[R] filtering in aggregate

[Antonio Olinto]

Hello all,

I have a data frame with year, month, species, fishing gear and catch 
(Y, M, S, F, C) and I want the sum of C by Y for species "A" and fishing 
gear "trawl".

I tried things like aggregate(C[S=="A" & F=="trawl"], list (Year = 
Y[S=="A" & F=="trawl"]), fun=sum), but it didn't worked.

To overcome this problem I did a subset as a new data frame and then I 
used aggregate. But I'm sure there's a way to apply a filter in data frame.

Thanks for any help. Best regards.

Antonio Olinto