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55 | <H3><A NAME="SECTION007651000000000000000"> | |
56 | 5.6.5.1 Mitigating round-off error with increased precision</A> | |
57 | </H3> | |
58 | ||
59 | <P> | |
60 | The use of decimal floating point eliminates decimal representation error | |
61 | (making it possible to represent <tt class="constant">0.1</tt> exactly); however, some | |
62 | operations can still incur round-off error when non-zero digits exceed the | |
63 | fixed precision. | |
64 | ||
65 | <P> | |
66 | The effects of round-off error can be amplified by the addition or subtraction | |
67 | of nearly offsetting quantities resulting in loss of significance. Knuth | |
68 | provides two instructive examples where rounded floating point arithmetic with | |
69 | insufficient precision causes the breakdown of the associative and | |
70 | distributive properties of addition: | |
71 | ||
72 | <P> | |
73 | <div class="verbatim"><pre> | |
74 | # Examples from Seminumerical Algorithms, Section 4.2.2. | |
75 | >>> from decimal import * | |
76 | >>> getcontext().prec = 8 | |
77 | ||
78 | >>> u, v, w = Decimal(11111113), Decimal(-11111111), Decimal('7.51111111') | |
79 | >>> (u + v) + w | |
80 | Decimal("9.5111111") | |
81 | >>> u + (v + w) | |
82 | Decimal("10") | |
83 | ||
84 | >>> u, v, w = Decimal(20000), Decimal(-6), Decimal('6.0000003') | |
85 | >>> (u*v) + (u*w) | |
86 | Decimal("0.01") | |
87 | >>> u * (v+w) | |
88 | Decimal("0.0060000") | |
89 | </pre></div> | |
90 | ||
91 | <P> | |
92 | The <tt class="module">decimal</tt> module makes it possible to restore the identities | |
93 | by expanding the precision sufficiently to avoid loss of significance: | |
94 | ||
95 | <P> | |
96 | <div class="verbatim"><pre> | |
97 | >>> getcontext().prec = 20 | |
98 | >>> u, v, w = Decimal(11111113), Decimal(-11111111), Decimal('7.51111111') | |
99 | >>> (u + v) + w | |
100 | Decimal("9.51111111") | |
101 | >>> u + (v + w) | |
102 | Decimal("9.51111111") | |
103 | >>> | |
104 | >>> u, v, w = Decimal(20000), Decimal(-6), Decimal('6.0000003') | |
105 | >>> (u*v) + (u*w) | |
106 | Decimal("0.0060000") | |
107 | >>> u * (v+w) | |
108 | Decimal("0.0060000") | |
109 | </pre></div> | |
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