I was wondering if I want to do the procedure only on even (or odd) rows, what should I add and where?

There are many different ways (certainly lots of them are more elegant than this one):

matrix = RandomReal[1, {6, 6}];
DeleteCases[Table[If[EvenQ[i + j], matrix[[i, j]]], {i, 1, 6}, {j, 1, 6}], Null,Infinity] // MatrixForm

Here's another option:

Table[Reap[Do[If[EvenQ[i + j], Sow[matrix[[i, j]]]], {j, 1, 6}]], {i, 1, 6}][[All, 2, 1]] // MatrixForm

This one might be the nicest of the three:

Pick[matrix, Table[If[EvenQ[i + j], 1, 0], {i, 1, 6}, {j, 1, 6}], 1]

Cheers,

Marco

Hi,

I don't think that the question is quite specific, but this might help. Given a matrix:

matrix = RandomReal[1, {5, 5}]

We can display it in TableForm

matrix // TableForm

Now we can take all entries which have only odd entries:

matrix[[1 ;; ;; 2, 1 ;; ;; 2]] // TableForm

or only the even ones:

matrix[[2 ;; ;; 2, 2 ;; ;; 2]] // TableForm

In fact, I can choose any index combination I like:

matrix[[{1, 2, 5}, {2, 4, 5}]] // TableForm

Cheers,

Marco