Changing the order in for-loop - matlab

Question

I have a matrix (with the size of A and B; suppose 100x100) and want to fill in with smaller matrix (or block) with the size of a and b (suppose 12x12).
As it is clear, the loop starts from "j" and then goes to the next row. Actually I want to use the same loop, by adding another variable to impose that it first complete the columns. Any idea that how I should define this new variable in the following loop to control the completion direction.
M = zeros(100,100);
for j = 1:12:100-12+1
for i = 1:12:100-12+1
block = rand(12,12);
M(i:i+11, j:j+11) = block;
imagesc(M); axis equal tight xy
pause(.1)
end;
end;

Answer 1

Why not just do
M = zeros(100,100);
for j = 1:12:100-12+1
for i = 1:12:100-12+1
block = rand(12,12);
M(i:i+11, j:j+11) = block;
imagesc(M); axis equal tight xy
pause(.1)
end;
end;
Now you will iterate over the i's first.
Incidentally, I recommend not using i and j as loop variables - they shadow the built in sqrt(-1) imaginary number...
update based on your comment, it seems you want to leave the order of i and j in the outer loop, and add "another parameter" to change the direction. The following code does all that. Is this what you are after?
M = zeros(100,100);
rowFirst = true; % set to false for "column first"
for i = 1:12:100-12+1
for j = 1:12:100-12+1
block = rand(12,12);
if rowFirst
M((0:11) + i, (0:11) + j) = block;
else
M((0:11) + j, (0:11) + i) = block;
end
imagesc(M); axis equal tight xy
pause(.1)
end
end
update 2 and now "even for non square matrix" (not tested, late at night):
M = zeros(100, 120);
rowFirst = true;
sz = size(M);
blockSize = 12;
v = 1:blockSize;
nrc = floor(sz / blockSize);
if rowFirst
nrc = reverse(nrc);
end
for ii = blockSize * (0:nrc(1)-1)
for jj = blockSize * (0:nrc(2)-1)
block = rand(blockSize*[1 1]);
if ~rowFirst
block = block';
end if
M(v + ii, v+jj) = block;
if rowFirst
imagesc(M);
else
imagesc(M');
end
axis equal tight xy
pause(0.1)
end
end
LAST TIME if you insist that the outer loop iterates over j and the inner loop over i, yet that in some instances j is the "faster moving" variable, you can do the following.
P = 120;
Q = 180;
M = zeros(P, Q); % not a square matrix
rowFirst = true; % a switch you can flip
blockSize = 15; % size of block
sz = floor(size(M)/blockSize); % number of iterations in j, i
nr = sz(1); nc = sz(2);
vv = 1:blockSize;
for jj = 0: (nc-1)
for ii = 0: (nr-1)
if(rowFirst)
kk = ii * blockSize;
ll = jj * blockSize;
else
nn = jj * nr + ii;
ll = mod(nn, nc);
kk = floor(nn / nc);
%ll = (nn - kk * nc);
fprintf(1, 'ii, jj, nn = [%d, %d, %d]: [kk, ll] = %d, %d\n', ii, jj, nn, kk, ll)
ll = ll * blockSize; kk = kk * blockSize;
% mod(nn, P);
end
M(kk+vv, ll+vv) = rand(blockSize*[1 1]);
imagesc(M);
axis tight equal xy;
pause(0.1);
end
end