Introduction to Computer Science

1. Introduction to Computer Science Unit 9 One Dimensional Arrays

2. Arrays • An array is a sequence of variables of identical type • It is used to store large amounts of data, without having to declare many individual variables counts [0] [1] [2] [3] [4] [5] [6]

3. Arrays • “Arrays bring to data what loops bring to action—power.” • Arrays give us a way of defining a structured collection of data, without having to name each component (as we would have to do with individual variables) • It gives us an important tool for implementing data abstraction

4. Old Example public static void main (String[ ] args) { int score, sumOfScores = 0, numberOfScores = 0; SimpleInput sinp = new SimpleInput(System.in); System.out.print(“Enter score (eof ends the data): “); score = sinp.readInt( ); int maxOfScores = score; int minOfScores = score; while ( !sinp.eof( ) ) { numberOfScores++; //new score sumOfScores = sumOfScores + score; //update sum System.out.print(“Enter score (eof ends the data): “); score = sinp.readInt( ); }…

5. That was a “stream-oriented” approach to data Once a value goes past, it’s gone—there’s only one variable, score, that holds input data. But suppose we want to read and average the input stream, and echo only values that are above average. We could: • Read the input stream twice, averaging the first time, printing large numbers the second (a file-oriented approach; not practical for human input) • Read and store the input values, summing them as they’re stored. Get average at end, then echo only the larger ones (this will use an array)

6. Variables and Arrays 95 264 3 score sumOfScores numberOfScores Simple variables give us a snapshot of the data. counts [0] [1] [2] [3] [4] [5] [6] An array lets us reinspect any part of the data stream.

7. A Random Access Data Structure Arrays are a random access data structure. Values can be stored or retrieved in any order we want. counts [0] [1] [2] [3] [4] [5] [6]

8. Things to Notice with an Array 1. What’s the array’s name? 2. What kinds of values can it hold? 3. How many values can it hold? 4. How do we refer to individual values in the array? We’ll now look at each of these answers for a Java array

9. Arrays Are a Special Kind of Object • An array contains a fixed number of variables of identical type • This fixed number is called the array’s length • Like any object, an array must first be declared, then allocated (created) • Remember that an array is an object, and this fact will help you

10. Declaring an Array Variable • Declare a variable to be an array:int[ ] counts; double[ ] scores; Time[ ] appointmentTimes; • The brackets tell Java that the variable (counts, scores, or appointmentTimes) will be an array

11. Answers to some questions • We’ve answered questions 1 and 2 with our declarations:1. What is the array’s name?2. What kinds of values can it hold?int[ ] counts; double[ ] scores; Time[ ] appointmentTimes; • First array’s name is “counts”, and it holds values of type int, etc.

12. Allocating (creating) an Array in the Heap • We create an array using the new notation, but with some variations • We have to tell new how many variables will be in the array object (use square brackets):counts = new int[10]; scores = new double[15]; appointmentTimes = new Time[10]; • In general, new type[ size ]creates an array of length size with variables of type type, returning an array object (in the heap); Answer to Question 3. How many values can it hold? (given during allocation)

13. Doing it All At Once • Obviously, like with any variable (simple or object), you can write it on one line:int[ ] counts = new int[10]; double[ ] scores = new double[15]; Time[ ] appointmentTimes = new Time[10]; • Now we have array objects sitting in the heap, called counts (that can hold 10 ints), scores (that can hold 15 doubles), and appointmentTimes (that can hold 10 Time objects)

14. Creation of an Array Object new type[ size ] • size can be any integer or integer expression:int i = 3; char[ ] name = new char[i];is just as good as:char[ ] name = new char[3]; • The memory in an array is automatically initialized for us (to 0 for numbers, false for booleans, or null for objects, as appropriate)

15. Let’s Look at counts counts [0] [1] [2] [3] [4] [5] [6] [7] [8] [9] • Place to hold 10 ints • Each int is held in a separate box • The subscript, or index, of the variables runs from 0 to 9 (the index always starts at 0) • Arrays can contain any type of value: simple value or (reference to) object

16. Answer to Question 4: How do we refer to individual values in the array? Use subscripts to obtain any specific variablecounts[0] is the first variable in countscounts[1] is the second variable in countscounts[9] is the last variable in countscounts[10] is an error! counts [0] [1] [2] [3] [4] [5] [6] [7] [8] [9]

17. They are Used LikeRegular Variables counts[0]counts[1]counts[9] These are all int variables, and can be used anywhere any int variable (like “score” or “sumOfScores”) is used:counts[1] = 17;score = counts[8];sumOfScores = (7 * ((counts[4] + 2)/8));

18. Java ChecksArray Bounds For You counts[10] • This does not exist, since the 10 elements in counts run from 0 to 9 • If you try and use this nonexistent value, Java throws anArrayIndexOutOfBoundsExceptionerror • More about these kinds of errors (exceptions) later

19. class Simple { public static void main (String[ ] args) { int[ ] counts = new int[10]; SimpleInput sinp = new SimpleInput(System.in); System.out.println(“Enter ten numbers: “); counts[0] = sinp.readInt( ); counts[1] = sinp.readInt( ); counts[2] = sinp.readInt( ); counts[3] = sinp.readInt( ); counts[4] = sinp.readInt( ); counts[5] = sinp.readInt( ); counts[6] = sinp.readInt( ); counts[7] = sinp.readInt( ); counts[8] = sinp.readInt( ); counts[9] = sinp.readInt( ); System.out.print(counts[9] + “ “ + counts[8] + “ “ + counts[7] + “ “ + counts[6] + “ “ + counts[5] + “ “ + counts[4] + “ “ + counts[3] + “ “ + counts[2] + “ “ + counts[1] + “ “ + counts[0] + “\n”); }} Of course we can do this with a loop

20. Using a Variable as an Index • If i is an integer variable, you can, for example, writecounts[i]to refer to one of the variables in counts • counts[2 * i]refers to counts[0] if i is 0, counts[2] if i is 1, etc.; illegal if i is outside the 0 to 4 range • counts[i/2]refers to counts[0] if i is 0 or 1, counts[1] if i is 2 or 3, etc.; illegal if i is outside 0 to 19

21. Alternative Way to Declare and Allocate an Array All At Once • Arrays can be initialized by giving a list of all their elements:int[ ] primes = {2, 3, 5, 7, 11, 13, 17, 19, 23, 29}; • The declaration is on the left side of the assignment • No explicit creation of the object is necessary; Java handles it automatically • It must be done in a single line; this is illegal:int[ ] primes;primes ={2, 3, 5, 7, 11, 13, 17, 19, 23, 29};

22. Let’s Use the Technique • Method that takes an integer and prints out that day of the week:final String[ ] NAME= {“”, “Sunday”, “Monday”, “Tuesday”, “Wednesday”, “Thursday”, “Friday”, “Saturday”};void printName (int day) { System.out.print(NAME[day]);}

23. What is the Effect of the Following? int[ ] a, b; a = new int[10]; a = b; • Since arrays are objects, a and b are references to objects (in the heap, as expected) • So now a and b refer to the same array • An array is a mutable object; changes made to the array are reflected in “both” a and b

24. Passing an Array as an Argument Between Methods public int firstMethod( ) { int[ ] a = new int[10]; a[0] = 9; secondMethod(a); return a[0];}public void secondMethod(int[ ] b) { b[0] = 5;} a is an array object variable. b receives a copy of a, and thus points to the same object. Changes to b are reflected in a. firstMethod( ) therefore returns 5.

25. The public Instance Variable “length” • Every array object has a public instance variable called length • It tells how many elements are in the array • Since it is public, it can be freely accessed; if a is an array, a.length says how many elements it has • Print all elements of the array counts:for (int i = 0; i < counts.length; i++) System.out.println( counts[i] );

26. A Small Note on Variable Scope • When we writefor (int i = 0; i < counts.length; i++) System.out.println( counts[i] );the variable i exists only for the scope of the for loop • It cannot be reused later, unless it is redeclared • This is just a convenience for writing loops

27. Simple Array Processing Loops • Copy all the elements from array b into array a:for (int i = 0; i < a.length; i++) a[i] = b[i]; • How does this differ froma = b;

28. Another Example • Initialize the array counts to contain the numbers 0, 10, 20, etc. up to 90: 0 40 10 20 50 60 80 counts 30 70 90 [0] [1] [2] [3] [4] [5] [6] [7] [8] [9] for (int i = 0; i < counts.length; i++) counts[i] = i * 10;

29. Read 10 numbers into array “counts” and print them in reverse order class Simple2 { public static void main (String[ ] args) { int[ ] counts = new int[10]; SimpleInput sinp = new SimpleInput(System.in); System.out.println(“Enter ten numbers: “); for (int i = 0; i < 10; i++) counts[i] = sinp.readInt( ); for (int i = 0; i < 10; i++) System.out.print(counts[9 - i] + “ “); System.out.println( ); }}

30. Another Way to Do It class Simple2 { public static void main (String[ ] args) { int[ ] counts = new int[10]; SimpleInput sinp = new SimpleInput(System.in); System.out.println(“Enter ten numbers: “); for (int i = 0; i < 10; i++) counts[i] = sinp.readInt( );for (int i = 9; i >= 0; i- -) System.out.print(counts[i] + “ “); System.out.println( ); }}

31. An Array’s Size is FixedWhen Created • An array cannot grow • Its size is determined when it is created • One simple way of dealing with this is to make a large array and only partially fill it • Let’s look at an example

32. Encapsulate reading and echoing an array in a class • We’ll make a class Collection, which will include an array and methods for handling the array • The client uses Collection by creating a Collection object, passing its constructor the (large) size of the array desired • Let’s look at how the client would use the Collection class, and how the class itself is defined

33. The client to the Collection class class MyClient { static final int INPUT_MAX = 1000; public static void main (String[ ] arg) { Collection c = new Collection(INPUT_MAX); c.readAndEcho( ); }}

34. class Collection { int[ ] _item; int _size = 0; public Collection (int number) { _item = new int[number]; } public void readAndEcho( ) { SimpleInput sinp = new SimpleInput(System.in); System.out.println(“Enter first number: “); int n = sinp.readInt( ); while ( !sinp.eof( ) ) { _item[_size] = n; _size++; System.out.println(“Enter next number: “); n = sinp.readInt( ); } System.out.println( ); for (int i = 0; i < _size; i++) System.out.println(_item[i]); }}

35. Another Example • Create program that reads in student name and two scores and prints average for each student (until end-of-file):Enter name and two exam grades: Bill5769Enter name and two exam grades: Scott9486Enter name and two exam grades: ^D or ^Z

36. The Expected Output Name AverageBill 63Scott 90 The client program:class MyClient { static final int INPUT_MAX = 100; public static void main (String[ ] arg) { GradeBook grades = new GradeBook(INPUT_MAX); grades.readAndAverage( ); }}

37. class GradeBook { String[ ] _names; int[ ] _exam1, _exam2; int _size = 0; public GradeBook (int number) { _names = new String[number]; _exam1 = new int[number]; _exam2 = new int[number]; } public void readAndAverage( ) { SimpleInput sinp = new SimpleInput(System.in); while ( true ) { System.out.println(“Enter name and two exam grades: “); _names[_size] = sinp.readString( ); if ( sinp.eof( ) ) break; _exam1[_size] = sinp.readInt( ); _exam2[_size] = sinp.readInt( ); _size++; } System.out.println( ); System.out.println(“\tName\tAverage”); for (int i = 0; i < _size; i++) System.out.println(“\t” + _names[i] + “\t\t” + (_exam1[i] + _exam2[i]) / 2); }}

38. Arrays Can Contain Objects • Arrays can contain any kind of data, not just integers and strings • So instead of building GradeBook the way we did, let’s create a Student class, and fill up the array with Student objects • Each object includes names and grades

39. Class Student, for making Student Objects class Student { private String _name; private int _exam1, _exam2; public Student ( ) { } public void setName (String s) { _name = s; } public void setExam1 (int n) { _exam1 = n; } public void setExam2 (int n) { _exam2 = n; } public String getName ( ) { return _name; } public int getExam1 ( ) { return _exam1; } public int getExam2 ( ) { return _exam2; } public int getAvg ( ) {return (_exam1+_exam2)/2; } }} can be omitted; Java will provide a default constructor that does nothing if we left this out

40. class GradeBook { Student[ ] _students; int _size = 0; public GradeBook (int number) { _students = new Student[number]; } public void readAndAverage( ) { SimpleInput sinp = new SimpleInput(System.in); String nextname; while ( true ) { System.out.println(“Enter name and two exam grades: “); nextname = sinp.readString( ); if ( sinp.eof( ) ) break; _students[_size] = new Student( ); _students[_size].setName(nextname); _students[_size].setExam1(sinp.readInt( )); _students[_size].setExam2(sinp.readInt( )); _size++; } System.out.println( ); System.out.println(“\tName\tAverage”); for (int i = 0; i < _size; i++) System.out.println(“\t” + _students[i].getName( ) + “\t\t” + _students[i].getAvg( )); }}