From 4631481dfdffab442f1295342c1c4e2663759060 Mon Sep 17 00:00:00 2001 From: Raymond Hettinger Date: Thu, 14 Feb 2008 10:46:57 +0000 Subject: Add fixed-point examples to the decimal FAQ --- Doc/library/decimal.rst | 33 ++++++++++++++++++++++++++++++--- 1 file changed, 30 insertions(+), 3 deletions(-) diff --git a/Doc/library/decimal.rst b/Doc/library/decimal.rst index 977e95d..7af200c 100644 --- a/Doc/library/decimal.rst +++ b/Doc/library/decimal.rst @@ -1565,9 +1565,36 @@ the :const:`Inexact` trap is set, it is also useful for validation:: Q. Once I have valid two place inputs, how do I maintain that invariant throughout an application? -A. Some operations like addition and subtraction automatically preserve fixed -point. Others, like multiplication and division, change the number of decimal -places and need to be followed-up with a :meth:`quantize` step. +A. Some operations like addition, subtraction, and multiplication by an integer +will automatically preserve fixed point. Others operations, like division and +non-integer multiplication, will change the number of decimal places and need to +be followed-up with a :meth:`quantize` step. + + >>> a = Decimal('102.72') # Initial fixed-point values + >>> b = Decimal('3.17') + >>> a + b # Addition preserves fixed-point + Decimal('105.89') + >>> a - b + Decimal('99.55') + >>> a * 42 # So does integer multiplication + Decimal('4314.24') + >>> (a * b).quantize(TWOPLACES) # Must quantize non-integer multiplication + Decimal('325.62') + >>> (b / a).quantize(TWOPLACES) # And quantize divisions + Decimal('0.03') + +In developing fixed-point applications, it is convenient to define functions +to handle the :meth:`quantize` step:: + + def mul(x, y, fp=TWOPLACES): + return (x * y).quantize(fp) + def div(x, y, fp=TWOPLACES): + return (x / y).quantize(fp) + + >>> mul(a, b) # Automatically preserve fixed-point + Decimal('325.62') + >>> div(b, a) + Decimal('0.03') Q. There are many ways to express the same value. The numbers :const:`200`, :const:`200.000`, :const:`2E2`, and :const:`.02E+4` all have the same value at -- cgit v0.12