Data Structures and Algorithm Analysis - Computer Science at ...

Sec. 4.4 Dictionaries 135/** Container class for a key-value pair */class KVpair {private Key k;private E e;}/** Constructors */KVpair(){ k = null; e = null; }KVpair(Key kval, E eval){ k = kval; e = eval; }/** Data member access functions */public Key key() { return k; }public E value() { return e; }Figure 4.32 Implementation for a class representing a key-value pair.The fundamental issue is that the key value for a record is not an intrinsic propertyof the record’s class, or of any field within the class. The key for a record isactually a property of the context in which the record is used.A truly general alternative is to explicitly store the key associated with a givenrecord, as a separate field in the dictionary. That is, each entry in the dictionarywill contain both a record and its associated key. Such entries are known as keyvaluepairs. It is typical that storing the key explicitly duplicates some field in therecord. However, keys tend to be much smaller than records, so this additionalspace overhead will not be great. A simple class for representing key-value pairsis shown in Figure 4.32. The insert method of the dictionary class supports thekey-value pair implementation because it takes two parameters, a record and itsassociated key for that dictionary.Now that we have defined the dictionary ADT and settled on the design approachof storing key-value pairs for our dictionary entries, we are ready to considerways to implement it. Two possibilities would be to use an array-based or linkedlist. Figure 4.33 shows an implementation for the dictionary using an (unsorted)array-based list.Examining class UALdict (UAL stands for “unsorted array-based list), we caneasily see that insert is a constant-time operation, because it simply inserts thenew record at the end of the list. However, find, and remove both require Θ(n)time in the average and worst cases, because we need to do a sequential search.Method remove in particular must touch every record in the list, because once thedesired record is found, the remaining records must be shifted down in the list tofill the gap. Method removeAny removes the last record from the list, so this is aconstant-time operation.on the keyword list triggered this appearance of the record. Thus, we cannot write a function thatextracts the key from such a record.