how to convert this code exposal to linear and how to find mape

function [a,g0] = getExpontialModel(f)

A = zeros(1,length(f)-1);

G_0 = zeros(1,length(f)-1);

for i=2:length(f)

if f(i-1) ~= 0

A(i-1) = f(i)/f(i-1);

else

A(i-1) = 1;

end

end

a = median(A); %more robust than mean function

for i=1:length(f)-1

G_0(i) = f(i)/a^i;

end

g0 = median(G_0);%more robust than mean function