使用data.table进行聚合 [英] Using data.table to aggregate

问题描述

在SO用户的多个建议后，我终于试图将我的代码转换为使用 data.table 。

After multiple suggestions from SO users, I am finally trying to convert my code over to using data.table.

library(data.table)
DT <- data.table(plate = paste0("plate",rep(1:2,each=5)),
             id = rep(c("CTRL","CTRL","ID1","ID2","ID3"),2),
             val = 1:10)

> DT
    plate   id val
1: plate1 CTRL   1
2: plate1 CTRL   2
3: plate1  ID1   3
4: plate1  ID2   4
5: plate1  ID3   5
6: plate2 CTRL   6
7: plate2 CTRL   7
8: plate2  ID1   8
9: plate2  ID2   9
10: plate2  ID3  10

我想要做的是取平均 DT [，val] / code>，当ID为CTRL时。

What I would like to do is take the average of DT[,val] by plate when the id is "CTRL".

我通常会聚合数据框，然后使用 match 将值映射回新列ctrl。

I would normally aggregate the data frame, then use match to map the values back to a new column, 'ctrl'.

使用 data.table 包可以得到：

DT[id=="CTRL",ctrl:=mean(val),by=plate]

> DT
    plate   id val ctrl
1: plate1 CTRL   1  1.5
2: plate1 CTRL   2  1.5
3: plate1  ID1   3   NA
4: plate1  ID2   4   NA
5: plate1  ID3   5   NA
6: plate2 CTRL   6  6.5
7: plate2 CTRL   7  6.5
8: plate2  ID1   8   NA
9: plate2  ID2   9   NA
10: plate2  ID3  10   NA

我需要的是：

DT <- data.table(plate = paste0("plate",rep(1:2,each=5)),
                 id = rep(c("CTRL","CTRL","ID1","ID2","ID3"),2),
                 val = 1:10,
                 ctrl = rep(c(1.5,6.5),each=5))

> DT
    plate   id val ctrl
1: plate1 CTRL   1  1.5
2: plate1 CTRL   2  1.5
3: plate1  ID1   3  1.5
4: plate1  ID2   4  1.5
5: plate1  ID3   5  1.5
6: plate2 CTRL   6  6.5
7: plate2 CTRL   7  6.5
8: plate2  ID1   8  6.5
9: plate2  ID2   9  6.5
10: plate2  ID3  10  6.5

最后，我想使用更复杂的值，但是我不知道如何选择特定的值，运行一些函数，然后使用数据框将这些值映射回相应的行。

Eventually I would like to use much more complicated selections of the values, but I do not know how to select specific values, run some function, then map those values back to the appropriate row using data frames.

推荐答案

这是你想要做的：

This is what you want to do:

DT[,ctrl:=mean(val[id=="CTRL"]),by=plate]

b $ b

     plate   id val ctrl
 1: plate1 CTRL   1  1.5
 2: plate1 CTRL   2  1.5
 3: plate1  ID1   3  1.5
 4: plate1  ID2   4  1.5
 5: plate1  ID3   5  1.5
 6: plate2 CTRL   6  6.5
 7: plate2 CTRL   7  6.5
 8: plate2  ID1   8  6.5
 9: plate2  ID2   9  6.5
10: plate2  ID3  10  6.5

代码 DT [id ==CTRL，ctrl：= mean（val），by = plate] 未对 id ==CTRL不是真的，因为当你使用 []的第一个参数时，第二个参数中的操作只对子集化的 data.table 进行。

Your original code DT[id=="CTRL",ctrl:=mean(val),by=plate] did not make an assignment for rows where id=="CTRL" was not true because, when you use the first argument of [, you are subsetting; the operations in the second argument are only done for the subsetted data.table.

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