为数据帧的每个组中的行创建序列号（计数器） [英] Create a sequential number (counter) for rows within each group of a dataframe

问题描述

如何在数据框架的每个组中生成唯一的ID号？以下是按personid分组的一些数据：

How can we generate unique id numbers within each group of a dataframe? Here's some data grouped by "personid":

personid date measurement
1         x     23
1         x     32
2         y     21
3         x     23
3         z     23
3         y     23

我希望为personid定义的每个子集中的每一行添加一个具有唯一值的id列，始终以 1 开头。这是我的预期输出：

I wish to add an id column with a unique value for each row within each subset defined by "personid", always starting with 1. This is my desired output:

personid date measurement id
1         x     23         1
1         x     32         2
2         y     21         1
3         x     23         1
3         z     23         2
3         y     23         3

推荐答案

错误地命名为 ave（）函数，参数 FUN = seq_along 会很好地完成 - 即使你的 personid 列不严格排序。

The misleadingly named ave() function, with argument FUN=seq_along, will accomplish this nicely -- even if your personid column is not strictly ordered.

df <- read.table(text = "personid date measurement
1         x     23
1         x     32
2         y     21
3         x     23
3         z     23
3         y     23", header=TRUE)

## First with your data.frame
ave(df$personid, df$personid, FUN=seq_along)
# [1] 1 2 1 1 2 3

## Then with another, in which personid is *not* in order
df2 <- df[c(2:6, 1),]
ave(df2$personid, df2$personid, FUN=seq_along)
# [1] 1 1 1 2 3 2

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