Vectorize loop for image manipulation

I have a black and white image (Ibw), and I want to take a circular region from it, and place it on a white background. I made a mask with a one in each position where I want to keep the pixel in the image (inside the circular region).

The code below works... but I want to vectorize it if possible. Right now I step across the image pixels one at a time, and if the mask has a one, I put the value in the black and white image onto the white field (Ifield).

 Ifield = ones(size(Ibw));
  for i = 1:1:rows
      for j = 1:1:cols
          if mask(i,j) > 0
              Ifield(i,j) = Ibw(i,j);
          else
              %Let Ifield keep its 1 (white)
          end
      end
  end

Answer 1

[Image Analyst]

Try this vectorized way:

Ifield = ones(size(Ibw));
Ifield(mask) = Ibw(mask);

[Clay Swackhamer]

It works- and it's much easier on the eyes. Thanks ImageAnalyst!