Problem 55220. Matrix Quadrants

Good problem!

function M = foursquare(N)
M = zeros(2*N);
for i=1:length(M)/2
for j=1:length(M)/2
M(i,j)=1;
j=j+1;
end
i=i+1;
end
for i=(length(M)/2)+1:length(M)
for j=(length(M)/2)+1:length(M)
M(i,j)=3;
j=j+1;
end
i=i+1;
end
end
Just wrote that program that gives me the right solution but I don't understand why it won't work.Any suggestions?

@Andreea you're hard-coding the value 3 on the twelfth line; the problem description says that the lower-left quadrant should contain N's instead.