I have a data set that contains a group an id variable and a pair id variable. Each observation is paired with another observation. I want to create a new variable that takes the id variable of its pair-partner and assign it as a new 'partner' variable.
Below I have an example data:
id <- 1:10
pair_id <- rep(1:5,2)
df <- cbind(id, pair_id)
> df
id pair_id
[1,] 1 1
[2,] 2 2
[3,] 3 3
[4,] 4 4
[5,] 5 5
[6,] 6 1
[7,] 7 2
[8,] 8 3
[9,] 9 4
[10,] 10 5
As I said above, I want to add a variable indicating on each id partner's id, where the partnership is identified with the pair_id. As an example, observations with id == 1 has pair_id == 1, which makes it partner to be the observation with id == 6, since they share pair_id.
So the end result should look like this:
id pair_id partner_id
[1,] 1 1 6
[2,] 2 2 7
[3,] 3 3 8
[4,] 4 4 9
[5,] 5 5 10
[6,] 6 1 1
[7,] 7 2 2
[8,] 8 3 3
[9,] 9 4 4
[10,] 10 5 5
Thanks!
You can reverse the id values for each pair_id :
library(dplyr)
df %>% group_by(pair_id) %>% mutate(partner_id = rev(id))
# id pair_id partner_id
# <int> <int> <int>
# 1 1 1 6
# 2 2 2 7
# 3 3 3 8
# 4 4 4 9
# 5 5 5 10
# 6 6 1 1
# 7 7 2 2
# 8 8 3 3
# 9 9 4 4
#10 10 5 5
The equivalent in base R :
df$partner_id <- with(df, ave(id, pair_id, FUN = rev))
and in data.table is
library(data.table)
setDT(df)[, partner_id := rev(id), pair_id]
data
df <- data.frame(id, pair_id)