Problem Solving using the Java Programming Language May 2010 Mok Heng Ngee Day 5: Arrays

Arrays We will only deal with arrays of primitive types in this course: – E.g. an array of ints or an array of booleans What is an array? – A collection of elements. – An array of ints is a collection of ints

Arrays (2) How to declare an array variable: int[] a; How to create the array itself using the new keyword: a = new int[3]; // creates an array for 3 ints This creates an array of 5 booleans: boolean[] b = new boolean[5];

Arrays (3) By default, all elements in the array will be 0, false or null (depending on the type) You can set the values in the array: a[0] = 30; a[1] = 40; a[2] = 50; Notice that the array index starts from 0! How to get values from the array: int x = a[2]; System.out.println(x); // will print out 50

Arrays (4) What is an ArrayIndexOutOfBoundsException? – When you try to use an index which is out of range – E.g.: setting the 4 th element of the array when there are only 3 elements: a[3] = 60; // index 3 refers to the 4 th element or retrieving an invalid element: int i = a[-2];

Arrays (5) Use.length to find out how many elements there are in an array: int []a = new int[15]; System.out.println ( a.length ); Can methods take in arrays and return arrays? public static int[] doSomething ( double[] x ){ … }

Try it Write a class which contains a main method which: – Declares & creates an array of 5 boolean values – Sets the values to true, false, true, false, true – Invokes the printArray method Write another method called printArray which: – Takes in an array of boolean values – Prints out the values in one row – Returns nothing

Arrays (6) “Lazy” way to initialize an array - the following 2 sections of code are identical: int []a = new int[3]; a[0] = 9; a[1] = 8; a[2] = 7; int []a= {9,8,7};

2D Arrays A 1D array can be viewed as a single row A 2D array is an array of arrays: – Can be viewed as a table of rows and columns Declare & create a 2D array of ints with 3 rows & 2 columns. int [][] a = new int[3][2];

2D Arrays (2) Setting values in a 2D array: a[0][0] = 3; a[0][1] = 9; Getting values from a 2D array: int x = a[2][1] ; Very flexible storage mechanism to represent a table of values

2D Arrays (3) a.length gives the number of rows a[0].length gives the number of columns in the 1 st row a[1].length gives the number of columns in the 2 nd row & so on

Example: getting the sum of all ints in a 2D array public class Test{ public static void main (String[] args){ int[][] a = new int [3][4]; // code to initialize values in a... int sum = 0; for (int i=0; i<a.length; i++){ for (int j=0; j<a[i].length; j++){ sum += a[i][j]; } System.out.println(sum); }

2D Arrays (4) “Lazy” way to initialize a 2D array: int [][]a = {{9,8},{1,7},{-1,2}};

Summary Day1 : Introduction to the JDK, variables, operators Day 2 : looping Day 3 : decisions Day 4 : methods & 1D arrays Day 5: 2D arrays

Objectives achieved? To exercise your systematic thinking skills in logical problem solving To introduce you to Java To let you experience SMU’s learning culture Have you had fun?