在R中按组添加索引（或计数器）到数据帧

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问题描述

我有一个df喜欢

ProjectID Dist
  1        x
  1        y
  2        z
  2        x
  2        h
  3        k
  ....     ....

我想添加第三列，以便每个ProjectID都有一个递增计数器：

I want to add a third column such that we have an incrementing counter for each ProjectID:

ProjectID Dist counter
  1        x     1
  1        y     2
  2        z     1
  2        x     2
  2        h     3
  1        k     3
  ....     ....

我看过 seq rank 以及其他几项，特别是要查看我是否可以使用 ddply 帮助：

I've had a look at seq rank and a couple of other bits particularly looking to see if I could use ddply to help:

df$counter <- ddply(df,.(projectID), function(x).....? )

我想我可以调整这个答案但更喜欢使用类似ddply的东西（我找不到相当于的cumsum但我认为这里的原理相同：）。这让我在列表中对事件进行索引（例如合并）。

I think I could adapt this answer How to create a counter/numeration by group? but would prefer something using something like ddply (I can't find an equivalent of cumsum but I think that's the same principle here: Create ascending series of integers by group in Pandas ). That'd let me index occurrences in a list (and e.g. merge on this).

推荐答案

A dplyr 解决方案非常简单：

A dplyr solution is quite simple:

library(dplyr)

df %>% group_by(ProjectID) %>% mutate(counter = row_number(ProjectID))


#  ProjectID Dist counter
#1         1    x       1
#2         1    y       2
#3         2    z       1
#4         2    x       2
#5         2    h       3
#6         1    k       3

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projectId