Introduction to arrays Array

1. Introduction to arrays Array One dimenstional array

2. outlines • What is an array • Why arrays • How to declare and initialise an array • How to use for loops to process arrays • Array index out of bounds exceptions

3. Array definition • An array is the memory of the computer used to store lots of data items of the same types. • An array is an ordered list of value

4. Array index index marks The array marks holds 10 marks: The 1st mark is indexed by 0 The last mark is indexed by 9(10-1)

5. Array indexwith N values index N-1 Nmarks The array Nmarks array holds N marks: The 1st mark is indexed by 0 The last mark is indexed by (N-1)

6. How to reference a value in an array? index N-1 99 Nmarks A particular value in an array is referenced using the array name followed by the index in brackets System.out.println(Nmarks[0]); will print 79, Nmarks[9]

7. How to reference a value in an array? A particular value in an array is referenced using the array name followed by the index in brackets index marks marks[0] refers to 79 where as marks[9] refers to 91

8. Array declaration • We first declare a as an array of integers. • int [] a ; • We the give it a space in the memory to hold 10 items(integes). • a new in[10];

9. Array declaration (cont) • We can also declare and assign a memory space at the same time • int [] a = new int[10];

10. Array declaration and initialisation • We can also declare and assign a memory space at the same time • int [] a = {10, 5, 6, 22, 11, 13, 15, 81, 8,26} ; 10 5 6 22 11 13 15 18 8 26

11. Array declaration(Cont) • An array can hold only objects of the same type specified in the declaration step. • Int [] intarr = new int[5]; • String [] starr= new String[5]; • Char [] charr= new Char[5]; • Doucle [] darr= new double[5]; • Boolean [] barr= new boolean[5];

12. Array declarationInitializer Lists (cont) • An initializer list can be used to instantiate and initialize an array in one step • The values are delimited by braces and separated by commas • Examples: • int[] units = {147, 323, 89, 933, 540, 269, 97, 114, 298, 476}; • char[] letterGrades = {'A', 'B', 'C', 'D', 'F'}; • String [] names= {“xxxx”, “yyy”, ”xx”, ”tt”,”dddddddd”};

13. Array declarationInitializer Lists (cont) • Note that when an initializer list is used: • the new operator is not used • no size value is specified • The size of the array is determined by the number of items in the initializer list • An initializer list can only be used in the declaration of an array

14. Array declaration (cont) • int[] x, y, z; • identifies that x, y and z are all arrays. • int x[], y, z; • identifies that only x is an array, y and z are simply integers.

15. Why arrays? Imagine you want to write a program that asks the user to enter in 50 student midterm marks. The program would then display each grade with the student’s ID number and then the average for that assignment. Write a program, MarksArray.java to achieve this?

16. Why arrays? (cont) Imagine you want to write a program that asks the user to enter in 50 student midterm marks. The program would then display each grade with the student’s ID number and then the average for that assignment in a nice report. How would you do it? • One way would be to provide 100 variables: • int mark1, mark2, mark3, (until 50) • int ID1, ID2, ID3, (until 50) • The array provides for a way to create only two variables: • int marks[] = new int[50]; • int IdNumbers[] = new int[50]; • This much easier then declaring 100 variables.

17. Array length • Int [] ar = new int [10]; • a.length = 10 • The values are a[0] to a[a.length-1]

18. Primes.java public class Primes { public static void main (String[] args) { int[] primes = {2, 3, 5, 7, 11, 13, 17, 19,23}; System.out.println ("Array length: " + primes.length); System.out.println ("The first few prime numbers are:"); for (int prime = 0; prime < primes.length; prime++) System.out.print (primes[prime] + " "); System.out.println (); } }

19. Array index out of bounds • Once an array is created, it has a fixed size • An index used in an array reference must specify a valid element • That is, the index value must be in bounds (0 to N-1) • The Java interpreter will throw an exception if an array index is out of bounds • This is called automatic bounds checking

20. Example • Int [] ar = new int [10]; • Ar[10]=86; • It prints an error • Exception in thread “main” java.lang.ArrayIndexOutOfBoundException:86

21. Example // StudentArray.java: store integers in arrays and access public class StudentArray { public static void main(String[ ] args) { int[] students = {55, 69, 70, 30, 80}; System.out.println("Array Length = " + students.length); System.out.println("Values Stored in Array:"); for ( int i=0; i < students.length; i+ +) System.out.println(students[ i] ); } }

22. Example • Write a java program that stores the first hundred positive integers in an array and prints them in a reverse order.

23. Example • Write a java program that stores that allows the user to enter 10 positive integers, stores them in an array. Then prints how many even number were enter if any.

24. Example Write a program that allows the user to enter 10 integers stores them in an array and search for largest and the smallest in the array.

25. Example • Write a java program that uses two array of length 10: arrID stores the students id and arrMark stores the students mark. For each students the program should print the student and id and its mark as follows: • Student id Mark • 12009 15

26. Example • Write a numbera program that generates 100 random integers between 0 to 99 and stores them any array and prints the largest, smallest and the average.

27. Example • Write a program where the user types in a number of Strings which are stored in an array ofStrings and then the program prints out all the longest Strings entered by the user.

28. Example • Write a program where the user types in a number of Strings stored in an array of Strings andthen the program prints out the string that has the most occurrences of the character ’a’.