Author: raymond.hettinger
Date: Thu Feb 7 21:09:43 2008
New Revision: 60656
Modified:
python/branches/py3k/Doc/library/decimal.rst
Log:
Simplify float conversion recipe.
Modified: python/branches/py3k/Doc/library/decimal.rst
==============================================================================
--- python/branches/py3k/Doc/library/decimal.rst (original)
+++ python/branches/py3k/Doc/library/decimal.rst Thu Feb 7 21:09:43 2008
@@ -1516,39 +1516,29 @@
A. Yes, all binary floating point numbers can be exactly expressed as a
Decimal. An exact conversion may take more precision than intuition would
-suggest, so trapping :const:`Inexact` will signal a need for more precision::
+suggest, so we trap :const:`Inexact` to signal a need for more precision::
- def floatToDecimal(f):
- "Convert a floating point number to a Decimal with no loss of
information"
- # Transform (exactly) a float to a mantissa (0.5 <= abs(m) < 1.0) and an
- # exponent. Double the mantissa until it is an integer. Use the
integer
- # mantissa and exponent to compute an equivalent Decimal. If this
cannot
- # be done exactly, then retry with more precision.
-
- mantissa, exponent = math.frexp(f)
- while mantissa != int(mantissa):
- mantissa *= 2.0
- exponent -= 1
- mantissa = int(mantissa)
-
- oldcontext = getcontext()
- setcontext(Context(traps=[Inexact]))
- try:
- while True:
- try:
- return mantissa * Decimal(2) ** exponent
- except Inexact:
- getcontext().prec += 1
- finally:
- setcontext(oldcontext)
+ def float_to_decimal(f):
+ "Convert a floating point number to a Decimal with no loss of
information"
+ n, d = f.as_integer_ratio()
+ with localcontext() as ctx:
+ ctx.traps[Inexact] = True
+ while True:
+ try:
+ return Decimal(n) / Decimal(d)
+ except Inexact:
+ ctx.prec += 1
-Q. Why isn't the :func:`floatToDecimal` routine included in the module?
+ >>> float_to_decimal(math.pi)
+ Decimal("3.141592653589793115997963468544185161590576171875")
+
+Q. Why isn't the :func:`float_to_decimal` routine included in the module?
A. There is some question about whether it is advisable to mix binary and
decimal floating point. Also, its use requires some care to avoid the
representation issues associated with binary floating point::
- >>> floatToDecimal(1.1)
+ >>> float_to_decimal(1.1)
Decimal("1.100000000000000088817841970012523233890533447265625")
Q. Within a complex calculation, how can I make sure that I haven't gotten a
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