Index handler: Difference between revisions

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== Abstract view ==
== Representation invariant ==


At any time, an object of an implementation of this abstract data structure is repesented by a positive integral number <math>N</math> and a subset <math>I</math> of <math>\{1,\ldots,N\}</math>, the '''current indexes'''.


'''Abstract data structure:'''
== Method ==
A variation of [[Sets and sequences#Maps|map]], where:
# The number of elements is bounded by some fixed positive [[Numbers#natural numbers|natural number]] <math>N</math>.
# The key type is the range <math>\{1,\ldots,N\}</math>.
# Instead of a method for inserting (key,value)-pairs, a value can be inserted alone, and the key for referring to the value is returned. As a precondition, less than <math>N</math> values must be currently stored in that index handler object.
# The methods to retrieve a value for a key and to remove a key and the associated value from the index handler object are basically identical to the corresponding methods of normal maps. A key may be reused by the index handler object after removal.


'''Implementation invariant:'''
'''Name:''' reserve index
# There is an array <math>A</math> with index range <math>1,...,N</math>, and the component type is the value type of the map.
# There is a [[Sets and sequences|set]] <math>U</math> (for unused) of [[Numbers#natural numbers|natural numbers]].
# For each <math>i\in\{1,...,N_\text{max}\}</math> not in <math>U</math>, <math>A[i]</math> is the value associated with key <math>i</math>.


== Methods ==
'''Precondition:''' <math>|I|<N</math>.


'''Return value:''' One of the indexes not in <math>I</math>.


# The constructor takes <math>N</math> as an argument, creates <math>A</math> and <math>U</math>, and stores all potential keys <math>1,\ldots,N</math> in <math>U</math>.
'''Postcondition:''' The returned index is inserted in <math>I</math>.
# The method for inserting a new value extracts a key <math>i</math> from <math>U</math>, stores the value at <mah>A[i]</math>and returns <math>i</math>.
 
# The method to look up a value for a given key <math>i</math> returns <math>A[i]</math>.
== Method ==
# The method to remove a key re-inserts the key in <math>U</math>.
 
'''Name:''' release index

Revision as of 13:32, 20 October 2014

Representation invariant

At any time, an object of an implementation of this abstract data structure is repesented by a positive integral number [math]\displaystyle{ N }[/math] and a subset [math]\displaystyle{ I }[/math] of [math]\displaystyle{ \{1,\ldots,N\} }[/math], the current indexes.

Method

Name: reserve index

Precondition: [math]\displaystyle{ |I|\lt N }[/math].

Return value: One of the indexes not in [math]\displaystyle{ I }[/math].

Postcondition: The returned index is inserted in [math]\displaystyle{ I }[/math].

Method

Name: release index

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