Subset data for the most recent month - r

Question

I have a .txt file
test.txt
V1 V2 Date
A B 2020-01-02
C D 2020-02-27
E F 2020-09-10
G H 2020-09-15
I want to subset data based on the most recent month.
I did this which does the job but I want to extract the most recent month automatically rather than typing in manually and then extract the data
test$month <- factor(format(test$Date, "%B"),levels = month.name)
test.subset <- test[test$month == "September"]

Answer 1

We can arrange the Date class column and filter the formated value by comparing it with the last one
library(dplyr)
test %>%
mutate(Date = as.Date(Date), Month = format(Date, '%B')) %>%
arrange(Date) %>%
filter(Month == last(Month)) %>%
select(-Month)
-output
# V1 V2 Date
#1 E F 2020-09-10
#2 G H 2020-09-15
data
test <- structure(list(V1 = c("A", "C", "E", "G"), V2 = c("B", "D", "F",
"H"), Date = c("2020-01-02", "2020-02-27", "2020-09-10", "2020-09-15"
)), class = "data.frame", row.names = c(NA, -4L))

Answer 2

Here is a base R option using subset + gsub
subset(
transform(
df,
ym = gsub("\\d+$", "", Date)
),
ym == max(ym),
select = -ym
)
which gives
V1 V2 Date
3 E F 2020-09-10
4 G H 2020-09-15
A data.table option
setDT(df)[
,
`:=`(Year = year(as.IDate(Date)), Month = month(as.Date(Date)))
][
.(max(Year), max(Month)),
on = .(Year, Month)
][
,
`:=`(Year = NULL, Month = NULL)
][]
gives
V1 V2 Date
1: E F 2020-09-10
2: G H 2020-09-15
Data
> dput(df)
structure(list(V1 = c("A", "C", "E", "G"), V2 = c("B", "D", "F",
"H"), Date = c("2020-01-02", "2020-02-27", "2020-09-10", "2020-09-15"
)), class = "data.frame", row.names = c(NA, -4L))

Answer 3

Using the structure shared by #ThomasIsCoding, and assuming the year is constant, one could just look for the row with the max month and filter for that:
# using datatable
library(data.table)
setDT(df)[month(as.IDate(Date)) == max(month(as.IDate(Date)))]
V1 V2 Date
1: E F 2020-09-10
2: G H 2020-09-15