为什么x未定义？

Question

请帮我摆脱以下错误：

error: 'x' undefined near line 3 column 8
error: called from
conv_test at line 3 column 2
MAIN at line 17 column 1
error: evaluating argument list element number 1
error: called from
conv_test at line 3 column 2
MAIN at line 17 column 1

在 MAIN.m 内部，我只需致电：

result_g_p1 = conv_test(@g_p1);

我的 g_p1.m：

function endpoint = g_p1(x)
    endpoint = -8/(x^3);
end

conv_test.m ：

function conv_test_result = conv_test(g)
    if (g(x) > 1) && (g(x) < 0)    <--- Line 3
        conv_test_result = fprintf('Convergent Test Result: The function is monotonically convergent! \n');
    elseif (g(x) > -1) && (g(x) < 0)
        conv_test_result = fprintf('Convergent Test Result: The function is oscillatory convergent! \n');
    else (g(x) > 1)
        conv_test_result = fprintf('Convergent Test Result: The function is divergent! \n');
    end
end

我不知道为什么x未定义，因为我明确地在g_p1内传递了conv_test。