How do I simplify this code (remove for loop)?

I'm trying to get in the habit of writing my code in terms of matrix operations and using as few for loops as possible. Is there a better way to write this code without the for loop?

for ii=1:N
  if (acos(dot(rel_vec,vec_array(:,ii))) > pi/2)
    vec_array(:,ii) = -vec_array(:,ii);
  end
end

vec_array is matrix containing N vectors (3xN). I compare each vector in the matrix to make sure they're within 180 degrees of some relative vector (rel_vec). If the vector is more than 180 degrees away, I multiply the vector by -1.

Answer 1

[Oleg Komarov]

idx = acos(rel_vec * vec_array) > pi/2;
rvec(:,idx) = -rvec(:,idx);

where rel_vec is 1 by 3.

Answer 2

[Josh]

Yah, that'll do it. I knew there was a much easier way to do it. Thanks!