R 按条件选择行并排序（包括重复项）

我试图为我的问题创建一个小例子。第一列是不同的日期，第二列降级发生在这些特定日期。每次降级我需要过滤3行：降级前一天、降级发生的当天和降级后一天

df <- data.frame(date = Sys.Date() - 19:0, dgrd = NA)
df$dgrd[c(4, 10, 11, 16)] <- "X" #small dataframe inclduing problematic downgrades

down <- which(!is.na(df$dgrd)) #select all days where downgrade occured
keep <- df[c(down-1, down, down+1), ] #select the specific days for each downgrade

print(keep)

date       dgrd
2017-09-26 <NA>
2017-10-02 <NA>
2017-10-03    X
2017-10-08 <NA>
2017-09-27    X
2017-10-03    X
2017-10-04    X
2017-10-09    X
2017-09-28 <NA>
2017-10-04    X
2017-10-05 <NA>
2017-10-10 <NA>

---

df您可能希望更改创建索引的方式，如下所示

down <- which(!is.na(df$dgrd))
index <- unlist(lapply(down, function(x) c(x - 1, x, x + 1)))
keep <- df[index, ]
keep
           date dgrd
3    2017-09-26 <NA>
4    2017-09-27    X
5    2017-09-28 <NA>
9    2017-10-02 <NA>
10   2017-10-03    X
11   2017-10-04    X
10.1 2017-10-03    X
11.1 2017-10-04    X
12   2017-10-05 <NA>
15   2017-10-08 <NA>
16   2017-10-09    X
17   2017-10-10 <NA>

down
down <- which(!is.na(df$dgrd))
index <- unlist(lapply(down, function(x) c(x - 1, x, x + 1)))
keep <- df[index, ]
keep
           date dgrd
3    2017-09-26 <NA>
4    2017-09-27    X
5    2017-09-28 <NA>
9    2017-10-02 <NA>
10   2017-10-03    X
11   2017-10-04    X
10.1 2017-10-03    X
11.1 2017-10-04    X
12   2017-10-05 <NA>
15   2017-10-08 <NA>
16   2017-10-09    X
17   2017-10-10 <NA>