EMB1006 JAVA Part 2 Arrays and onwards Jonathan-Lee Jones

Overview Array Recap Using an Array Example Finding the lowest and highest values in an array Defensive Programming

What is an Array As mentioned previously, an array is a method for storing multiple values that are linked together. The name of the array is a pointer to the first address in memory. An array can be of any type, primitive or object (even user defined object for example student records) and can even be made up of other arrays to give a 2 dimensional, 3 dimensional or even greater array.

An Example Array Arrays can be initialised as empty arrays, or just assigned values. To initialise as an empty array we use the following structure: Int N = 100; Int[] testArray = new int[N]; To Initialise by assigning values we use the following structure: Int[] testArray = {1,2,3,4,5,6,7,8,9};

An Example Array class Deck { public static void main(final String args[]) { final String suit[] = {"Clubs", "Diamonds", "Hearts", "Spades" }; final String rank[] = {"2", "3", "4", "5", "6", "7", "8", "9", "10", "Jack", "Queen", "King", "Ace" }; final int nr_suits = suit.length; final int nr_ranks = rank.length; final int nr_cards = nr_suits * nr_ranks; final String deck[] = new String[nr_cards]; for (int r = 0, base_index = 0; r < nr_ranks; ++r, base_index+=nr_suits) { for (int s=0; s < nr_suits; ++s) deck[base_index+s] = rank[r] + " of " + suit[s]; }

Shuffling the Deck Using an Array for (int i = 0; i < nr_cards; ++i) { final int rand = i + (int) (Math.random() * (nr_cards-i)); final String t = deck[rand]; deck[rand] = deck[i]; deck[i] = t; } The above code snippet will perform a shuffle on the deck. This is the best method to use with the tools you have for creating a mathematically fair shuffle. You can test this if you want! Below is how you print contents of array (remember printing out the name just prints the memory address pointer!) for (int i = 0; i < nr_cards; ++i) System.out.println(deck[i]);

Finding the Lowest and Highest Values Write a short piece of code to find both the highest and lowest numbers from an array of size N, and print these out, alongside their position in the array. You may need the following:- Integer.MIN_VALUE; Integer.MAX_VALUE; Array.length; For Loop Integer.parseInt();

Finding the Lowest and Highest Values class minmax{ public static void main(String[] args){ int N = args.length; int[]numbers = new int[N]; int min = Integer.MAX_VALUE; int max = Integer.MIN_VALUE; int maxPos =0; int minPos =0; for(int i=0;i<N;++i){ numbers[i]=Integer.parseInt(args[i]); if (numbers[i]>max){ max=numbers[i]; maxPos=i; } if (numbers[i]<min){ min=numbers[i]; minPos=i;}} System.out.println("Max = " + max + " at position " + maxPos +"."); System.out.println("Min = " + min + " at position " + minPos +"."); }} What would happen if zero arguments were entered? What else could go wrong?

Validating Input, Defensive programming When you allow the user to input values into the program, you inevitably invite errors at run time. If you ask for an int and they give a string for example. A large number of these can be avoided by using various scanner tools, but how would you validate command line input? You are tasked with writing a program to take in 3 numbers from the command line, then add the first 2 together then divide by the third. It is a trivial task, but you would be surprised how many problems may occur. Try to make sure that the program will not crash by throwing a run time exception, and also the program gives the expected values. What do you need to check to ensure this.

Validating Input, Defensive programming class threeNums{ public static void main(String[] args){ int a = Integer.parseInt(args[0]); int b = Integer.parseInt(args[1]); int c = Integer.parseInt(args[2]); double ans = (a+b)/c; System.out.println(“ans = ” + ans); }

Validating Input, Defensive programming lass threeNums{ public static void main(final String[] args){ final int a = Integer.parseInt(args[0]); final int b = Integer.parseInt(args[1]); final int c = Integer.parseInt(args[2]); double ans = (a+b)/c; System.out.println("ans = " + ans); }

Validating Input, Defensive programming Possible Errors that can occur:- 1.Not enough input arguments (program requires 3, what happens if it gets 2?) 2.If the third input is 0 what will happen? 3.What happens if a= 2,000,000,000 and b = 1,999,999,999? 4.What happens if the user inputs the following values by mistake:- 4 8u 22? Is there anything else that can go wrong with this code? What if the following numbers are input 1, 2 & 2? What output do you expect, and what do you get?