MATLAB: Q is a 3D array of size 3x3xn. I want to use the next value of Q i.e. Q(:,:,2) and so on, every time j is even. Is there a way to do it in MATLAB.

for loopif statement

for j=1:length(p)

stress(:,:,j)=(Q(:,:,1)*strain(:,:,j));
    if mod(j, 2) == 0
  % j is even
     else
  % j is odd
    end

end

If I understood correctly,

for j = 1:numel(p)
   stress(:, :, j) = Q(:, :, 1+floor(j/2)) * strain(:, :, j);
end

You could try the following.

J=5;
while J<13
    if J>=7
        j=1:6;
    else
        j=1:J;
    end
    z=63*pi.*(J-j); %piecewise multiplication
    disp(z)
    J=J+1;
end

This satisfies the conditions of j less than or equal to J and j less than 7.

Note the .* operation (see link)

When j is not changing in size, i.e. after the first two iterations of the loop, you could in fact get a solution therein without any loops:

j=1:6;
J=[7:13]';
z=63*pi.*(J-j)

Replace

A=sort(x);
[A,T]=sort(x);
Q=R(T);

Q = Q(randperm(65536, 65536));

Then:

Q4 = reshape(Q, 4, 4, numel(Q)/16)

Now Q4(:, :, n) is the n.th matrix.