DEMYSTIFYING PROGRAMMING: CHAPTER SIXTEEN ADD AND SEARCH AN ARRAY (TOC DETAILED)

Transcription

1 DEMYSTIFYING PROGRAMMING: CHAPTER SIXTEEN ADD AND SEARCH AN ARRAY (TOC DETAILED) Chapter Sixteen: Add and Search an Array... 1 Objectives Design and Implementation... 1 An Array-based List can include any class... 1 Add and Search an Array-based List Add... 1 Inline Exercise: Search... 2 Inline Exercise:... 3 Comparing Objects... 3 Inline Exercise:... 4 Challenge: Patterns... 4 The add pattern... 4 The search pattern Exercises... 4 Design Exercises... 4 Complete the Code Exercises... 4 Code to Mend... 5 Code Exercises... 5 Index... 5

2 Chapter Sixteen: Add and Search an Array Objectives Can a List contain class objects? How are elements of an array ordered? How can an element be found in an array? 16.1 Design and Implementation An Array-based List can include any class An array is a collection of homogeneous elements in a linear order. An array may be a collection of integers, names, or even dogs. The class Dog includes three data items (name, breed, and age). Therefore, an array of Dog class elements would include at position 0 three pieces of information (a name, a breed, and an age). These three items are a single element of Dog class stored in a single position of the array. To declare an array of Dog type is as easy as declaring it for any other type. The following declares, instantiates, and stores a new Dog object in an array of Dog type. Dog[] mydogs; mydogs = new Dog[10]; mydogs[0] = Dog( Fido, Labrador, 4); Add and Search an Array-based List It is becoming clear that although there are many similarities between a List as a data structure and an array as a language structure, the two are not identical. A List is responsible for keeping track of its top and its end point. A List cannot allow any gaps in its list. When a List is supported with an array, the array may have several empty positions until its capacity is full Add Keeping track of the top of the array-based List is easy. The top of a List is the 0 th position of the array. However, the bottom of the List will not be the length of the array unless the array is full. So the programmer must add a variable to track the count of items on the list. This count is the number of items on the list and points to the next available position in the array for adding to the list. Adding to the list means adding to the position indicated by the count variable and incrementing it. In this way count indicates first where to place the next item and after incrementation shows the new count or correct number of items on the list. To code this solution using an array requires the creating of an empty array and a variable initialized to zero, count. The example NameList.java illustrates this process. Study NameList.java for more details. Demystifying Programming Search, Remove, Sort Page 1

3 Figure 1: NameList.java Add method The method addone gets the user s information from the textfield and assigns it to the next available position in the array-based List, increments the counter, and clears the user s typing from the textfield. Inline Exercise: Copy TestDog.java from CH15 to a CH16 folder and create an array of Dog type in TestDog.java. Add and initialize a counter to 0. In the actionperformed, create a new Dog using information provided by the user. Place this new dog at position counter and increment the counter. When show is pressed use a for-loop to show the entire array. Since an additional Show button and method will be necessary, add a new JPanel for buttons and the btnshow and showall method. Theoretically, a List can contain and infinite number of items. In practice, every list has some limit on the number of items it can contain because of its implementation. In an array-based list, the size of the list is limited to the length of the array. The length of the array is provided at the time the array is instantiated. In NameList, the namelist array is an instance of length 10. Therefore, no more than ten items can be placed on the list. What happens if the user attempts to place more than ten items on the list? Experiment and find out. Notice the command prompt (black box) under your applet if you are running code in Appletviewer/Textpad. To prevent this error from occurring the programmer must verify that count is not greater than length BEFORE attempting to add a new element. The most efficient time to make this test is immediately before calling addone in the event handler. Figure 2: namelist is full 16.3 Search After adding a few more names to namelist it is time to search the name list for a specific key (name). Which name should be searched for? That is a choice we want the user to make through a textfield. Once we have the key item to search for, beginning at the top search one item at a time until a match is found or the search reaches the count. If the search reaches the count, the item was not found. Moving through the List from top to bottom is called iteration. Iteration is easy with an array-based list. The showall method of NameList already does this iteration. Figure 3: Iteration with showall (NameList.java) Demystifying Programming Search, Remove, Sort Page 2

4 Observe this linear search in NameList.java. Figure 4: Linear search in NameList.java 1. The search method must first have a key as the goal of the search. 2. It must have a mechanism for iterating through the array. 3. It must compare the key to the current item of the array. 4. It must signal the position at which the item was found or not. 1. The key is gained on line Iteration uses the for-loop on line Matching is done with if condition on line The return statements signal position or failure (-1). Most of this process is familiar from programming previously done. Step 3, although it may look strange, is familiar too. To compare int or float type use the comparative operator = = (double equals). To compare two Strings, the comparative operator equals is used on line 91. The operator is dotted to the first String, and the second String is placed in the operator s parentheses. Other String operators are available, but this one servers our purpose. If an identical match is found, the position of the matching item is immediately returned and the search terminates. If the iterator moves through the entire list without finding a match, the loop-terminates and the return of -1 signals failure. This linear search termination method assumes no duplicates will be in the list. Search returns either -1 for failure or an integer greater than -1 as the position at which the key is located. How can this return value be used? What a programmer does with this returned position or failure integer is a matter of choice. The code in NameList.java stores the returned int as position and displays relevant information in the textarea. Figure 5: btnfind from actionperformed of NameList.java Inline Exercise: Copy Sales.java from chapter 10 and modify it so that it has a JPanel for buttons and a btnfind. Add a search method. The comparative operator for numbers is not.equals( ). Comparing Objects When an object represents a complex cluster of information, the programmer must make a decision regarding which attribute should be compared. The Dog class used earlier included attributes for name, breed, and age. When search through a list of Dog objects, it is necessary to decide which attribute should be compared. If the programmer wishes to search by name, the getname method will be used in the comparison. If a search by breed is required, the getbreed method will be used in the comparison. If searching by name the txtname.gettext method captures the name, and it is stored in String key. To compare this key with the name attribute of the Dog object stored at each location the Dog s method getname will be required. Demystifying Programming Search, Remove, Sort Page 3

5 if(doglist[x].getname().equals(key)) The equals String operator is used to compare two Strings. The Dog getname method returns a String, the name of the dog object located at the current x. In database theory a primary key is used to distinquish each tuple (row or object) from every other. If your object includes an attribute or field that serves as primary key, compare on the basis of this unique attribute. This discussion assumes that the list contains no duplicate entries. What will happen if the list contains duplicates? Inline Exercise: Use TestDog.java from the previous Inline exercise. Add a button for search and a search method. This method will use getname to match the key. Since name is a String, this will require the String comparative operation, equals. Challenge: Save TestDog.java as TestDogBreed.java. Add a second button and search method to search by breed Patterns The add pattern doglist[count] = new Dog(name, breed, age); count++; 1. create new item 2. store at count position 3. increment count The search pattern String key = txtname.gettext(); for(int x=top; x<count; x++){ if(doglist[x].getname().equals(key)) return x; } return -1; } 1. establish key 2. step through array 3. match item to key 4. if found return position 5. if not found return Exercises Design Exercises 1. Design a class called Inventory which will include ID, Name, OnHand as count of those in items in inventory. This class should have the appropriate constructor and observer methods. Also add a method to transform the OnHand value. 2. Design a class called WareHouse which will allow users to create new Inventory items. This is a GUI class and provides buttons for Add, Show, and Sort. Complete the Code Exercises 3. Modify the WareHouse.java file to include the ability to add a new inventory item. 4. Modify the WareHouse.java file to include a showall method. Demystifying Programming Search, Remove, Sort Page 4

6 Code to Mend 5. The file TownList.java is missing several code segments. The statements //missing code indicates where they should be. Please mend this code and test it. Code Exercises 6. If you have a course project to which these methods could be applied, do it. If you need to create a small project to which to apply these methods, do it. Index actionperformed...2, 3 add...1, 2, 4 array...1, 2, 3 attribute...3 attributes...3 condition...3 constructor...4 counter...2 data structure...1 decision...3 declare...1 double...3 element...1, 2 elements...1 equals...3, 4 event...2 event handler...2 float...3 for-loop...2, 3 gettext... 3 GUI... 4 initialize... 2 int 3 iterator... 3 JPanel... 2, 3 length... 1, 2 List... 1, 2 loop... 3 methods... 4, 5 new...1, 2, 4 observer... 4 parentheses... 3 process... 1, 3 return... 3 search...2, 3, 4 String... 3, 4 variable... 1 Demystifying Programming Search, Remove, Sort Page 5