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Tim Peters writes: 

1. Fixes float divmod so that the quotient it returns is always an integral
value.

2. Fixes float % and float divmod so that the remainder always gets the
right sign (the current code uses a "are the signs different?" test that
doesn't work half the time <wink> when the product of the divisor and the
remainder underflows to 0).
This commit is contained in:
Guido van Rossum 1999-05-06 14:26:34 +00:00
parent 8e40759d5a
commit 9263e78ff2
1 changed files with 19 additions and 7 deletions
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26
Objects/floatobject.c
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@ -359,7 +359,7 @@ float_rem(v, w)
PyFloatObject *w; PyFloatObject *w;
{ {
double vx, wx; double vx, wx;
double /* div, */ mod; double mod;
wx = w->ob_fval; wx = w->ob_fval;
if (wx == 0.0) { if (wx == 0.0) {
PyErr_SetString(PyExc_ZeroDivisionError, "float modulo"); PyErr_SetString(PyExc_ZeroDivisionError, "float modulo");
@ -368,10 +368,10 @@ float_rem(v, w)
PyFPE_START_PROTECT("modulo", return 0) PyFPE_START_PROTECT("modulo", return 0)
vx = v->ob_fval; vx = v->ob_fval;
mod = fmod(vx, wx); mod = fmod(vx, wx);
/* div = (vx - mod) / wx; */ /* note: checking mod*wx < 0 is incorrect -- underflows to
if (wx*mod < 0) { 0 if wx < sqrt(smallest nonzero double) */
if (mod && ((wx < 0) != (mod < 0))) {
mod += wx; mod += wx;
/* div -= 1.0; */
} }
PyFPE_END_PROTECT(mod) PyFPE_END_PROTECT(mod)
return PyFloat_FromDouble(mod); return PyFloat_FromDouble(mod);
@ -383,7 +383,7 @@ float_divmod(v, w)
PyFloatObject *w; PyFloatObject *w;
{ {
double vx, wx; double vx, wx;
double div, mod; double div, mod, floordiv;
wx = w->ob_fval; wx = w->ob_fval;
if (wx == 0.0) { if (wx == 0.0) {
PyErr_SetString(PyExc_ZeroDivisionError, "float divmod()"); PyErr_SetString(PyExc_ZeroDivisionError, "float divmod()");
@ -392,13 +392,25 @@ float_divmod(v, w)
PyFPE_START_PROTECT("divmod", return 0) PyFPE_START_PROTECT("divmod", return 0)
vx = v->ob_fval; vx = v->ob_fval;
mod = fmod(vx, wx); mod = fmod(vx, wx);
/* fmod is typically exact, so vx-mod is *mathemtically* an
exact multiple of wx. But this is fp arithmetic, and fp
vx - mod is an approximation; the result is that div may
not be an exact integral value after the division, although
it will always be very close to one.
*/
div = (vx - mod) / wx; div = (vx - mod) / wx;
if (wx*mod < 0) { /* note: checking mod*wx < 0 is incorrect -- underflows to
0 if wx < sqrt(smallest nonzero double) */
if (mod && ((wx < 0) != (mod < 0))) {
mod += wx; mod += wx;
div -= 1.0; div -= 1.0;
} }
/* snap quotient to nearest integral value */
floordiv = floor(div);
if (div - floordiv > 0.5)
floordiv += 1.0;
PyFPE_END_PROTECT(div) PyFPE_END_PROTECT(div)
return Py_BuildValue("(dd)", div, mod); return Py_BuildValue("(dd)", floordiv, mod);
} }
static double powu(x, n) static double powu(x, n)