function v=prefix(ss)
% Allocate a stack, stk
% Get the length of ss and set it to n
for j=n:-1:1
            % Get the jth element of ss and set it to c
            if is_digit(c)
                stk=push(str2num(c),stk)  %push c to stack stk.
            end
            if is_perator(c)
                  if stk.top >= 2
                       %pop an element from stk and set it to d1
                       %pop an element from stk and set it to d2
                       v=evaluat(c,num2str(d1),num2str(d1));%apply operand c to d1 and d2 and set the result to v
                       % push v to stack stk
                  else
                      display('an invalid prefix expression');
                  end
            end
end



