Spring 2008

Mark Fontenotmfonten@engr.smu.edu

CSE 1341 - HonorsPrinciples of Computer Science I

Note Set 10

Note Set 10 Overview

ArraysArray ExamplesVariable-length argument lists

Example 1Declare an int array of length 20 and initialize each element

to the square of its subscript location.

int[] arr = new int [20];for (int i = 0; i < arr.length; i++) arr[i] = i * i;

Shuffle/Randomize the elements of an arrayRandomize the elements of the array arr from the previous

example

Random r = new Random();for (int i = 0; i < arr.length; i++){ int j = r.nextInt (arr.length); //swap element i with element j int temp = arr[i]; arr[i] = arr[j]; arr[j] = temp;}

Rolling DiceUse an array and random number generator to count the

outcomes of rolling a pair of dice 10,000 times. Print the frequencies.

Random r = new Random();int[] frequency = new int[13];

for (int i = 0; i < 10000; i++) { frequency[ r.nextInt(11) + 2 ] ++;

System.out.println(“%s%10s\n”, “Total”, “Frequency”);

for (int i = 2; i < frequency.length; i++) System.out.printf(“%4d%10d\n”, i, frequency[i]);

Variable Length Argument ListsEver notice that you can send 1, 2, 3, or more arguments to

printf?

It is possible to implement a method that takes a variable number of arguments

public static double average(double... numbers) { double total = 0.0; for (double d : numbers) total += d; return total/numbers.length;

}

use the ellipsis to indicatevariable length argument

numbers acts like an array and can bemanipulated as such

Rules of Variable Length Argument Lists

parameter with ellipsis must be at the end of the parameter list

can only be one ellipsis in a parameter listpublic static void foo(int... vals, int x, int y);//Don’t you dare!public static void bar(int... vals, int… otherVals);//NO NO!

This is sometimes called a vararg for variable-length argument list

public static double average(double... numbers)

Array IssuesArrays are statically sized

once they are declared, they cannot change sizeyou can allocate a new, larger array and copy the contents of

the old array over. can be computationally expensive especially for large array or if this

function is performed often.

If you attempt to access past the end of an array, you’ll get a ArrayOutOfBoundsremember that x[i] is a reference to a memory location so if you access an invalid subscript i the you would be trying to

access memory you do not have access to OR the reference is NULL and thus can’t be accessed.