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Issue #11888: Add log2 function to math module. Patch written by Mark 

Dickinson.
This commit is contained in:
Victor Stinner 2011-05-09 01:01:09 +02:00
parent 3de49192aa
commit fa0e3d52d6
6 changed files with 234 additions and 16 deletions
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7
Doc/library/math.rst
View File

@ -184,6 +184,13 @@ Power and logarithmic functions
result is calculated in a way which is accurate for *x* near zero.
.. function:: log2(x)
Return the base-2 logarithm of *x*.
.. versionadded:: 3.3
.. function:: log10(x)
Return the base-10 logarithm of *x*. This is usually more accurate

45
Doc/whatsnew/3.3.rst
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@ -76,6 +76,33 @@ New module: :mod:`faulthandler`.
* :envvar:`PYTHONFAULTHANDLER`
* :option:`-X` ``faulthandler``
math
----
The :mod:`math` module has a new function:
* :func:`~math.log2`: return the base-2 logarithm of *x*
(Written by Mark Dickinson in :issue:`11888`).
nntplib
-------
The :class:`nntplib.NNTP` class now supports the context manager protocol to
unconditionally consume :exc:`socket.error` exceptions and to close the NNTP
connection when done::
>>> from nntplib import NNTP
>>> with nntplib.NNTP('news.gmane.org') as n:
... n.group('gmane.comp.python.committers')
...
('211 1454 1 1454 gmane.comp.python.committers', '1454', '1', '1454', 'gmane.comp.python.committers')
>>>
(Contributed by Giampaolo Rodolà in :issue:`9795`)
os
--
@ -96,21 +123,6 @@ os
(Patch submitted by Giampaolo Rodolà in :issue:`10784`.)
nntplib
-------
The :class:`nntplib.NNTP` class now supports the context manager protocol to
unconditionally consume :exc:`socket.error` exceptions and to close the NNTP
connection when done::
>>> from nntplib import NNTP
>>> with nntplib.NNTP('news.gmane.org') as n:
... n.group('gmane.comp.python.committers')
...
('211 1454 1 1454 gmane.comp.python.committers', '1454', '1', '1454', 'gmane.comp.python.committers')
>>>
(Contributed by Giampaolo Rodolà in :issue:`9795`)
sys
---
@ -120,10 +132,11 @@ sys
(:issue:`11223`)
signal
------
* The :mod:`signal` module has a new functions:
* The :mod:`signal` module has new functions:
* :func:`~signal.pthread_sigmask`: fetch and/or change the signal mask of the
calling thread (Contributed by Jean-Paul Calderone in :issue:`8407`) ;

114
Lib/test/math_testcases.txt
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@ -517,3 +517,117 @@ expm10306 expm1 1.79e308 -> inf overflow
-- weaker version of expm10302
expm10307 expm1 709.5 -> 1.3549863193146328e+308
-------------------------
-- log2: log to base 2 --
-------------------------
-- special values
log20000 log2 0.0 -> -inf divide-by-zero
log20001 log2 -0.0 -> -inf divide-by-zero
log20002 log2 inf -> inf
log20003 log2 -inf -> nan invalid
log20004 log2 nan -> nan
-- exact value at 1.0
log20010 log2 1.0 -> 0.0
-- negatives
log20020 log2 -5e-324 -> nan invalid
log20021 log2 -1.0 -> nan invalid
log20022 log2 -1.7e-308 -> nan invalid
-- exact values at powers of 2
log20100 log2 2.0 -> 1.0
log20101 log2 4.0 -> 2.0
log20102 log2 8.0 -> 3.0
log20103 log2 16.0 -> 4.0
log20104 log2 32.0 -> 5.0
log20105 log2 64.0 -> 6.0
log20106 log2 128.0 -> 7.0
log20107 log2 256.0 -> 8.0
log20108 log2 512.0 -> 9.0
log20109 log2 1024.0 -> 10.0
log20110 log2 2048.0 -> 11.0
log20200 log2 0.5 -> -1.0
log20200 log2 0.25 -> -2.0
log20200 log2 0.125 -> -3.0
log20200 log2 0.0625 -> -4.0
-- values close to 1.0
log20300 log2 1.0000000000000002 -> 3.2034265038149171e-16
log20301 log2 1.0000000001 -> 1.4426951601859516e-10
log20302 log2 1.00001 -> 1.4426878274712997e-5
log20310 log2 0.9999999999999999 -> -1.6017132519074588e-16
log20311 log2 0.9999999999 -> -1.4426951603302210e-10
log20312 log2 0.99999 -> -1.4427022544056922e-5
-- tiny values
log20400 log2 5e-324 -> -1074.0
log20401 log2 1e-323 -> -1073.0
log20402 log2 1.5e-323 -> -1072.4150374992789
log20403 log2 2e-323 -> -1072.0
log20410 log2 1e-308 -> -1023.1538532253076
log20411 log2 2.2250738585072014e-308 -> -1022.0
log20412 log2 4.4501477170144028e-308 -> -1021.0
log20412 log2 1e-307 -> -1019.8319251304202
-- huge values
log20500 log2 1.7976931348623157e+308 -> 1024.0
log20501 log2 1.7e+308 -> 1023.9193879716706
log20502 log2 8.9884656743115795e+307 -> 1023.0
-- selection of random values
log20600 log2 -7.2174324841039838e+289 -> nan invalid
log20601 log2 -2.861319734089617e+265 -> nan invalid
log20602 log2 -4.3507646894008962e+257 -> nan invalid
log20603 log2 -6.6717265307520224e+234 -> nan invalid
log20604 log2 -3.9118023786619294e+229 -> nan invalid
log20605 log2 -1.5478221302505161e+206 -> nan invalid
log20606 log2 -1.4380485131364602e+200 -> nan invalid
log20607 log2 -3.7235198730382645e+185 -> nan invalid
log20608 log2 -1.0472242235095724e+184 -> nan invalid
log20609 log2 -5.0141781956163884e+160 -> nan invalid
log20610 log2 -2.1157958031160324e+124 -> nan invalid
log20611 log2 -7.9677558612567718e+90 -> nan invalid
log20612 log2 -5.5553906194063732e+45 -> nan invalid
log20613 log2 -16573900952607.953 -> nan invalid
log20614 log2 -37198371019.888618 -> nan invalid
log20615 log2 -6.0727115121422674e-32 -> nan invalid
log20616 log2 -2.5406841656526057e-38 -> nan invalid
log20617 log2 -4.9056766703267657e-43 -> nan invalid
log20618 log2 -2.1646786075228305e-71 -> nan invalid
log20619 log2 -2.470826790488573e-78 -> nan invalid
log20620 log2 -3.8661709303489064e-165 -> nan invalid
log20621 log2 -1.0516496976649986e-182 -> nan invalid
log20622 log2 -1.5935458614317996e-255 -> nan invalid
log20623 log2 -2.8750977267336654e-293 -> nan invalid
log20624 log2 -7.6079466794732585e-296 -> nan invalid
log20625 log2 3.2073253539988545e-307 -> -1018.1505544209213
log20626 log2 1.674937885472249e-244 -> -809.80634755783126
log20627 log2 1.0911259044931283e-214 -> -710.76679472274213
log20628 log2 2.0275372624809709e-154 -> -510.55719818383272
log20629 log2 7.3926087369631841e-115 -> -379.13564735312292
log20630 log2 1.3480198206342423e-86 -> -285.25497445094436
log20631 log2 8.9927384655719947e-83 -> -272.55127136401637
log20632 log2 3.1452398713597487e-60 -> -197.66251564496875
log20633 log2 7.0706573215457351e-55 -> -179.88420087782217
log20634 log2 3.1258285390731669e-49 -> -161.13023800505653
log20635 log2 8.2253046627829942e-41 -> -133.15898277355879
log20636 log2 7.8691367397519897e+49 -> 165.75068202732419
log20637 log2 2.9920561983925013e+64 -> 214.18453534573757
log20638 log2 4.7827254553946841e+77 -> 258.04629628445673
log20639 log2 3.1903566496481868e+105 -> 350.47616767491166
log20640 log2 5.6195082449502419e+113 -> 377.86831861008250
log20641 log2 9.9625658250651047e+125 -> 418.55752921228753
log20642 log2 2.7358945220961532e+145 -> 483.13158636923413
log20643 log2 2.785842387926931e+174 -> 579.49360214860280
log20644 log2 2.4169172507252751e+193 -> 642.40529039289652
log20645 log2 3.1689091206395632e+205 -> 682.65924573798395
log20646 log2 2.535995592365391e+208 -> 692.30359597460460
log20647 log2 6.2011236566089916e+233 -> 776.64177576730913
log20648 log2 2.1843274820677632e+253 -> 841.57499717289647
log20649 log2 8.7493931063474791e+297 -> 989.74182713073981

22
Lib/test/test_math.py
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@ -649,6 +649,28 @@ class MathTests(unittest.TestCase):
n= 2**90
self.assertAlmostEqual(math.log1p(n), math.log1p(float(n)))
@requires_IEEE_754
def testLog2(self):
self.assertRaises(TypeError, math.log2)
# Check that we get exact equality for log2 of powers of 2.
actual = [math.log2(2.0**n) for n in range(-324, 1024)]
expected = [float(n) for n in range(-324, 1024)]
self.assertEqual(actual, expected)
# Check some integer values
self.assertEqual(math.log2(1), 0.0)
self.assertEqual(math.log2(2), 1.0)
self.assertEqual(math.log2(4), 2.0)
# Large integer values
self.assertEqual(math.log2(2**1023), 1023.0)
self.assertEqual(math.log2(2**1024), 1024.0)
self.assertEqual(math.log2(2**2000), 2000.0)
self.assertRaises(ValueError, math.log2, -1.5)
self.assertRaises(ValueError, math.log2, NINF)
self.assertTrue(math.isnan(math.log2(NAN)))
def testLog10(self):
self.assertRaises(TypeError, math.log10)
self.ftest('log10(0.1)', math.log10(0.1), -1)

3
Misc/NEWS
View File

@ -140,6 +140,9 @@ Core and Builtins
Library
-------
- Issue #11888: Add log2 function to math module. Patch written by Mark
Dickinson.
- Issue #12012: ssl.PROTOCOL_SSLv2 becomes optional.
- Issue #8407: The signal handler writes the signal number as a single byte

59
Modules/mathmodule.c
View File

@ -577,6 +577,55 @@ m_log(double x)
}
}
/*
log2: log to base 2.
Uses an algorithm that should:
(a) produce exact results for powers of 2, and
(b) be monotonic, assuming that the system log is monotonic.
*/
static double
m_log2(double x)
{
if (!Py_IS_FINITE(x)) {
if (Py_IS_NAN(x))
return x; /* log2(nan) = nan */
else if (x > 0.0)
return x; /* log2(+inf) = +inf */
else {
errno = EDOM;
return Py_NAN; /* log2(-inf) = nan, invalid-operation */
}
}
if (x > 0.0) {
double m;
int e;
m = frexp(x, &e);
/* We want log2(m * 2**e) == log(m) / log(2) + e. Care is needed when
* x is just greater than 1.0: in that case e is 1, log(m) is negative,
* and we get significant cancellation error from the addition of
* log(m) / log(2) to e. The slight rewrite of the expression below
* avoids this problem.
*/
if (x >= 1.0) {
return log(2.0 * m) / log(2.0) + (e - 1);
}
else {
return log(m) / log(2.0) + e;
}
}
else if (x == 0.0) {
errno = EDOM;
return -Py_HUGE_VAL; /* log2(0) = -inf, divide-by-zero */
}
else {
errno = EDOM;
return Py_NAN; /* log10(-inf) = nan, invalid-operation */
}
}
static double
m_log10(double x)
{
@ -1622,6 +1671,15 @@ PyDoc_STRVAR(math_log_doc,
Return the logarithm of x to the given base.\n\
If the base not specified, returns the natural logarithm (base e) of x.");
static PyObject *
math_log2(PyObject *self, PyObject *arg)
{
return loghelper(arg, m_log2, "log2");
}
PyDoc_STRVAR(math_log2_doc,
"log2(x)\n\nReturn the base 2 logarithm of x.");
static PyObject *
math_log10(PyObject *self, PyObject *arg)
{
@ -1894,6 +1952,7 @@ static PyMethodDef math_methods[] = {
{"log", math_log, METH_VARARGS, math_log_doc},
{"log1p", math_log1p, METH_O, math_log1p_doc},
{"log10", math_log10, METH_O, math_log10_doc},
{"log2", math_log2, METH_O, math_log2_doc},
{"modf", math_modf, METH_O, math_modf_doc},
{"pow", math_pow, METH_VARARGS, math_pow_doc},
{"radians", math_radians, METH_O, math_radians_doc},