我有一个数据框架，如下所示：
df <- data.frame("id" = c(111,111,111,222,222,222,222,333,333,333,333),
"Location" = c("A","B","A","A","C","B","A","B","A","A","A"),
"Encounter" = c(1,2,3,1,2,3,4,1,2,3,4))
id Location Encounter
1 111 A 1
2 111 B 2
3 111 A 3
4 222 A 1
5 222 C 2
6 222 B 3
7 222 A 4
8 333 B 1
9 333 A 2
10 333 B 3
11 333 A 4
我基本上是想为每个id组创建一个二进制标志，该标志位于先前的Encounter中。因此，它看起来像：
id Location Encounter Flag
1 111 A 1 0
2 111 B 2 0
3 111 A 3 1
4 …