Problem with parallel loops

Matlab does not let me to perform the following parfor loop as it warns me that valid indices for A is restricted. How can I overcome this? Any suggestions would be appreciated.

A = zeros(4,4);
a = [1 1 1 2 2 3];
b = [2 3 4 3 4 4];
parfor i = 1:6
 A(a(i),b(i)) = corr(C(a(i),:),D(b(i),:));
end

C and D can be any matrices.

Answer 1

[Matt J]

If a and b are small

      s=zeros(1,6);

      Ca=C(a,:).';
      Db=D(b,:).';

      parfor i = 1:6
       s(i) = corr(Ca(:,i).',Db(:,i).');
      end

      A=sparse(a,b,s,4,4);

If C and D are small,

      s=zeros(1,6);
      C=C.';
      D=D.';

      parfor i = 1:6
       s(i) = corr(C(:,a(i).'), D(:,b(i)).');
      end

      A=sparse(a,b,s,4,4);

Answer 2

[Kian]

Thank you Matt, I really appreciate your help.