Placing Blocks of fixed size to random positions in a bigger matrix

How do you place a smaller matrix(block) of fixed size in a bigger matrix at random positions?

[Bharathwaj]

what if the placement of the smaller blocks should not overlap with the existing blocks ?

Answer 1

[Andrei Bobrov]

M = zeros(5)
m = randi(4,3)

sM = size(M);
sm = size(m);
k=sM-sm+1;
iM = reshape(1:numel(M),sM(1),[]);
iw = iM(1:k(1),1:k(2));
im =  bsxfun(@plus,(0:sm(1)-1).',0:sM(1):sM(1)*(sm(2)-1));
M(iw(randi(numel(m)))+im) = m

Answer 2

[Fangjun Jiang]

a=rand(3);
A=magic(10);

[m,n]=size(a);
[M,N]=size(A);

i=floor((M-m)*rand)+1;
j=floor((N-n)*rand)+1;

A(i+1:i+m,j+1:j+n)=a

[Bharathwaj]

what if the placement of the smaller blocks should not overlap with the existing blocks ?