如何在每个组中随机选择仅一行 [英] How to randomly choose only one row in each group

问题描述

说我有一个数据帧，如下所示：

Say I have a dataframe as follows:

df <- data.frame(Region = c("A","A","A","B","B","C","D","D","D","D"),
                          Combo = c(1,2,3,1,2,1,1,2,3,4))
> df
   Region Combo
1       A     1
2       A     2
3       A     3
4       B     1
5       B     2
6       C     1
7       D     1
8       D     2
9       D     3
10      D     4

我想为每个地区（A，B，C，D）随机选择一个可能的组合

What I would like to do, is for each Region (A,B,C,D) randomly choose only one of the possible combos for that region.

如果所选组合用二进制变量表示，则可能看起来像这样：

If the chosen combination were indicated by a binary variable, it would look something potentially like this:

   Region Combo RandomlyChosen
1       A     1              1
2       A     2              0
3       A     3              0
4       B     1              0
5       B     2              1
6       C     1              1
7       D     1              0
8       D     2              0
9       D     3              1
10      D     4              0

我知道示例函数，但只是不知道如何在每个区域内只选择一个组合。

I'm aware of the sample function, but just don't know how to choose only one combo within each region.

我经常使用data.table，因此任何欢迎使用该解决方案。尽管同样欢迎不使用data.table的解决方案。

I reglarly use data.table, so any solutions using that are welcome. Though solutions not using data.table are equally welcome.

谢谢！

推荐答案

在普通R语言中，您可以在 tapply（）中使用 sample（）：

In plain R you can use sample() within tapply():

df$Chosen <- 0
df[-tapply(-seq_along(df$Region),df$Region, sample, size=1),]$Chosen <- 1
df
   Region Combo Chosen
1       A     1      0
2       A     2      1
3       A     3      0
4       B     1      1
5       B     2      0
6       C     1      1
7       D     1      0
8       D     2      0
9       D     3      1
10      D     4      0

请注意-（-selected_row_number）避免当一组有单个行号时避免从1采样到n的技巧

Note the -(-selected_row_number) trick to avoid sampling from 1 to n when there is a single row number for one group

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