How to extract value at "final visit" from a data set with repeated measurement in R?

Question

Suppose I have a data frame with repeated measurements:

 >m
 id  age    diagnosis
  1   4         0
  1   7         1
  1   9         0
  2   6         1
  4   9         1
  4   10        0

Diagnosis is not time-invariant. How can I extract the diagnosis result at final visit (oldest age) to get something like this?

id  age    diagnosis
 1   9         0
 2   6         1
 4   10        0

Answer 1

You could try data.table's last()

library(data.table)
as.data.table(df)[, last(.SD), by = id]
#    id age diagnosis
# 1:  1   9         0
# 2:  2   6         1
# 3:  4  10         0

Or with dplyr slice(., n())

library(dplyr)
slice(group_by(df, id), n())
# Source: local data frame [3 x 3]
# Groups: id [3]
#
#      id   age diagnosis
#   (int) (int)     (int)
# 1     1     9         0
# 2     2     6         1
# 3     4    10         0