Question

How can I subset the following example data frame to only return one
observation for the earliest occurance [i.e. min(year)] of each id?

id <- c("A", "A", "C", "D", "E", "F")
year <- c(2000, 2001, 2001, 2002, 2003, 2004)
qty  <- c(100, 300, 100, 200, 100, 500)
df=data.frame(year, qty, id)

In the example above there are two observations for the “A” id at years 2000 and 2001. In the case of duplicate id’s, I would like the subset data frame to only include the the first occurance (i.e. at 2000) of the observations for the duplicate id.

df2 = subset(df, ???)

This is what I am trying to return:

df2

year qty id
2000 100  A
2001 100  C
2002 200  D
2003 100  E
2004 500  F

Any assistance would be greatly appreciated.

Answer 1

You can aggregate on minimum year + id, then merge with the original data frame to get qty:

df2 <- merge(aggregate(year ~ id, df1, min), df1)

# > df2
#   id year qty
# 1  A 2000 100
# 2  C 2001 100
# 3  D 2002 200
# 4  E 2003 100
# 5  F 2004 500