mirrors/cpython
1
0
Fork 0
mirror of https://github.com/python/cpython.git synced 2025-08-04 08:59:19 +00:00
Code Issues Projects Releases Packages Wiki Activity Actions 11

Return reasonable results for math.log(long) and math.log10(long) (we were

Browse source 
getting Infs, NaNs, or nonsense in 2.1 and before; in yesterday's CVS we
were getting OverflowError; but these functions always make good sense
for positive arguments, no matter how large).
This commit is contained in:
Tim Peters 2001-09-05 00:53:45 +00:00
parent 63c9453929
commit 785261684e
3 changed files with 104 additions and 7 deletions
Download patch file Download diff file Expand all files Collapse all files

68
Modules/mathmodule.c
View file

@ -1,6 +1,7 @@
/* Math module -- standard C math library functions, pi and e */
#include "Python.h"
#include "longintrepr.h"
#ifndef _MSC_VER
#ifndef __STDC__
@ -136,10 +137,6 @@ FUNC2(fmod, fmod,
" x % y may differ.")
FUNC2(hypot, hypot,
"hypot(x,y)\n\nReturn the Euclidean distance, sqrt(x*x + y*y).")
FUNC1(log, log,
"log(x)\n\nReturn the natural logarithm of x.")
FUNC1(log10, log10,
"log10(x)\n\nReturn the base-10 logarithm of x.")
#ifdef MPW_3_1 /* This hack is needed for MPW 3.1 but not for 3.2 ... */
FUNC2(pow, power,
"pow(x,y)\n\nReturn x**y (x to the power of y).")
@ -231,6 +228,69 @@ static char math_modf_doc [] =
"Return the fractional and integer parts of x. Both results carry the sign\n"
"of x. The integer part is returned as a real.";
/* A decent logarithm is easy to compute even for huge longs, but libm can't
do that by itself -- loghelper can. func is log or log10, and name is
"log" or "log10". Note that overflow isn't possible: a long can contain
no more than INT_MAX * SHIFT bits, so has value certainly less than
2**(2**64 * 2**16) == 2**2**80, and log2 of that is 2**80, which is
small enough to fit in an IEEE single. log and log10 are even smaller.
*/
static PyObject*
loghelper(PyObject* args, double (*func)(double), char *name)
{
PyObject *arg;
char format[16];
/* See whether this is a long. */
format[0] = 'O';
format[1] = ':';
strcpy(format + 2, name);
if (! PyArg_ParseTuple(args, format, &arg))
return NULL;
/* If it is long, do it ourselves. */
if (PyLong_Check(arg)) {
double x;
int e;
x = _PyLong_AsScaledDouble(arg, &e);
if (x <= 0.0) {
PyErr_SetString(PyExc_ValueError,
"math domain error");
return NULL;
}
/* Value is ~= x * 2**(e*SHIFT), so the log ~=
log(x) + log(2) * e * SHIFT.
CAUTION: e*SHIFT may overflow using int arithmetic,
so force use of double. */
x = func(x) + func(2.0) * (double)e * (double)SHIFT;
return PyFloat_FromDouble(x);
}
/* Else let libm handle it by itself. */
format[0] = 'd';
return math_1(args, func, format);
}
static PyObject *
math_log(PyObject *self, PyObject *args)
{
return loghelper(args, log, "log");
}
static char math_log_doc[] =
"log(x) -> the natural logarithm (base e) of x.";
static PyObject *
math_log10(PyObject *self, PyObject *args)
{
return loghelper(args, log10, "log10");
}
static char math_log10_doc[] =
"log10(x) -> the base 10 logarithm of x.";
static PyMethodDef math_methods[] = {
{"acos", math_acos, METH_VARARGS, math_acos_doc},
{"asin", math_asin, METH_VARARGS, math_asin_doc},