CompSci 125 Lecture 09. Chapter 5: while, break and continue statements Iterators and the ArrayList Class

Transcription

1 CompSci 125 Lecture 09 Chapter 5: while, break and continue statements Iterators and the ArrayList Class

2 Homework Update HW3 Due 9/20 HW4 Due 9/27 Exam-1 10/2

3 Programming Assignment Update p1: Traffic Applet due Sept 21 (Submit problem fixed) p2: Find Parking due Oct 5

4 The while Loop

5 Introduction to the while loop Sometimes we wish to execute the same statements over and over again We use a while loop to repeatedly execute a statement over and over again while some condition remains true

6 The while loop while (condition) statement; Java repeatedly executes statement while condition evaluates true! Java resumes normal execution of the program (with the following statement, not illustrated) after condition evaluates false

7 Use code block with while loop while (condition) {!!.!!.!!.! }! Java repeatedly executes the statements inside the code block while condition evaluates true Java resumes normal execution of the program (following the code block) after condition evaluates false

8 Counter-Controlled Repetition: Counter Variable Determines When to Exit the Loop int i=1;! while (i <= 10) {!!System.out.printf( i = %d\n, i);! }!!i = i + 1;! Prints the integers from inclusive, each on its own line What would happen if you forgot to increment i?

9 Sentinel-Controlled Repetition: Sentinel Variable Determines When to Exit the Loop!.!!.!!.!!int x = stdin.nextint();!//initialize sentinel!!while (x > 0) {!!!//Check sentinel!!!.!!!.!!!.!!!x = stdin.nextint();!//update sentinel!!}!

10 Comparing the if, if else, and while Statements if (condition) s1;! if (condition) s1; else s2;! while (condition) s1;! Condition determines whether to execute an optional statement Condition selects one of two statements to execute Repeatedly executes statement while condition remains true! Decide whether to print a warning message if a checking account is overdrawn Decide whether to turn on a cooler or a heater depending upon whether a temperature is too hot or too cold Execute a code block exactly 10 times Execute a code block until the user indicates they are finished

11 The break and continue Statements

12 Use break to Exit a Loop Early int i=1, sum=0;! while(i<=10) {! int x = stdin.nextint();! if (x<0) break;! sum+=x;! i++;! }! System.out.println( Sum= +sum);! Break causes loop to terminate immediately Execution resumes with the first statement following code block

13 Use continue to skip statements in a loop s code block int i=1, sum=0;! while(i<=10) {! int x = stdin.nextint();! if (x<0) continue;! sum+=x;!!! i++;! }! System.out.println( Sum= +sum);! Continue passes control to bottom of the loop Skips statements between continue and the bottom of the code block Execution continues by re-evaluating while s condition!

14 Iterators

15 Collection AKA a container An object that groups/holds other objects known as elements Usually models a real-world group of related objects Implemented by many built-in classes

16 Collection Methods Add an element to the collection Remove an element from the collection Remove all the elements in the collection Calculate the size of the collection Determine if a specified element resides in the collection Etc

17 Collection Example: Set Unordered collection Cannot contain duplicate elements Examples: Cards in a poker hand Courses you are taking Photographs of friends ENGL 101 MATH 175 CS 125 UF 100

18 Collection Example: List Ordered collection AKA sequence Each element has a unique numerical position in the list known as its index Duplicate element values typically ok May allow empty (null) elements 0 Bashful 1 Sleepy 2 Dopey 3 Grumpy 4 5 Doc 6 Happy

19 Collection Example: Map Cannot contain duplicate elements Elements map a key to a value Example: Map of Student Ids to Student Names Amy Smith Bob Jones Maria Zuazua Floyd Crow Sidi Ali Fran White Alex Wang

20 Iterator A family of built-in methods for processing each element of a collection Methods implemented by many built-in classes You determine what the processing will be (e.g. print value of an element)

21 Iterator Methods Access the next element in a collection Determine if more elements exist Remove element from a collection Etc (there are variations of the above)

22 Scanner Implements an Iterator next() nextint() nextdouble() hasnext() hasnextint() hasnextdouble()

23 Class ArrayList java.util.arraylist Resizable implementation of List Implements Iterable Implements Collection Allows empty (null) elements Generic 0 Bashful 1 Sleepy 2 Dopey 3 Grumpy 4 Sneezy 5 Doc 6 Happy

24 Motivation for Generics ArrayList shouldn t define the type of the elements in the collection Might want an ArrayList of Strings, Integers, Doubles, etc Generics allow you (rather than the built-in Java class) to specify the element type

25 Generics Example ArrayList x = new ArrayList<String>();!! x.add( Bashful );! x.add( Sleepy );! String dwarffriend = x.get(0);! String anotherdwarf = x.get(1);!!!!!!