Extract n rows after string in R - r

Question

I Would like to extract the next 'n' rows after I find a string in R.
For example, let's say I have the following data frame:
df<-as.data.frame(rep(c("a","b","c","d","e","f"),10))
I would like to extract every row that includes "b", as well as the next two rows (in this example, I would like to extract rows with "b", or "c", or "d")
BUT, please, I don't want to specify "c" and "d", I just want the next two rows after "b" as well (in my real data the next two rows are not consistent).
I've tried many things, but no success.. Thanks in advance! Nick

Answer 1

You can find the indices of rows with b and then use those and the next two of each, something like this:
df <- data.frame(col1=rep(c("a","b","c","d","e","f"),3), col2=letters[1:18], stringsAsFactors = FALSE)
df
col1 col2
1 a a
2 b b
3 c c
4 d d
5 e e
6 f f
7 a g
8 b h
9 c i
10 d j
11 e k
12 f l
13 a m
14 b n
15 c o
16 d p
17 e q
18 f r
bs <- which(df$col1=="b")
df[sort(bs+rep(0:2, each=length(bs)),] #2 is the number of rows you want after your desired match (b).
col1 col2
2 b b
3 c c
4 d d
8 b h
9 c i
10 d j
14 b n
15 c o
16 d p
I added a second column to illustrate the dataframe better, otherwise a vector would be returned.

Answer 2

My "SOfun" package has a function called getMyRows which does what you ask for, with the exception of returning a list instead of a data.frame.
I had left the result as a list to make it easier to handle some edge cases, like where the requests for rows would overlap. For example, in the following sample data, there are two consecutive "b" values. There's also a "b" value in the final row.
df <- data.frame(col1 = c("a", "b", "b",
rep(c("a", "b", "c", "d", "e", "f"), 3), "b"),
col2 = letters[1:22])
library(SOfun)
getMyRows(df, which(df$col1 == "b"), 0:2, TRUE)
# [[1]]
# col1 col2
# 2 b b
# 3 b c
# 4 a d
#
# [[2]]
# col1 col2
# 3 b c
# 4 a d
# 5 b e
#
# [[3]]
# col1 col2
# 5 b e
# 6 c f
# 7 d g
#
# [[4]]
# col1 col2
# 11 b k
# 12 c l
# 13 d m
#
# [[5]]
# col1 col2
# 17 b q
# 18 c r
# 19 d s
#
# [[6]]
# col1 col2
# 22 b v
The usage is essentially:
Specify the data.frame.
Specify the index positions to use as the base. Here, we want all rows where "col1" equals "b" to be our base index position.
Specify the range of rows interested in. -1:3, for example, would give you one row before to three rows after the base.
TRUE means that you are specifying the starting points by their numeric indices.