分组观察数 [英] Observation number by group

问题描述

在 R 中，我有一个数据框，其中的观测值由几个值描述，其中一个值是一个因子.我已按此因素对数据集进行排序，并想添加一列，在该列中我将获得对因素的每个级别的多个观察结果，例如

In R I have a data frame with observations described by several values one of which is a factor. I have sorted the dataset by this factor and would like to add a column in which I would get a number of observation on each level of the factor e.g.

factor   obsnum
a        1
a        2
a        3
b        1
b        2
b        3
b        4
c        1
c        2
...

在 SAS 中，我这样做:

In SAS I do it with something like:

data logs.full;
    set logs.full;
    count + 1;
    by cookie;
    if first.cookie then count = 1;
run;

我如何在 R 中实现这一点?

How can I achieve that in R?

谢谢，

推荐答案

使用rle(游程编码)和sequence:

x <- c("a", "a", "a", "b", "b", "b", "b", "c", "c")

data.frame(
    x=x,
    obsnum = sequence(rle(x)$lengths) 
)

  x obsnum
1 a      1
2 a      2
3 a      3
4 b      1
5 b      2
6 b      3
7 b      4
8 c      1
9 c      2

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