问题描述
请问有人给我解释 .I 的正确用法，用于返回数据表的行号吗？
Would someone please explain to me the correct usage of .I for returning the row numbers of a data.table?
我有这样的数据：
require(data.table)
DT <- data.table(X=c(5, 15, 20, 25, 30))
DT
# X
# 1: 5
# 2: 15
# 3: 20
# 4: 25
# 5: 30
我想返回行索引的向量，其中 i 中的条件为 TRUE ，例如哪些行的 X 大于20。
I want to return a vector of row indices where a condition in i is TRUE, e.g. which rows have an X greater than 20.
DT[X > 20]
# rows 4 & 5 are greater than 20
要获取索引，我尝试过：
To get the indices, I tried:
DT[X > 20, .I]
# [1] 1 2
...但显然我这样做是错误的，因为这只会返回一个包含1的向量（返回的行数）。 （我以为 .N 到底是干什么的？）。
...but clearly I am doing it wrong, because that simply returns a vector containing 1 to the number of returned rows. (Which I thought was pretty much what .N was for?).
对不起，如果这看起来非常基础，但是我只能在data.table文档中找到 .I 和 .N 的含义，而不是如何使用它们。
Sorry if this seems extremely basic, but all I have been able to find in the data.table documentation is WHAT .I and .N do, not HOW to use them.
推荐答案
如果只需要行号而不是行本身，则使用哪个= TRUE ，不是 。我。
If all you want is the row numbers rather than the rows themselves, then use which = TRUE, not .I.
DT[X > 20, which = TRUE]
# [1] 4 5
优化 i 的好处，例如快速联接或使用自动索引。 哪个=真使它只返回行号就早返回。
That way you get the benefits of optimization of i, for example fast joins or using an automatic index. The which = TRUE makes it return early with just the row numbers.
这是<$的手动输入c $ c> which data.table中的参数：
Here's the manual entry for the which argument inside data.table :
解释：
注意存在特定关系在 .I 和 DT [i = ..， j = ..，by = ..]
即， .I 是子表的行号的向量。
Explanation:
Notice there is a specific relationship between .I and the i = .. argument in DT[i = .., j = .., by = ..]Namely, .I is a vector of row numbers of the subsetted table.
### Lets create some sample data
set.seed(1)
LL <- sample(LETTERS[1:5], 20, TRUE)
DT <- data.table(X=LL)
看一下将整个表设置为子表与只设置的子集之间的区别。I
look at the difference between subsetting the whole table, and subsetting just .I
DT[X == "B", .I]
# [1] 1 2 3 4 5 6
DT[ , .I[X == "B"] ]
# [1] 1 2 5 11 14 19
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