如何重置循环?

大家好我有这个数据集

library(dplyr)
library(tidyr)
input<- frame_data(
  ~member_id,   ~fill_date   ,  ~drug, ~days_supply, 
  "603",       "02/17/2005",    "a",    30,         
  "603",       "06/13/2005",    "a",    30,       
  "603",       "08/11/2005",    "a",    30,           
  "603",       "06/12/2006",    "b",    15,
  "603",       "05/09/2006",    "b",    30
  )

我试图创建一个名为“time”的变量,它指示变量“drug”出现的时间。所以输出应该如下所示

output<- frame_data(
  ~member_id,   ~fill_date   ,  ~drug, ~days_supply, ~time,
  "603",       "02/17/2005",    "a",    30,           1,
  "603",       "06/13/2005",    "a",    30,           2,
  "603",       "08/11/2005",    "a",    30,           3,
  "603",       "06/12/2006",    "b",    15,           1,
  "603",       "05/09/2006",    "b",    30            2
  )

换句话说,我在寻找一种循环,它可以在每次“drug”变量更改时重置 我试过这个密码

time<-1
i<-2
j<-1
while (i <=nrow(input)){
  if (input[i,3,drop=]==input[i-1,3,drop=]){
    j<-i
    time<-c(time,j)
  }else{
    j<-1
    time<-c(time,j)
  }
  i<-i+1
}

但它当然不起作用,因为无法重置i,因为它在同一时间验证条件时指示行索引。

谢谢你的帮助