CS 231 Data Structures and Algorithms Fall Arrays Lecture 07 - September 19, Prof. Zadia Codabux

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1 CS 231 Data Structures and Algorithms Fall 2018 Arrays Lecture 07 - September 19, 2018 Prof. Zadia Codabux 1

2 Agenda Arrays For Each Loop 2D Arrays 2

3 Administrative None 3

4 Arrays 4

Array Data structure which stores a fixed-size sequential collection of elements of the same type. ArrayLists vs. Arrays Java arrays are of a fixed length.

5 Array Data structure which stores a fixed-size sequential collection of elements of the same type. ArrayLists vs. Arrays Java arrays are of a fixed length. After arrays are created, they cannot grow or shrink, which means that you must know in advance how many elements an array will hold. ArrayLists are created with an initial size. When this size is exceeded, the collection is automatically enlarged. When objects are removed, the array may be shrunk. (Reference: 5

6 Arrays In Java, there are a variety of ways of creating a new array. One way is to first create the array and then fill it: Die[] dice = new Die[5]; /*Declaring an array variable dice, creating an array, and assigning the reference of the array to Die */ //Die dice[] = new Die[5] also works but not preferred way for(int i = 0; i < 5; i++) { System.out.println(dice[i] ); 6

7 Arrays We now have Dice[0], Dice[1], Dice[2], Dice[3], Dice[4] (Note: index starts with 0) Notice that the array was created but the slots were not filled with Die objects. Instead, it is full of the keyword null. We need to create 5 new Die objects and add them to the array. for(int i = 0; i < 5; i++) { dice[i] = new Die(); Note that you have to specify the array size when you create it. Java arrays are fixed size and cannot be increased or decreased! Arrays can have size up to 2 billion (more precisely, 2^31-1). 7

8 Arrays double[] mylist = new double[10]; 8

9 Arrays To save space, the Java language creators added a second way to create an array, for when you know ahead of time all the values to be placed in the array initially: int[] values = new int[]{3,1,4,1,5,9,2,6,5,3,5,8,9,7,9; int[] values = {3,1,4,1,5,9,2,6,5,3,5,8,9,7,9; 9

10 Arrays Code: Die.java public static void main(string[] args) { Die[] dice = new Die[5]; for(int i = 0; i < 5; i++) dice[i] = new Die(); for(die d : dice) //foreach loop (d gets successively each value in dice) d.roll(); for(int i = 0; i < 5; i++) System.out.println(dice[i].getFaces()); for(die d : dice) same as for (int i=0; i < dice.length; i++) //type is Die object 10

11 ForEach Loop for ( type variable : array) { <body of loop> iterates through each item in the array stores each item in a variable executes the body of the loop. for(die d : dice) //iterates through each die object in the array, store each die object in variable d and executes the loop statements. 11

12 2D Arrays Code: TestArray.java What if you need bigger arrays than 2 billion? One way is to have arrays of arrays int[][] A = new int[3][4]; //3 rows and 4 columns This creates an array with 3 slots, each of which is an array of length 4. Each element in a multi-dimensional array is itself an array 12

13 2D Arrays 13

14 2D Arrays Code: TestArray.java (matrix 1-4) What do the following lines of code do? int[][] matrix = new int[5][4]; matrix[0][1] = 3; for (int i = 0; i < 4; i++) { matrix[0][i] = 1; for (int i = 0; i < 5; i++) { for (int j = 0; j < 4; j++) { matrix[i][j] = i * j; for (int i = 0; i < matrix.length; i++) { for (int j = 0; j < matrix[0].length; j++) { System.out.print(matrix[i][j] + " "); System.out.println(); 14

15 Jagged Array We don't need all inner arrays to be the same length. Each row in a 2D array is a separate one-dimensional array, and each of those arrays can have a different length. int[][] m = new int[2][]; m[0] = new int[]{1,2,3; m[1] = new int[]{1,2; 15

Declaring and ini,alizing 2D arrays

Declaring and ini,alizing 2D arrays 4 2D Arrays (Savitch, Chapter 7.5) TOPICS Multidimensional Arrays 2D Array Allocation 2D Array Initialization TicTacToe Game // se2ng up a 2D array final int M=3, N=4;