xlswrite with formula in loop function

I have a large cell (2500,3) array which I want to export as a .xls file, but I want to include a formula ('=SUM(B1:C1)') which can be used in excel, The code below is working fine but I want the "B1" and "C1" to increase by 1 for each loop ('=SUM(B1:C1)', '=SUM(B2:C2)', '=SUM(B2:C2)' etc). How can I do this??? My present loop code is:

for II = 1:length(out)
    out{II,4}='=SUM(B1:C1)';
  end

xlswrite('Book1.xls',out)

Answer 1

[Walter Roberson]

      out{II,4} = sprintf('=SUM(B%d:C%d)', II, II);

[JB]

PERFECT, thanks Walter Roberson

Answer 2

[Fangjun Jiang]

num2str(II), e.g. num2str(3)