From 1124e71368c73b592020b4896fb1c5d371efbcef Mon Sep 17 00:00:00 2001 From: Mark Dickinson Date: Wed, 28 Jan 2009 21:25:58 +0000 Subject: [PATCH] Issue #4707: round(x, n) now returns an integer when x is an integer. Previously it returned a float. --- Lib/test/test_builtin.py | 6 +- Lib/test/test_long.py | 75 ++++++++++++++++++++ Misc/NEWS | 3 + Objects/longobject.c | 148 +++++++++++++++++++++++++++++++++------ Python/bltinmodule.c | 18 +++-- 5 files changed, 217 insertions(+), 33 deletions(-) diff --git a/Lib/test/test_builtin.py b/Lib/test/test_builtin.py index 99785dc30e6..284f07e2ec0 100644 --- a/Lib/test/test_builtin.py +++ b/Lib/test/test_builtin.py @@ -1068,9 +1068,9 @@ class BuiltinTest(unittest.TestCase): self.assertEqual(round(8), 8) self.assertEqual(round(-8), -8) self.assertEqual(type(round(0)), int) - self.assertEqual(type(round(-8, -1)), float) - self.assertEqual(type(round(-8, 0)), float) - self.assertEqual(type(round(-8, 1)), float) + self.assertEqual(type(round(-8, -1)), int) + self.assertEqual(type(round(-8, 0)), int) + self.assertEqual(type(round(-8, 1)), int) # test new kwargs self.assertEqual(round(number=-8.0, ndigits=-1), -10.0) diff --git a/Lib/test/test_long.py b/Lib/test/test_long.py index 2acb3fa240b..0e070902ece 100644 --- a/Lib/test/test_long.py +++ b/Lib/test/test_long.py @@ -896,6 +896,81 @@ class LongTest(unittest.TestCase): self.assertEqual((a+1).bit_length(), i+1) self.assertEqual((-a-1).bit_length(), i+1) + def test_round(self): + # check round-half-even algorithm. For round to nearest ten; + # rounding map is invariant under adding multiples of 20 + test_dict = {0:0, 1:0, 2:0, 3:0, 4:0, 5:0, + 6:10, 7:10, 8:10, 9:10, 10:10, 11:10, 12:10, 13:10, 14:10, + 15:20, 16:20, 17:20, 18:20, 19:20} + for offset in range(-520, 520, 20): + for k, v in test_dict.items(): + got = round(k+offset, -1) + expected = v+offset + self.assertEqual(got, expected) + self.assert_(type(got) is int) + + # larger second argument + self.assertEqual(round(-150, -2), -200) + self.assertEqual(round(-149, -2), -100) + self.assertEqual(round(-51, -2), -100) + self.assertEqual(round(-50, -2), 0) + self.assertEqual(round(-49, -2), 0) + self.assertEqual(round(-1, -2), 0) + self.assertEqual(round(0, -2), 0) + self.assertEqual(round(1, -2), 0) + self.assertEqual(round(49, -2), 0) + self.assertEqual(round(50, -2), 0) + self.assertEqual(round(51, -2), 100) + self.assertEqual(round(149, -2), 100) + self.assertEqual(round(150, -2), 200) + self.assertEqual(round(250, -2), 200) + self.assertEqual(round(251, -2), 300) + self.assertEqual(round(172500, -3), 172000) + self.assertEqual(round(173500, -3), 174000) + self.assertEqual(round(31415926535, -1), 31415926540) + self.assertEqual(round(31415926535, -2), 31415926500) + self.assertEqual(round(31415926535, -3), 31415927000) + self.assertEqual(round(31415926535, -4), 31415930000) + self.assertEqual(round(31415926535, -5), 31415900000) + self.assertEqual(round(31415926535, -6), 31416000000) + self.assertEqual(round(31415926535, -7), 31420000000) + self.assertEqual(round(31415926535, -8), 31400000000) + self.assertEqual(round(31415926535, -9), 31000000000) + self.assertEqual(round(31415926535, -10), 30000000000) + self.assertEqual(round(31415926535, -11), 0) + self.assertEqual(round(31415926535, -12), 0) + self.assertEqual(round(31415926535, -999), 0) + + # should get correct results even for huge inputs + for k in range(10, 100): + got = round(10**k + 324678, -3) + expect = 10**k + 325000 + self.assertEqual(got, expect) + self.assert_(type(got) is int) + + # nonnegative second argument: round(x, n) should just return x + for n in range(5): + for i in range(100): + x = random.randrange(-10000, 10000) + got = round(x, n) + self.assertEqual(got, x) + self.assert_(type(got) is int) + for huge_n in 2**31-1, 2**31, 2**63-1, 2**63, 2**100, 10**100: + self.assertEqual(round(8979323, huge_n), 8979323) + + # omitted second argument + for i in range(100): + x = random.randrange(-10000, 10000) + got = round(x) + self.assertEqual(got, x) + self.assert_(type(got) is int) + + # bad second argument + bad_exponents = ('brian', 2.0, 0j, None) + for e in bad_exponents: + self.assertRaises(TypeError, round, 3, e) + + def test_main(): support.run_unittest(LongTest) diff --git a/Misc/NEWS b/Misc/NEWS index f591976f8d0..3a7fec51d3b 100644 --- a/Misc/NEWS +++ b/Misc/NEWS @@ -12,6 +12,9 @@ What's New in Python 3.1 alpha 0 Core and Builtins ----------------- +- Issue #4707: round(x, n) now returns an integer if x is an integer. + Previously it returned a float. + - Issue #4753: By enabling a configure option named '--with-computed-gotos' on compilers that support it (notably: gcc, SunPro, icc), the bytecode evaluation loop is compiled with a new dispatch mechanism which gives diff --git a/Objects/longobject.c b/Objects/longobject.c index 76986fd4ee1..5476e1978d7 100644 --- a/Objects/longobject.c +++ b/Objects/longobject.c @@ -3643,32 +3643,140 @@ long__format__(PyObject *self, PyObject *args) PyUnicode_GET_SIZE(format_spec)); } - static PyObject * long_round(PyObject *self, PyObject *args) { -#define UNDEF_NDIGITS (-0x7fffffff) /* Unlikely ndigits value */ - int ndigits = UNDEF_NDIGITS; - double x; - PyObject *res; - - if (!PyArg_ParseTuple(args, "|i", &ndigits)) - return NULL; + PyObject *o_ndigits=NULL, *temp; + PyLongObject *pow=NULL, *q=NULL, *r=NULL, *ndigits=NULL, *one; + int errcode; + digit q_mod_4; - if (ndigits == UNDEF_NDIGITS) + /* Notes on the algorithm: to round to the nearest 10**n (n positive), + the straightforward method is: + + (1) divide by 10**n + (2) round to nearest integer (round to even in case of tie) + (3) multiply result by 10**n. + + But the rounding step involves examining the fractional part of the + quotient to see whether it's greater than 0.5 or not. Since we + want to do the whole calculation in integer arithmetic, it's + simpler to do: + + (1) divide by (10**n)/2 + (2) round to nearest multiple of 2 (multiple of 4 in case of tie) + (3) multiply result by (10**n)/2. + + Then all we need to know about the fractional part of the quotient + arising in step (2) is whether it's zero or not. + + Doing both a multiplication and division is wasteful, and is easily + avoided if we just figure out how much to adjust the original input + by to do the rounding. + + Here's the whole algorithm expressed in Python. + + def round(self, ndigits = None): + """round(int, int) -> int""" + if ndigits is None or ndigits >= 0: + return self + pow = 10**-ndigits >> 1 + q, r = divmod(self, pow) + self -= r + if (q & 1 != 0): + if (q & 2 == r == 0): + self -= pow + else: + self += pow + return self + + */ + if (!PyArg_ParseTuple(args, "|O", &o_ndigits)) + return NULL; + if (o_ndigits == NULL) return long_long(self); - /* If called with two args, defer to float.__round__(). */ - x = PyLong_AsDouble(self); - if (x == -1.0 && PyErr_Occurred()) + ndigits = (PyLongObject *)PyNumber_Index(o_ndigits); + if (ndigits == NULL) return NULL; - self = PyFloat_FromDouble(x); - if (self == NULL) - return NULL; - res = PyObject_CallMethod(self, "__round__", "i", ndigits); + + if (Py_SIZE(ndigits) >= 0) { + Py_DECREF(ndigits); + return long_long(self); + } + + Py_INCREF(self); /* to keep refcounting simple */ + /* we now own references to self, ndigits */ + + /* pow = 10 ** -ndigits >> 1 */ + pow = (PyLongObject *)PyLong_FromLong(10L); + if (pow == NULL) + goto error; + temp = long_neg(ndigits); + Py_DECREF(ndigits); + ndigits = (PyLongObject *)temp; + if (ndigits == NULL) + goto error; + temp = long_pow((PyObject *)pow, (PyObject *)ndigits, Py_None); + Py_DECREF(pow); + pow = (PyLongObject *)temp; + if (pow == NULL) + goto error; + assert(PyLong_Check(pow)); /* check long_pow returned a long */ + one = (PyLongObject *)PyLong_FromLong(1L); + if (one == NULL) + goto error; + temp = long_rshift(pow, one); + Py_DECREF(one); + Py_DECREF(pow); + pow = (PyLongObject *)temp; + if (pow == NULL) + goto error; + + /* q, r = divmod(self, pow) */ + errcode = l_divmod((PyLongObject *)self, pow, &q, &r); + if (errcode == -1) + goto error; + + /* self -= r */ + temp = long_sub((PyLongObject *)self, r); Py_DECREF(self); - return res; -#undef UNDEF_NDIGITS + self = temp; + if (self == NULL) + goto error; + + /* get value of quotient modulo 4 */ + if (Py_SIZE(q) == 0) + q_mod_4 = 0; + else if (Py_SIZE(q) > 0) + q_mod_4 = q->ob_digit[0] & 3; + else + q_mod_4 = (PyLong_BASE-q->ob_digit[0]) & 3; + + if ((q_mod_4 & 1) == 1) { + /* q is odd; round self up or down by adding or subtracting pow */ + if (q_mod_4 == 1 && Py_SIZE(r) == 0) + temp = (PyObject *)long_sub((PyLongObject *)self, pow); + else + temp = (PyObject *)long_add((PyLongObject *)self, pow); + Py_DECREF(self); + self = temp; + if (self == NULL) + goto error; + } + Py_DECREF(q); + Py_DECREF(r); + Py_DECREF(pow); + Py_DECREF(ndigits); + return self; + + error: + Py_XDECREF(q); + Py_XDECREF(r); + Py_XDECREF(pow); + Py_XDECREF(self); + Py_XDECREF(ndigits); + return NULL; } static PyObject * @@ -3773,8 +3881,8 @@ static PyMethodDef long_methods[] = { {"__ceil__", (PyCFunction)long_long, METH_NOARGS, "Ceiling of an Integral returns itself."}, {"__round__", (PyCFunction)long_round, METH_VARARGS, - "Rounding an Integral returns itself.\n" - "Rounding with an ndigits arguments defers to float.__round__."}, + "Rounding an Integral returns itself.\n" + "Rounding with an ndigits argument also returns an integer."}, {"__getnewargs__", (PyCFunction)long_getnewargs, METH_NOARGS}, {"__format__", (PyCFunction)long__format__, METH_VARARGS}, {"__sizeof__", (PyCFunction)long_sizeof, METH_NOARGS, diff --git a/Python/bltinmodule.c b/Python/bltinmodule.c index f87fdd2a79d..5597bc728e5 100644 --- a/Python/bltinmodule.c +++ b/Python/bltinmodule.c @@ -1717,15 +1717,14 @@ For most object types, eval(repr(object)) == object."); static PyObject * builtin_round(PyObject *self, PyObject *args, PyObject *kwds) { -#define UNDEF_NDIGITS (-0x7fffffff) /* Unlikely ndigits value */ static PyObject *round_str = NULL; - int ndigits = UNDEF_NDIGITS; + PyObject *ndigits = NULL; static char *kwlist[] = {"number", "ndigits", 0}; PyObject *number, *round; - if (!PyArg_ParseTupleAndKeywords(args, kwds, "O|i:round", - kwlist, &number, &ndigits)) - return NULL; + if (!PyArg_ParseTupleAndKeywords(args, kwds, "O|O:round", + kwlist, &number, &ndigits)) + return NULL; if (Py_TYPE(number)->tp_dict == NULL) { if (PyType_Ready(Py_TYPE(number)) < 0) @@ -1746,15 +1745,14 @@ builtin_round(PyObject *self, PyObject *args, PyObject *kwds) return NULL; } - if (ndigits == UNDEF_NDIGITS) - return PyObject_CallFunction(round, "O", number); + if (ndigits == NULL) + return PyObject_CallFunction(round, "O", number); else - return PyObject_CallFunction(round, "Oi", number, ndigits); -#undef UNDEF_NDIGITS + return PyObject_CallFunction(round, "OO", number, ndigits); } PyDoc_STRVAR(round_doc, -"round(number[, ndigits]) -> floating point number\n\ +"round(number[, ndigits]) -> number\n\ \n\ Round a number to a given precision in decimal digits (default 0 digits).\n\ This returns an int when called with one argument, otherwise the\n\