Why do we use iteration 10. Iteration 1

Computer Science Dept Va Tech August 2002 ©2002 D Barnette, B Keller & P Schoenhoff

110. Iteration

Programming in Java

Why do we use iteration

! So far we’ve done projects with a very limited number of objects! What if an unknown number of data records?! What if you had an account management system that had to deal with 1000

transactions?

Couldn’t rely on fixed code segmentto process specific number of moves.

It would be silly to copy and paste code toprocess a transaction 1000 times.


210. Iteration

Programming in Java

Iteration Terms

! Definitions:Before considering specifics we define some general terms that apply to any iteration construct:

pass (or iteration) - one complete execution of the bodyloop entry - the point where flow of control passes to the bodyloop test - the point at which the decision is made to (re)enter

the body, or notloop exit - the point at which the iteration ends and control

passes to the next statement after the looptermination condition - the condition that causes the iteration to stop

When designing a loop, it is important to clearly identify each of these.

Understanding the termination condition, and what guarantees it will eventually occur, are particularly vital.


310. Iteration

Programming in Java

The While Statement

Syntax: while ( condition )body

1. Test condition2. if condition evaluates to false, terminate loop and

continue with statement after body3. if condition is true, execute body statement and

start over

IMPORTANT: The body must have the “power” to change the outcome of the condition!!!

Semantics:

while <bool expr>

<loop body>

true

false


410. Iteration

Programming in Java

The While Statement - variations

while ( someCondition )doSomething;

while (someOtherCondition) {doSomethingA;someObject.doSomethingB();doSomethingC;

}

while (someCondition);doSomethingElse;

Fine!

Fine!(notice thebrackets)

Problem!!Why?


510. Iteration

Programming in Java

Example problem:

! An input file stores employee work data. – In the file, one line will contain employee information, and the next will contain

the number of hours the employee worked. – There are an unknown number of employees.

! We can use the static method Employee.readIn(bufRdr)…to get employee data, and the non-static method

public String calcPay(int hours)

…to compute and return that employee’s pay.


610. Iteration

Programming in Java

Example – what’s wrong here?

int hours = 0;Employee e;

while ( e != null ) {e = Employee.readIn(br);

hours = Integer.parseInt(br.readLine());System.out.println(”Employee ” + e.getName() +

”has earned ” + e.calcPay(hours));}


710. Iteration

Programming in Java

Example – what’s wrong here?

int hours = 0;Employee e = Employee.readIn(br); // OK! Fixed!

while ( e != null ) {hours = Integer.parseInt(br.readLine());System.out.println(”Employee ” + e.getName() +

”has earned ” + e.calcPay(hours));}

Infinite loop!“e” never changes!


810. Iteration

Programming in Java

Example – this will work!

int hours = 0;Employee e = Employee.readIn(br);

while ( e != null ) {hours = Integer.parseInt(br.readLine());System.out.println(”Employee ” + e.getName() +

”has earned ” + e.calcPay(hours));e = Employee.readIn(br);

}

Called a“priming read”


910. Iteration

Programming in Java

Input Loop Pattern

int hours = 0;Employee e = Employee.readIn(br);while ( e != null ) {

hours = Integer.parseInt(br.readLine());System.out.println(”Employee ” + e.getName() +

”has earned ” + e.calcPay(hours));e = Employee.readIn(br);

}

First read input

Test input OK

Read input for next iteration

How does this loop terminate?


1010. Iteration

Programming in Java

Another Example

int i;String tmp;tmp = br.readLine();while ( tmp != null ) {

i = Integer.parseInt(tmp);System.out.print ( i * i );temp = br.readLine();

}

What does this code do, exactly ?


1110. Iteration

Programming in Java

Input Loop Pattern

read first itemwhile ( input is OK or item valid ) {

process itemread next item

}

Some observations:

- It is possible that the body of the loop will never be executed.

- It is possible that the loop test condition will become false in the middle of an iteration. Even so, the remainder of the loop body will be executed before the loop test is performed and iteration stops.

- The loop test must involve at least one variable whose value is (potentially) changed by at least one statement within the loop body.


1210. Iteration

Programming in Java

Example:

! Song library – files in the format:TitleArtistTitleArtist(etc…)

LaylaEric ClaptonRevolutionBeatlesYesterdayBeatlesHouse of the Rising SunThe AnimalsLike a Rolling StoneDylan, Bob(...)

File: “classicRock.lib”


1310. Iteration

Programming in Java

Example:

! The DJ wants to be able to query a song database by typing an artist and getting all matching songs.

! First bit of code:

public static Song read(BufferedReader br)

throws Exception {

String title = br.readLine();

if (title == null)

return null;

String artist = br.readLine();

return new Song(title, artist);

}


1410. Iteration

Programming in Java

Example:

! For each query, we’ll need to open the database and read all the way through.

! Possible use of our “SongLibrary” class:

SongLibrary rock = new SongLibrary(“classicRock.lib”);

String artist = myInterIO.promptAndRead(“Artist? ”)

while (artist!=“STOP”){

rock.lookUp(artist);

artist = myInterIO.promptAndRead(“Artist? ”)

}


1510. Iteration

Programming in Java

Example:

public void lookup(String artist)

throws Exception {

BufferedReader br = new BufferedReader(new InputStreamReader(

new FileInputStream(songFileName)));

Song song = Song.read(br);

while (song != null){

if (artist.equals(song.getArtist() ) )

System.out.println(song.getTitle() );

song = Song.read(br);

}

}


1610. Iteration

Programming in Java

Loop Design Considerations

! Questions that one should consider carefully when coding a loop:

- What is the condition that terminates the loop?

- How should the condition be initialized?

- How should the condition be updated?

- What guarantees this condition will eventually occur?

- What is the process to be repeated?

- How should the process be initialized?

- How should the process be updated?

- What is the state of the program on exiting the loop?

- Are "boundary conditions" handled correctly?


1710. Iteration

Programming in Java

Maintaining multiple values

! The SongLibrary example allowed us to read and process an indeterminate number of records, but…

! We could only have one record, (object), in memory at any one time.! What if we wanted to read a list and then:

– reverse it– sort it– build a concordance

We’d have to have the whole list in memory at once!


1810. Iteration

Programming in Java

The Vector class

! Just like a mathematical vector stores an ordered collection of numeric values, a Java Vector object stores an ordered collection of objects.String s;

Vector v = new Vector();

s = br.readLine();

while (s != null) {

v.addElement(s);

s.br.readLine();

}

System.out.print(V.size());

This allows us to Read and store an indeterminate number of strings (or any type of object), but how do we then read or process them?

The Vector addElement() method is used for storing elements in the vector.The Vector size() method returns the number of elements in the vector.The Vector isEmpty() method returns false if any elements are in the vector.Many other vector methods exist in the Vector class for use.

The Vector class is a container class.


1910. Iteration

Programming in Java

The Enumeration class

! The Vector class has a method, elements() , that returns an Enumeration object.

! The Enumeration class has the methods:boolean hasMoreElements();

someType nextElement();

The enumeration class models the behavior of traversing the elements of the collection, in this case the vector. Other Java container/collection classes also use the enumeration class to provide traversal services for visiting and processing each element of the collection.


2010. Iteration

Programming in Java


Enumeration myEnum = v.elements();

while (myEnum.hasMoreElements()){

String s = (String) myEnum.nextElement();

System.out.print(s);

}

This is an example of the traversal pattern using an enumeration to visit and process each element stored in the collection in sequence.


2110. Iteration

Programming in Java


Type cast!

Enumeration myEnum = v.elements();

while (myEnum.hasMoreElements() ){

String s = (String) myEnum.nextElement();

System.out.print(s);

}

The typecast is required to assign the reference returned by nextElement() to a specific reference variable since by default nextElement() returns a generic object type reference.


2210. Iteration

Programming in Java

The SongLibrary class, again

! Using the Vector and Enumeration classes, we could modify our SongLibraryclass so that we only have to read the database file once! Then we can query our vector object as many times as we want!

! See text, pages 284-285 for example.

! Exercise: plan out a means by which you could read in & query the song database, make additions, and then re-save to an updated file!


2310. Iteration

Programming in Java

Example: a “Set” class

! Problem: Create a container class to implement the mathematical concept of a Set.

! We need the following behavior (methods):– Set (constructor)– contains (test for membership in the set)– isEmpty (see if this is the “null set”)– addElement (as name implies)– copy (to make a copy of the set)– size (number of elements in the set)– elements (returns an enumeration)– union (returns a set containing only elements in both)– intersection (returns a set containing all elements of each)– print (as name implies)

Since the Set class will also be a container class consistency should be maintained with the other container classes, (e.g., the Vector class). Thus the method names: addElement(), size(), & elements() were selected.


2410. Iteration

Programming in Java

Example: a “Set” class

! … so we’ll only need the followinginstance variable in the Set class:

Vector v;

– Note: the book uses this example, and makes several errors, because sometimes it calls this instance variable “theElements” and sometimes it calls it “vector”. We’ll just call it “v”.


2510. Iteration

Programming in Java

Set class – “size” method

public int size(){

return v.size();

}

A very simple method!


2610. Iteration

Programming in Java

Set class – “elements” method

public Enumeration elements(){

return v.elements();

}

Another simple one!


2710. Iteration

Programming in Java

Set class – “addElement” method

Public member method of the Set class.

public void addElement(Object s){

if ( ! contains(s) )

v.addElement(s);

}

The behavior of the: addElement(), method is one aspect of the the Set class that is different from the vector class. The other behaviors that distinguish Sets from vectors is the union and intersection methods. .


2810. Iteration

Programming in Java

Set class – “intersection” method

public Set intersection(Set other){

Set interSet = new Set();

Enumeration enum = this.v.elements();

while ( enum.hasMoreElemets() ){

Object elem = enum.nextElement();

if ( other.contains(elem) )

interSet.addElement(elem);

}

return interSet;

}


2910. Iteration

Programming in Java

Set class – “union” method

Public member method of the Set class.

public Set union(Set other){

Set unionSet = other.copy();

Enumeration enum = this.v.elements();

while ( enum.hasMoreElemets() ){

unionSet.addElement(enum.nextElement());

}

return unionSet ;

}


3010. Iteration

Programming in Java

Set class – “copy” method

public Set copy (){

Set target = new Set();

Enumeration source = v.elements();

while (source.hasMoreElements())

target.addElement( source.nextElement() );

return target;

}

Why is the copy method necessary for the Set class. Why not allow the user to simply code:

Set A, B;//… add elements to Set AB = A;


3110. Iteration

Programming in Java

Set class – “contains” method

public boolean contains(Object o){

Enumeration enum = v.elements();

while ( enum.hasMoreElements() ){

Object item = enum.nextElement();

if (item.equals(o)) return true;

}

return false;

}

Note: the object class represents all object types. The object class contains a general equals method for comparing any two objects. In reality the general equals method does not compare any two objects. It relies upon the specific classes providing their own equals method which is automagically used. The Java predefined classes, (String, Integer), provides their own equalsmethods. To store user defined objects in the Set class we must provide an appropriate and specific equals method in our classes.

Documents

Why do we use iteration 10. Iteration 1