使用上一行的值创建具有布尔条件的计数器变量 [英] create counter variable with Boolean condition using value from the previous row

问题描述

我想根据组 变量用户和 True或False 创建计数器变量c strong>变量B 。

I want to create a counter variable c based on the group variable user and True or False variable B.

DT <- data.table(time=c(1,2,3,1,1,2,3,1,1,1),user=c(1,1,1,2,3,3,3,4,4,5), B=c('t','f','t','f','f','t','t','t','t','t'))
DT

变量c的所需输出 $

    time user B C
 1:    1    1 t 1
 2:    2    1 f 1
 3:    3    1 t 2
 4:    1    2 f 0
 5:    1    3 f 0
 6:    2    3 t 1
 7:    3    3 t 2
 8:    1    4 t 1 
 9:    2    4 t 2
10:    1    5 t 1

变量c是B组中的计数器。 变量c 的逻辑（NOT代码）如下。

variable c is a counter within the group when B is true. The logic (NOT code) of variable c is as follow. The sequence do matter as you can see from the time variable.

 if time=1 and b=='f' {c=0}
    else 
    {
      if b=='t'{c=previous[c]+1} 
      else {c=previous[c]}
    }


  #if there is no variable b, the counter can be created using dplyr:     
          group_by(user)%>%mutate(c=seq_along(user))
  #or data.table
         DT[, c := seq_len(.N), by = user]
  # we can use data.table function shift() combined with for loop but i want to avoid for loop, it is slow and I have 300,000 rows.

推荐答案

我们按'user'， cumsum 逻辑向量（ B ==t）和assign（：=

We group by 'user', cumsum the logical vector (B=="t") and assign (:= ) the output to 'C'.

DT[, C:= cumsum(B=="t"), by = user]
DT
#    time user B C
# 1:    1    1 t 1
# 2:    2    1 f 1
# 3:    3    1 t 2
# 4:    1    2 f 0
# 5:    1    3 f 0
# 6:    2    3 t 1
# 7:    3    3 t 2
# 8:    1    4 t 1
# 9:    2    4 t 2
#10:    1    5 t 1

b $ b

---

同样的逻辑可以应用于 dplyr 方法

library(dplyr)
DT %>%
   group_by(user) %>%
   mutate(C = cumsum(B == "t"))

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