：

> data.frame(with(mydf, table(grp, categ, condition)))
  grp categ condition Freq
1   A     X         P    1
2   B     X         P    1
3   A     Y         P    1
4   B     Y         P    1
5   A     X         Q    1
6   B     X         Q    1
7   A     Y         Q    1
8   B     Y         Q    1
> 
> data.table(with(mydf, table(grp, categ, condition)))
   V1
1:  1
2:  1
3:  1
4:  1
5:  1
6:  1
7:  1
8:  1
> 

我是在这里犯了一些错误，还是需要更正data.table命令来获取其他变量？这里不太可能有漏洞。有两个变量，它可以正常工作：

> data.table(with(mydf, table(grp, categ)))
   categ grp N
1:     A   X 2
2:     B   X 2
3:     A   Y 2
4:     B   Y 2
> 
> 
> data.frame(with(mydf, table(grp, categ)))
  grp categ Freq
1   A     X    2
2   B     X    2
3   A     Y    2
4   B     Y    2

谢谢你的帮助。

Answer 1

在data.table v1.9.5的提交#1760中修复。关闭#1043。