Operation of specific elements of a matrix

I have a matrix of dimensions 3000x3.

I need to compute statistics for the elements (1,1),(4,1),(7,1), (10,1),...,(3000,1)

how can I code this statistic i.e mean? It is basically the mean of the elements of the first colum, which are in rows 1,4,7,10...3000 i.e i+3?

Answer 1

[madhan ravi]

mean(matrix(1:3:end,1))