按组计算变量列表的总和 [英] Calculate sum of a list of variables by group

问题描述

我有一个data.table有一个键和约100个数字行，其中一个设置为键。

I have a data.table with one key and about 100 numeric rows, one of which is set to key. I would like to create a new variable that contains summation of each numeric rows, grouped by key.

例如，我的数据现在是

ID Count1 Count2 Count3
1   1      3      0
1   3      3      3
2   1      2      1
3   1      1      2

我想要的是：

ID Count1 Count2 Count3
1   4      6      3
2   1      2      1
3   1      1      2

我已经尝试了这么多方法来获得这个。我知道我可以做：

I have tried so many ways to get this. I know I can do:

Y <- X[, list(Count=sum(Count1), Count2=sum(Count2), Count3=sum(Count3), by = ID]

非常感谢您的帮助。

以下是生成测试数据的代码：

Here is a code to generate test data:

ID <-c(rep(210, 9), rep(3917,6))
Count1 <- c(1,1,0,1,3,1,4,1,1,1,1,1,1,0,1)
Count2 <- c(1,0,0,1,0,1,0,1,1,1,1,1,1,0,1)
Count3 <- c(1,0,0,1,0,1,0,1,1,1,1,1,1,0,1)
x <- data.table(ID, Count1, Count2, Count3)
setkey(x, ID)

推荐答案

不匹配您提供的示例，但不管怎样 - 您可以利用 data.table（）有一个名为 .SD的操作符 forsubset of data。This should work：

Your test data doesn't match the example you gave, but regardless - you can take advantage of the fact that data.table() has an operator named .SD for "subset of data. So this should work:

x[, lapply(.SD, sum), by = ID]
#----
     ID Count Count2 Count3
1:  210    13      5      5
2: 3917     5      5      5

这实际上包含在FAQ：type vignette（datatable-faq，package =data .table）或找到它在线。

This is actually covered in the FAQ: type vignette("datatable-faq", package="data.table") or find it online.

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