[R] Basic help needed: group bunch of lines in a list (matrix)

[Emmanuel Levy]

Hello,

I'd like to group the lines of a matrix so that:
A 1.0 200
A 3.0 800
A 2.0 200
B 0.5 20
B 0.9 50
C 5.0 70

Would give:
A 2.0 400
B 0.7 35
C 5.0 70

So all lines corresponding to a letter (level), become a single line
where all the values of each column are averaged.

I've done that with a loop but it doesn't sound right (it is very
slow). I imagine there is a
sort of "apply" shortcut but I can't figure it out.

Please note that it is not exactly a matrix I'm using, the function
"typeof" tells me it's a list, however I access to it like it was a
matrix.

Could someone help me with the right function to use, a help topic or
a piece of code?

Thanks,

  Emmanuel