|
Data Structures and Algorithms
with Object-Oriented Design Patterns in Python |
There is a special family of operators for comparing sets.
Consider two sets, say S and T.
We say that S is a subset of T,
written 
,
if every element of S is also an element of T.
If there is at least one element of T that is not also an element of S,
we say that S is a
proper subset of T,
written 
.
We can also reverse the order in which the expressions are written
to get 
or 
,
which indicates that T is a
(proper) superset
of S.
The set comparison operators follow the rule that if
and 
then 
,
which is analogous to a similar property of numbers:
.
However, set comparison is unlike numeric comparison in that there
exist sets S and T for which neither nor !
For example, clearly this is the case for 
and 
.
Mathematically, the relation 
is called
a partial order
because there exist some pairs of sets for which neither
nor holds;
whereas the relation 
(among integers, say) is a total order.
Program 
defines the methods __eq__ and __le__.
The former tests for equality
and the latter determines whether the relation holds
between self and set.
Both operators return a bool result.
The worst-case running time of each of these operations is clearly O(N).
Program: SetAsArray class __eq__ and __le__ methods.
A complete repertoire of comparison methods would also include
methods to compute 
, 
, 
, and 
.
These operations follow directly from the implementation
shown in Program (Exercise ).
Copyright © 2003, 2004 by Bruno R. Preiss, P.Eng. All rights reserved.