有条件重新启动的行的增量计数 [英] Increment count over rows with conditional restarting

问题描述

当要满足现有列中的条件时，我想增加一个从1开始的计数。

I would like to increment a count that restarts from 1 when a condition in an existing column is met.

例如，我有以下数据框：

For example I have the following data frame:

df <- data.frame(x1 = c(10, 100, 200, 300, 87, 90, 45, 80), 
                 x2 = c("start", "a", "b", "c", "start", "k", "l", "o"))

我想创建 x3 ，每次 x2 ==开始 。

结果数据框应如下所示：

The resulting data frame should look like this:

   x1    x2 x3
1  10 start  1
2 100     a  2
3 200     b  3
4 300     c  4
5  87 start  1
6  90     k  2
7  45     l  3
8  80     o  4

我猜有R中提供了一般解决方案的现有功能。有人能指出我正确的方向吗？

I'm guessing there are existing functions in R that give a general solution. Can anyone point me in the right direction?

推荐答案

使用基数R：

df$x3 <- with(df, ave(x1, cumsum(x2 == 'start'), FUN = seq_along))

给予：

> df
   x1    x2 x3
1  10 start  1
2 100     a  2
3 200     b  3
4 300     c  4
5  87 start  1
6  90     k  2
7  45     l  3
8  80     o  4

---

或使用 dplyr 或 data.table 软件包：

library(dplyr)
df %>% 
  group_by(grp = cumsum(x2 == 'start')) %>% 
  mutate(x3 = row_number())

library(data.table)
# option 1
setDT(df)[, x3 := rowid(cumsum(x2 == 'start'))][]
# option 2
setDT(df)[, x3 := 1:.N, by = cumsum(x2 == 'start')][]

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