ldexp和frexp如何在python中工作？

Question

python frexp和ldexp函数将浮点数拆分为尾数和指数.有人知道这个过程是暴露实际的浮动结构,还是需要python来进行昂贵的对数调用？

Answer 1

Python 2.6的math.frexp只是直接调用底层C库frexp.我们必须假设C库只是直接使用浮动表示的部分,而不是计算是否可用(IEEE 754).

static PyObject *
math_frexp(PyObject *self, PyObject *arg)
{
        int i;
        double x = PyFloat_AsDouble(arg);
        if (x == -1.0 && PyErr_Occurred())
                return NULL;
        /* deal with special cases directly, to sidestep platform
           differences */
        if (Py_IS_NAN(x) || Py_IS_INFINITY(x) || !x) {
                i = 0;
        }
        else {  
                PyFPE_START_PROTECT("in math_frexp", return 0);
                x = frexp(x, &i);
                PyFPE_END_PROTECT(x);
        }
        return Py_BuildValue("(di)", x, i);
}

PyDoc_STRVAR(math_frexp_doc,
"frexp(x)\n"
"\n"
"Return the mantissa and exponent of x, as pair (m, e).\n"
"m is a float and e is an int, such that x = m * 2.**e.\n"
"If x is 0, m and e are both 0.  Else 0.5 <= abs(m) < 1.0.");