如何在多列数据表上滚动 [英] How to rollapply over a multi column data table

问题描述

我想在多列数据表上使用rollapply函数，也就是说，我希望能够独立使用每一列，例如让我们考虑以下datable:

I would like to use the rollapply function over a multi column datatable, namely I would like to be able to use each column independantly for instance let's consider the following datable :

> DT = data.table(x=rep(c("a","b","c"),each=3), y=c(1,3,6), v=1:9)
> DT
   x y v
1: a 1 1
2: a 3 2
3: a 6 3
4: b 1 4
5: b 3 5
6: b 6 6
7: c 1 7
8: c 3 8
9: c 6 9

然后，我想使用rollapply作为滚动子集，以便计算第2列和第3列的3个元素的滚动平均值并将其存储到外部变量中:

Then I would like to use rollapply as a rolling subset in order to work out the rolling mean over 3 element of columns 2 and 3 and store them into external variables :

> r1= NA; r2 = NA
> ft=function(x) { r1=mean(x[,2,with=FALSE]) ; r2=mean(x[,3,with=FALSE]) }
> rollapply(DT, width=3, ft)
 Error in x[, 2, with = FALSE] : incorrect number of dimensions 

除了我得到的这个不方便的错误之外，为什么它不起作用?

Except I got this error which isn't handy, why isn't it working ?

输出为:

> r1
[1] 3.333333 3.333333 3.333333 3.333333 3.333333 3.333333 3.333333
> r2
[1] 2 3 4 5 6 7 8

推荐答案

您快到了并且可以:

lapply(DT[,2:3], function(x) rollapply(x,width=3, FUN=mean))
#$y
#[1] 3.333333 3.333333 3.333333 3.333333 3.333333 3.333333 3.333333

#$v
#[1] 2 3 4 5 6 7 8

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