Getting the mean value for every Id in a data frame

Question

Imagine I have a data frame with 2 columns

Id    Value
12    13
32    3
6022  11
9142  231
12    23
119   312
...

and I want to get the mean va

Answer 1

Just for completeness basic solution is tapply:

tapply(data$Value, data$Id, mean)

(or using with as with(data, tapply(Value, Id, mean)))