[R] How to write and analyze data with 3 dimensions

Suppose I have data organized in the following way: (P_i, M_j, S_k) where i, j and k and indexes for sets. I would like to analyze the data to get for example the following information: what is the average over k for (P_i, M_j) or what is the average over j and k for P_i. My question is what woul

[R] Reading and analyzing data with two headers

I have data that I would like to analyze in R with the following format: *,M1,,M2, *,S1,S2,S1,S2, p1,m1s1v1,m1s2v1,m2s1v1,m2s2v1 p2,m1s1v2,m1s2v2,m2s1v2,m2s2v2 ..,...,...,..., so I have a method M1 and M2 and each of these have attributes that I would like to analyze for the different problem