通过引用对R中的向量进行子分配 [英] Sub-assign by reference on vector in R
问题描述
我可以通过引用对子原子向量进行子分配吗?
当然不能在1列data.table中使用:=
。
Can I use sub-assign by reference on atomic vectors somehow?
Of course without wrapping it in 1 column data.table to use :=
.
library(data.table)
N <- 5e7
x <- sample(letters, N, TRUE)
X <- data.table(x = x)
upd_i <- sample(N, 1L, FALSE)
system.time(x[upd_i] <- NA_character_)
# user system elapsed
# 0.11 0.06 0.17
system.time(X[upd_i, x := NA_character_])
# user system elapsed
# 0.00 0.00 0.03
R6可以帮助我开放R6解决方案,因为它是我的一个已经。
我已经检查了< -
R6
对象仍会复制: gist 。
If R6 can help on that I'm open for R6 solution as it is one of my dep already.
I've already checked that <-
inside R6
object still makes copy: gist.
推荐答案
在最新的R版本(3.1-3.1.2 +左右)中,向量的赋值不会被复制。你不会看到通过运行OP的代码,而原因是以下。因为你重用 x
并将其分配给某个其他对象,R不会通知在该点复制了 x
并且必须假设它不会(在上面的特定情况下,我认为它会是好的改变在 data.table :: data.table
和通知R已经进行了复制,但这是一个单独的问题 - data.frame
遭受同样的问题),因为它复制 x
第一次使用。如果您稍微改变命令的顺序,您会看到没有区别:
In most recent R versions (3.1-3.1.2+ or so), assignment to a vector does not copy. You will not see that by running OP's code though, and the reason for that is the following. Because you reuse x
and assign it to some other object, R is not notified that x
is copied at that point, and has to assume that it won't be (in the particular case above, I think it'll be good to change it in data.table::data.table
and notify R that a copy has been made, but that's a separate issue - data.frame
suffers from same issue), and because of that it copies x
on first use. If you change the order of the commands a bit, you'd see no difference:
N <- 5e7
x <- sample(letters, N, TRUE)
upd_i <- sample(N, 1L, FALSE)
# no copy here:
system.time(x[upd_i] <- NA_character_)
# user system elapsed
# 0 0 0
X <- data.table(x = x)
system.time(X[upd_i, x := NA_character_])
# user system elapsed
# 0 0 0
# but now R will copy:
system.time(x[upd_i] <- NA_character_)
# user system elapsed
# 0.28 0.08 0.36
(旧答案,大多留作为好奇)
(old answer, mostly left as a curiosity)
你实际上可以使用 data.table
:=
运算符修改你的矢量到位(我认为你需要R版本3.1+避免在列表
):
You actually can use the data.table
:=
operator to modify your vector in place (I think you need R version 3.1+ to avoid the copy in list
):
modify.vector = function (v, idx, value) setDT(list(v))[idx, V1 := value]
v = 1:5
address(v)
#[1] "000000002CC7AC48"
modify.vector(v, 4, 10)
v
#[1] 1 2 3 10 5
address(v)
#[1] "000000002CC7AC48"
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