Prellocate memory to unknown size array ?

 numfiles = 3;

 A = [];
 for i = 1:numfiles
    f = fopen(sprintf('F%d', i), 'r'); 
    D = fscanf(f, '%d %d %f',[3, inf]); % Prellocate memory.
    D = D';
    P(i).element = D;
    A = [A; D]; %Prellocate memory 
    fclose(f);

 end

 B = zeros(size(A(:,1:2)));
 U = setdiff(A(:,1:2),B,'rows');

 C = [];
 for j = 1:numfiles
     Ua(j).ele = setdiff(U(:,1:2), P(j).element(:,1:2),'rows');%Prellocate memory
     Ua_z(j).ele = [Ua(j).ele , zeros(size(Ua(j).ele,1),1)]; % Prellocate memory
     AU(j).ele = [P(j).element ; Ua_z(j).ele]; % Prellocate memory
     AU_sorted(j).ele = sortrows(AU(j).ele); %Prellocate memory
     C = [C, AU_sorted(j).ele(:,3)];
 end
 CU = [U(:,1:2), C]; % Horizontally concatenate U and AU_sorted(j)