[R] Problem to generate training data set and test data set

[Aimin Yan]

I have a full data set like this:

    aa bas    aas bms   ams bcu        acu     omega       y
1 ALA   0 127.71   0 69.99   0 -0.2498560  79.91470 outward
2 PRO   0  68.55   0 55.44   0 -0.0949008  76.60380 outward
3 ALA   0  52.72   0 47.82   0 -0.0396550  52.19970 outward
4 PHE   0  22.62   0 31.21   0  0.1270330 169.52500  inward
5 SER   0  71.32   0 52.84   0 -0.1312380   7.47528 outward
6 VAL   0  12.92   0 22.40   0  0.1728390 149.09400  inward
......................................................................................


aa have 19 levels, and there are different number of observation for each 
levels.
I want to pick 75% of observations of each levels randomly to generate a 
training set,
and 25% of observation of each levels to generate a testing set.

Does anyone know to do this?

Thanks

Aimin Yan