A Revised Rational Proposal

Dan Bishop danb_83 at yahoo.com
Sun Dec 26 21:05:23 CET 2004


Steven Bethard wrote:
> Dan Bishop wrote:
> > Mike Meyer wrote:
> >>
> >>PEP: XXX
> >
> > I'll be the first to volunteer an implementation.
>
> Very cool.  Thanks for the quick work!
>
> For stdlib acceptance, I'd suggest a few cosmetic changes:

No problem.

"""Implementation of rational arithmetic."""

from __future__ import division

import decimal as decimal
import math as _math

def _gcf(a, b):
"""Returns the greatest common factor of a and b."""
a = abs(a)
b = abs(b)
while b:
a, b = b, a % b
return a

class Rational(object):
"""This class provides an exact representation of rational numbers.

All of the standard arithmetic operators are provided.  In
mixed-type
expressions, an int or a long can be converted to a Rational
without
loss of precision, and will be done as such.

Rationals can be implicity (using binary operators) or explicity
(using float(x) or x.decimal()) converted to floats or Decimals;
this may cause a loss of precision.  The reverse conversions can be
done without loss of precision, and are performed with the
from_exact_float and from_exact decimal static methods.  However,
because of rounding error in the original values, this tends to
produce
"ugly" fractions.  "Nicer" conversions to Rational can be made with
approx_smallest_denominator or approx_smallest_error.
"""
def __init__(self, numerator, denominator=1):
"""Contructs the Rational object for numerator/denominator."""
if not isinstance(numerator, (int, long)):
raise TypeError('numerator must have integer type')
if not isinstance(denominator, (int, long)):
raise TypeError('denominator must have integer type')
if not denominator:
raise ZeroDivisionError('rational construction')
factor = _gcf(numerator, denominator)
self._n = numerator // factor
self._d = denominator // factor
if self._d < 0:
self._n = -self._n
self._d = -self._d
def __repr__(self):
if self._d == 1:
return "Rational(%d)" % self._n
else:
return "Rational(%d, %d)" % (self._n, self._d)
def __str__(self):
if self._d == 1:
return str(self._n)
else:
return "%d/%d" % (self._n, self._d)
def __hash__(self):
try:
return hash(float(self))
except OverflowError:
return hash(long(self))
def __float__(self):
return self._n / self._d
def __int__(self):
if self._n < 0:
return -int(-self._n // self._d)
else:
return int(self._n // self._d)
def __long__(self):
return long(int(self))
def __nonzero__(self):
return bool(self._n)
def __pos__(self):
return self
def __neg__(self):
return Rational(-self._n, self._d)
def __abs__(self):
if self._n < 0:
return -self
else:
return self
def __add__(self, other):
if isinstance(other, Rational):
return Rational(self._n * other._d + self._d * other._n,
self._d * other._d)
elif isinstance(other, (int, long)):
return Rational(self._n + self._d * other, self._d)
elif isinstance(other, (float, complex)):
return float(self) + other
elif isinstance(other, _decimal.Decimal):
return self.decimal() + other
else:
return NotImplemented
__radd__ = __add__
def __sub__(self, other):
if isinstance(other, Rational):
return Rational(self._n * other._d - self._d * other._n,
self._d * other._d)
elif isinstance(other, (int, long)):
return Rational(self._n - self._d * other, self._d)
elif isinstance(other, (float, complex)):
return float(self) - other
elif isinstance(other, _decimal.Decimal):
return self.decimal() - other
else:
return NotImplemented
def __rsub__(self, other):
if isinstance(other, (int, long)):
return Rational(other * self._d - self._n, self._d)
elif isinstance(other, (float, complex)):
return other - float(self)
elif isinstance(other, _decimal.Decimal):
return other - self.decimal()
else:
return NotImplemented
def __mul__(self, other):
if isinstance(other, Rational):
return Rational(self._n * other._n, self._d * other._d)
elif isinstance(other, (int, long)):
return Rational(self._n * other, self._d)
elif isinstance(other, (float, complex)):
return float(self) * other
elif isinstance(other, _decimal.Decimal):
return self.decimal() * other
else:
return NotImplemented
__rmul__ = __mul__
def __truediv__(self, other):
if isinstance(other, Rational):
return Rational(self._n * other._d, self._d * other._n)
elif isinstance(other, (int, long)):
return Rational(self._n, self._d * other)
elif isinstance(other, (float, complex)):
return float(self) / other
elif isinstance(other, _decimal.Decimal):
return self.decimal() / other
else:
return NotImplemented
__div__ = __truediv__
def __rtruediv__(self, other):
if isinstance(other, (int, long)):
return Rational(other * self._d, self._n)
elif isinstance(other, (float, complex)):
return other / float(self)
elif isinstance(other, _decimal.Decimal):
return other / self.decimal()
else:
return NotImplemented
__rdiv__ = __rtruediv__
def __floordiv__(self, other):
truediv = self / other
if isinstance(truediv, Rational):
return truediv._n // truediv._d
else:
return truediv // 1
def __rfloordiv__(self, other):
return (other / self) // 1
def __mod__(self, other):
return self - self // other * other
def __rmod__(self, other):
return other - other // self * self
def _divmod__(self, other):
return self // other, self % other
def __cmp__(self, other):
if other == 0:
return cmp(self._n, 0)
else:
return cmp(self - other, 0)
def __pow__(self, other):
if isinstance(other, (int, long)):
if other < 0:
return Rational(self._d ** -other, self._n ** -other)
else:
return Rational(self._n ** other, self._d ** other)
else:
return float(self) ** other
def __rpow__(self, other):
return other ** float(self)
def decimal(self):
"""Return a Decimal approximation of self in the current
context."""
return _decimal.Decimal(self._n) / _decimal.Decimal(self._d)
@staticmethod
def from_exact_float(x):
"""Returns the exact Rational equivalent of x."""
mantissa, exponent = _math.frexp(x)
mantissa = int(mantissa * 2 ** 53)
exponent -= 53
if exponent < 0:
return Rational(mantissa, 2 ** (-exponent))
else:
return Rational(mantissa * 2 ** exponent)
@staticmethod
def from_exact_decimal(x):
"""Returns the exact Rational equivalent of x."""
sign, mantissa, exponent = x.as_tuple()
sign = (1, -1)[sign]
mantissa = sign * reduce(lambda a, b: 10 * a + b, mantissa)
if exponent < 0:
return Rational(mantissa, 10 ** (-exponent))
else:
return Rational(mantissa * 10 ** exponent)
@staticmethod
def approx_smallest_denominator(x, tolerance):
"""Returns a Rational approximation of x.
Minimizes the denominator given a constraint on the error.

x = the float or Decimal value to convert
tolerance = maximum absolute error allowed,
must be of the same type as x
"""
tolerance = abs(tolerance)
n = 1
while True:
m = int(round(x * n))
result = Rational(m, n)
if abs(result - x) < tolerance:
return result
n += 1
@staticmethod
def approx_smallest_error(x, maxDenominator):
"""Returns a Rational approximation of x.
Minimizes the error given a constraint on the denominator.

x = the float or Decimal value to convert
maxDenominator = maximum denominator allowed
"""
result = None
minError = x
for n in xrange(1, maxDenominator + 1):
m = int(round(x * n))
r = Rational(m, n)
error = abs(r - x)
if error == 0:
return r
elif error < minError:
result = r
minError = error
return result

def divide(x, y):
"""Same as x/y, but returns a Rational if both are ints."""
if isinstance(x, (int, long)) and isinstance(y, (int, long)):
return Rational(x, y)
    else:
        return x / y




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