How to find the next occurrence in a data.frame in R? - r

Question

Assume we have an email dataset with a sender and a recipient in every row. We want to find the next occurrence in the dataset for which the sender and the recipient are interchanged. So if sender==x & recipient==y, we are looking for the next row that has sender==y & recipient==x. Subsequently, we want to calculate the difference between counts for those observations. See the column diff_count for the desired output.
# creating the data.frame
id = 1:10
sender = c(1, 2, 3, 2, 3, 1, 2, 1, 2, 3)
recipient = c(2, 1, 2, 3, 1, 2, 3, 3, 1, 1)
count = c(1, 4, 5, 7, 12, 17, 24, 31, 34, 41)
df <- data.frame(id, sender, recipient, count)
# output should look like this
df$diff_count <- c(3, 13, 2, NA, 19, 17, NA, 10, NA, NA)
If there are no more observations that satisfy the requirement, then we simply fill in NA. Solution should be relatively easy with tidyverse, but I seem not to be able to do it.

Answer 1

Another dplyr-way without a custom function but several self joins:
library(dplyr)
data %>%
left_join(data,
by = c("sender" = "recipient", "recipient" = "sender"),
suffix = c("", ".y")) %>%
filter(id < id.y) %>%
group_by(id) %>%
slice_min(id.y) %>%
ungroup() %>%
mutate(diff_count = count.y - count) %>%
right_join(data) %>%
select(-matches("\\.(y|x)")) %>%
arrange(id)
returns
Joining, by = c("id", "sender", "recipient", "count")
# A tibble: 10 x 5
id sender recipient count diff_count
<int> <dbl> <dbl> <dbl> <dbl>
1 1 1 2 1 3
2 2 2 1 4 13
3 3 3 2 5 2
4 4 2 3 7 NA
5 5 3 1 12 19
6 6 1 2 17 17
7 7 2 3 24 NA
8 8 1 3 31 10
9 9 2 1 34 NA
10 10 3 1 41 NA

Answer 2

There should be easier ways, but below is one way using a custom function in tidyverse style:
library(dplyr)
calc_diff <- function(df, send, recp, cnt) {
df %>%
slice_tail(n = nrow(df) - cur_group_rows()) %>%
filter(sender == send, recipient == recp) %>%
slice_head(n = 1) %>%
pull(count) %>%
{ifelse(length(.) == 0, NA, .)} %>%
`-`(., cnt)
}
df %>%
rowwise(id) %>%
mutate(diff_count = calc_diff(df,
send = recipient,
recp = sender,
cnt = count))
#> # A tibble: 10 x 5
#> # Rowwise: id
#> id sender recipient count diff_count
#> <int> <dbl> <dbl> <dbl> <dbl>
#> 1 1 1 2 1 3
#> 2 2 2 1 4 13
#> 3 3 3 2 5 2
#> 4 4 2 3 7 NA
#> 5 5 3 1 12 19
#> 6 6 1 2 17 17
#> 7 7 2 3 24 NA
#> 8 8 1 3 31 10
#> 9 9 2 1 34 NA
#> 10 10 3 1 41 NA
Created on 2021-08-20 by the reprex package (v2.0.1)