How to see n rows after a specific row - r

Question

I'm trying to track user actions but I'd like to see what they do AFTER a specific event. How do I get the next n amount of lines?
For example below, I'd like to know what the user is doing after they "Get mushroom" to see if they are eating it. I'd want to reference the "Get mushroom" for each User and see the next few lines after that.
User Action
Bob Enter chapter 1
Bob Attack
Bob Jump
Bob Get mushroom
Bob Open inventory
Bob Eat mushroom
Bob Close inventory
Bob Run
Mary Enter chapter 1
Mary Get mushroom
Mary Attack
Mary Jump
Mary Attack
Mary Open inventory
Mary Close inventory
I'm not sure how to approach this after grouping by users. Expected results would be something like the below if i wanted 3 lines below
User Action
Bob Get mushroom # Action I want to find and the next 3 lines below it
Bob Open inventory
Bob Eat mushroom
Bob Close inventory
Mary Get mushroom # Action I want to find and the next 3 lines below it
Mary Attack
Mary Jump
Mary Attack
Thank you.

Answer 1

Two alternatives with dplyr and data.table:
library(dplyr)
df1 %>%
group_by(User) %>%
slice(rep(which(Action == 'Get-mushroom'), each=4) + 0:3)
library(data.table)
setDT(df1)[df1[, rep(.I[Action == 'Get-mushroom'], each=4) + 0:3, User]$V1]
both result in:
User Action
1: Bob Get-mushroom
2: Bob Open-inventory
3: Bob Eat-mushroom
4: Bob Close-inventory
5: Mary Get-mushroom
6: Mary Attack
7: Mary Jump
8: Mary Attack

Answer 2

Try this:
df
User Action
1 Bob Enterchapter1
2 Bob Attack
3 Bob Jump
4 Bob Getmushroom
5 Bob Openinventory
6 Bob Eatmushroom
7 Bob Closeinventory
8 Bob Run
9 Mary Enterchapter1
10 Mary Getmushroom
11 Mary Attack
12 Mary Jump
13 Mary Attack
14 Mary Openinventory
15 Mary Closeinventory
indices <- which(df$Action == 'Getmushroom')
n <- 3
# ensure that x + n does not go beyond the #rows of df
do.call(rbind, lapply(indices, function(x)df[x:min(x+n, nrow(df)),]))
User Action
4 Bob Getmushroom
5 Bob Openinventory
6 Bob Eatmushroom
7 Bob Closeinventory
10 Mary Getmushroom
11 Mary Attack
12 Mary Jump
13 Mary Attack

Answer 3

First find out the indices which has the term Get mushroom using which
You can use lapply on every indices and get the next 3 indices using seq.
args <- which(df$Action == "Get mushroom")
df[unlist(lapply(args, function(x) seq(x, x+3))), ]
# User Action
#4 Bob Get mushroom
#5 Bob Open inventory
#6 Bob Eat mushroom
#7 Bob Close inventory
#10 Mary Get mushroom
#11 Mary Attack
#12 Mary Jump
#13 Mary Attack
Or a similar approach (as suggested by #Sotos in comments)
df[sapply(args, function(x) seq(x, x+3)), ]
This sapply solution would work on dataframe and not on data.table as it does not accept 2-column matrix.
For it to work on data.table, you can unlist it using c
df[c(sapply(args, function(x) seq(x, x+3))), ]