Nearest-neighbor interpolation algorithm in MATLAB

I am trying to write my own function for scaling up an input image by using the Nearest-neighbor interpolation algorithm. The bad part is I am able to see how it works but cannot find the algorithm itself. I will be grateful for any help.

Here's what I tried for scaling up the input image by a factor of 2:

function output = nearest(input)
[x,y]=size(input);
output = repmat(uint8(0),x*2,y*2);
[newwidth,newheight]=size(output);
for i=1:y
    for j=1:x
        xloc = round ((j * (newwidth+1)) / (x+1));
        yloc = round ((i * (newheight+1)) / (y+1));
        output(xloc,yloc) = input(j,i);
    end
end

Here is the output after Mark's suggestion