Problem Solving using the Java Programming Language

17-21 May 2010Mok 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 4th element of the array

when there are only 3 elements: a[3] = 60; // index 3 refers to the 4th elementor 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 1st row• a[1].length gives the number of columns in

the 2nd 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}};

9 81 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?