How to aggregate some columns while keeping other columns in R?

Question

I have a data frame like this:

     id  no  age
1    1   7   23
2    1   2   23
3    2   1   25
4    2   4   25
5    3   6   23
6    3   1   23

Answer 1

Even better, data.table:

library(data.table)
# convert your object to a data.table (by reference) to unlock data.table syntax
setDT(DF)
DF[  , .(sum_no = sum(no), unq_age = unique(age)), by = id]