相关疑难解决方法(0)

我一直在玩C99的四倍精度长双.据我所知,(平台特定的)numpy支持long double和128bit float.

我遇到了一些我无法解释的事情.

鉴于:

>>> import numpy as np

计算需要超过64位但小于128位的数字表示为整数:

>>> 2**64+2
18446744073709551618          # note the '8' at the end
>>> int(2**64+2)
18446744073709551618          # same obviously

如果我在C99 128位长双倍中计算相同的数字,我得到18446744073709551618.000000

现在,如果我使用numpy long double:

>>> a=np.longdouble(2)
>>> b=np.longdouble(64)
>>> a**b+a
18446744073709551618.0              # all good...

这些不正确的结果怎么样:

>>> np.longdouble(2**64+2)
18446744073709551616.0             # Note '6'; appears 2**64 not done in long double
>>> np.longdouble(int(2**64+2))
18446744073709551616.0             # can't force the use of a Python long
>>> n=int(2**64+2)
>>> np.longdouble(n)
18446744073709551616.0 …

我正在尝试使用np.longdouble我的Python代码中的dtype,并且我正在尝试使用NumPy来操作从使用Cython编译的C模块获得的长双精度.

假设我这样做:

import numpy as np

print np.finfo(np.longdouble)
Machine parameters for float128
---------------------------------------------------------------------
precision= 18   resolution= 1e-18
machep=   -63   eps=        1.08420217249e-19
negep =   -64   epsneg=     5.42101086243e-20
minexp=-16382   tiny=       3.36210314311e-4932
maxexp= 16384   max=        1.18973149536e+4932
nexp  =    15   min=        -max
---------------------------------------------------------------------


a = np.longdouble(1e+346)

a
Out[4]: inf

b = np.longdouble(1e+347)

b
Out[6]: inf

c = a/b
/usr/lib/python2.7/site-packages/spyderlib/widgets/externalshell/start_ipython_kernel.py:1:
RuntimeWarning: invalid value encountered in longdouble_scalars
  # -*- coding: utf-8 -*-

c
Out[8]: nan

a.dtype, b.dtype, c.dtype
Out[9]: (dtype('float128'), dtype('float128'), dtype('float128'))

本质上,它与此 …