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

Alternatively, you could use ddply from plyr package:

require(plyr)
ddply(df,.(id,age),summarise,no = sum(no))

In this particular example the results are identical. However, this is not always the case, the difference between the both functions is outlined here. Both functions have their uses and are worth exploring, which is why I felt this alternative should be mentioned.