a = input('Enter a positive number: ');
err = 0;
tolerance = 1.e-3;
if a<=0
    err=1;
else
    current_est = a/2;
    new_est = current_est/2 + a/(2*current_est);
    while abs(new_est - current_est) > tolerance
        current_est = new_est;
        new_est = current_est/2 + a/(2*current_est);
    end % ends while loop
end % ends if structure
if err
    disp('You must enter a positive number')
else
    fprintf('The square root of %g is approximately %7.3f \n',a,new_est)
end