dplyr mutuate仅在不同的列==特定值时添加新列值

我有一个具有以下结构的数据框（摘要示例，不是实际的）

dput(df1)
structure(list(MedID = c(111, 111, 111, 111, 111, 111, 222, 222, 
222, 222, 222), Service = structure(c(1L, 1L, 2L, 1L, 1L, 3L, 
3L, 2L, 1L, 1L, 3L), .Label = c("Acute care", "Ext care", "Outpt 
care"), class = "factor"), AdmitDate = structure(c(16832, 16861, 
16892, 16922, 16953, 16983, 17181, 17212, 17240, 17271, 17301), class 
= "Date"), Flag = c(0, 0, 99, 0, 0, 0, 0, 99, 0, 0, 0)), .Names = 
c("MedID", "Service", "AdmitDate", "Flag"), row.names = c(NA, -11L), 
class = "data.frame")
> df1
    MedID  Service   AdmitDate  Flag
1    111 Acute care 2016-02-01    0
2    111 Acute care 2016-03-01    0
3    111   Ext care 2016-04-01   99
4    111 Acute care 2016-05-01    0
5    111 Acute care 2016-06-01    0
6    111 Outpt care 2016-07-01    0
7    222 Outpt care 2017-01-15    0
8    222   Ext care 2017-02-15   99
9    222 Acute care 2017-03-15    0
10   222 Acute care 2017-04-15    0
11   222 Outpt care 2017-05-15    0

我希望使用dplyr，group_by（MedID）和mutate在新数据帧中添加一个列（让我们称之为df2中的Flag2），这样在每个患者（MedID）中df2 $ Flag2列== 1，用于其中的每个后续行唯一的MedID，但只有在df1 $ Flag列== 99之后，否则df2 $ Flag2列得到0.如果在MedID的第一行中df1 $ Flag == 99，我可以根据需要编码，但否则我的代码要么仅在df1 $ Flag == 99的行中在df2 $ Flag2中生成1，或者对于给定MedID中的所有行生成1，其中df1 $ Flag == 99.所需的输出为：

dput(df2)
structure(list(MedID = c(111, 111, 111, 111, 111, 111, 222, 222, 
222, 222, 222), Service = structure(c(1L, 1L, 2L, 1L, 1L, 3L, 
3L, 2L, 1L, 1L, 3L), .Label = c("Acute care", "Ext care", "Outpt 
care"), class = "factor"), AdmitDate = structure(c(16832, 16861, 
16892,16922, 16953, 16983, 17181, 17212, 17240, 17271, 17301), class 
= "Date"),Flag = c(0, 0, 99, 0, 0, 0, 0, 99, 0, 0, 0), Flag2 = c(0, 
0, 1, 1, 1, 1, 0, 1, 1, 1, 1)), .Names = c("MedID", "Service", 
"AdmitDate", "Flag", "Flag2"), row.names = c(NA, -11L), class = 
"data.frame")
> df2
    MedID    Service  AdmitDate Flag Flag2
1    111 Acute care 2016-02-01    0     0
2    111 Acute care 2016-03-01    0     0
3    111   Ext care 2016-04-01   99     1
4    111 Acute care 2016-05-01    0     1
5    111 Acute care 2016-06-01    0     1
6    111 Outpt care 2016-07-01    0     1
7    222 Outpt care 2017-01-15    0     0
8    222   Ext care 2017-02-15   99     1
9    222 Acute care 2017-03-15    0     1
10   222 Acute care 2017-04-15    0     1
11   222 Outpt care 2017-05-15    0     1

这是代码的一个片段示例，但由于它没有正确执行而不完整...我是否需要将mutate嵌套在For循环中，这看起来像是混合的R编码？ :(注意：df1 $ Flag每个MedID只能== 99次，我认为应该会更容易 .

`df2 <- df1 %>% `
    `group_by(MedID) %>%`
    `mutate(Flag2 = ifelse(df1$Flag == 99, 1, 0))`