You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: dace_correxp.m
    path of file: ./repos/PlatEMO/PlatEMO/Algorithms/Multi-objective optimization/MO-L2SMEA/krg/dace
    
    the code of the file until where you have to start completion: function  [r, dr] = dace_correxp(theta, d)
%Exponential correlation function
%
%           n
%   r_i = prod exp(-theta_j * |d_ij|)
%          j=1
%
% If length(theta) = 1, then the model is isotropic:
% theta_j = theta(1), j=1,...,n
%
% Call:    r = dace_correxp(theta, d)
%          [r, dr] = dace_correxp(theta, d)
%
% theta :  parameters in the correlation function
% d     :  m*n matrix with differences between given data points
% r     :  correlation
% dr    :  m*n matrix with the Jacobian of r at x. It is
%          assumed that x is given implicitly by d(i,:) = x - S(i,:),
%          where S(i,:) is the i'th design site.

% hbn@imm.dtu.dk
% Last update April 12, 2002

    
function  [r, dr] = dace_correxp(theta, d)
%Exponential correlation function
%
%           n
%   r_i = prod exp(-theta_j * |d_ij|)
%          j=1
%
% If length(theta) = 1, then the model is isotropic:
% theta_j = theta(1), j=1,...,n
%
% Call:    r = dace_correxp(theta, d)
%          [r, dr] = dace_correxp(theta, d)
%
% theta :  parameters in the correlation function
% d     :  m*n matrix with differences between given data points
% r     :  correlation
% dr    :  m*n matrix with the Jacobian of r at x. It is
%          assumed that x is given implicitly by d(i,:) = x - S(i,:),
%          where S(i,:) is the i'th design site.

% hbn@imm.dtu.dk
% Last update April 12, 2002

[m n] = size(d);  % number of differences and dimension of data
lt = length(theta);
if  lt == 1,  theta = repmat(theta,1,n);
elseif  lt ~= n
    error(sprintf('Length of theta must be 1 or %d',n))
else
    theta = theta(:).';
end