Matlab: Zoom out by pixel reduction

%zoom out by pixel reduction
close all;
clear all;
aa=imread('ranahamid.jpg');
a=rgb2gray(aa);
c=size(a)
for i = 1:2:c(1)
    for j = 1:2:c(2)
        b((i+1)/2,(j+1)/2)=a(i,j);
    end
end

figure()
imshow(a)
figure()
imshow(b)
imwrite(b,'b.jpg');