select first and last occurences of a value in a given month - r

Question

I have a dataset that records the changes in a group from a certain ID, in a given month.
In the example, in july, the ID 5 changed from group 2 to group 1, then from group 1 to 2, and so on.
I need to get only the first and the last changes made in this ID/month.
ID groupTO groupFROM MONTH
5 2 1 6
5 1 2 7
5 2 1 7
5 3 2 7
5 1 3 7
5 2 1 8
5 1 2 8
5 2 1 8
6 1 2 6
6 3 1 6
6 2 1 7
6 3 2 8
6 1 3 8
In this case, i need the results to be:
ID groupTO groupFROM MONTH
5 2 1 6
5 1 2 7
5 1 3 7
5 2 1 8
5 2 1 8
6 1 2 6
6 3 1 6
6 2 1 7
6 3 2 8
6 1 3 8
If i remove the duplicates (ID/MONTH), i can get the first occurence, but how do i get the last one?

Answer 1

Here's an easy way you can do with dplyr;
library(dplyr)
# Create data
dt <-
data.frame(Id = c(rep(5, 8), rep(6, 5)),
groupTO = c(2, 1, 2, 3, 1, 2, 1, 2, 1, 3, 2, 3, 1),
groupFROM = c(1, 2, 1, 2, 3, 1, 2, 1, 2, 1, 1, 2, 3),
MONTH = c(6, 7, 7, 7, 7, 8, 8, 8, 6, 6, 7, 8, 8))
dt %>%
# Group by ID and month
group_by(Id, MONTH) %>%
# Get first and last row
slice(c(1, n())) %>%
# To remove cases where first is same as last
distinct()
# # A tibble: 9 x 4
# # Groups: Id, MONTH [6]
# Id groupTO groupFROM MONTH
# <dbl> <dbl> <dbl> <dbl>
# 5 2 1 6
# 5 1 2 7
# 5 1 3 7
# 5 2 1 8
# 6 1 2 6
# 6 3 1 6
# 6 2 1 7
# 6 3 2 8
# 6 1 3 8

Answer 2

Using data.table
library(data.table)
unique(setDT(df1)[, .SD[c(1, .N)], .(ID, MONTH)])
# ID MONTH groupTO groupFROM
#1: 5 6 2 1
#2: 5 7 1 2
#3: 5 7 1 3
#4: 5 8 2 1
#5: 6 6 1 2
#6: 6 6 3 1
#7: 6 7 2 1
#8: 6 8 3 2
#9: 6 8 1 3
data
df1 <- structure(list(ID = c(5L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 6L, 6L,
6L, 6L, 6L), groupTO = c(2L, 1L, 2L, 3L, 1L, 2L, 1L, 2L, 1L,
3L, 2L, 3L, 1L), groupFROM = c(1L, 2L, 1L, 2L, 3L, 1L, 2L, 1L,
2L, 1L, 1L, 2L, 3L), MONTH = c(6L, 7L, 7L, 7L, 7L, 8L, 8L, 8L,
6L, 6L, 7L, 8L, 8L)), class = "data.frame", row.names = c(NA,
-13L))

Answer 3

Here is a base R solution using split
dfout <- do.call(rbind,c(make.row.names = F,
lapply(split(df,df[c("Id","MONTH")],lex.order = T),
function(v) if (nrow(v)==1) v[1,] else v[c(1,nrow(v)),])))
such that
> dfout
Id groupTO groupFROM MONTH
1 5 2 1 6
2 5 1 2 7
3 5 1 3 7
4 5 2 1 8
5 5 2 1 8
6 6 1 2 6
7 6 3 1 6
8 6 2 1 7
9 6 3 2 8
10 6 1 3 8```

Answer 4

A base R way using ave where we select 1st and last row for each ID and MONTH and select the unique rows in the dataframe.
unique(subset(df, ave(groupTO == 1, ID, MONTH, FUN = function(x)
seq_along(x) %in% c(1, length(x)))))
# ID groupTO groupFROM MONTH
#1 5 2 1 6
#2 5 1 2 7
#5 5 1 3 7
#6 5 2 1 8
#9 6 1 2 6
#10 6 3 1 6
#11 6 2 1 7
#12 6 3 2 8
#13 6 1 3 8