Java Arrays. What could go wrong here? Motivation. Array Definition 9/28/18. Mohammad Ghafari

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1 9/28/8 Java Arrays What could go wrong here? int table[][] = new int[2][]; // some code int sum = ; for (int i = ; i < 2; i++) for (int j = ; j < ; j++) int value = table[i][j]; sum += value; Mohammad Ghafari 6 Motivation Array Definition Write a program that computes the average of three numbers. q A data structure for storing a fixed number of elements. q Elements are of the same type and share the same name. import java.util.scanner; // Scanner in = new Scanner(System.in); q Each element is accessed using its relative position in the array. int num = in.nextint(); // handle InputMismatchException int num2 = in.nextint(); int num = in.nextint(); Array of four elements: int total = num+ num2+ num; double average = total/; 2 Index Element of an array 8

2 9/28/8 Set up an array Arrays of Primitives You have to tell Java what kind of data is going into your array, and how many elements the array has. boolean status[]; Name of an array null status = new boolean[]; false false false status[2] = true; false false true nums[] = ; datatype name[] = new datatype[size]; Array size Type of an array and its elements datatype name[]; name = new datatype[size] 9 boolean status[] = new boolean[]; status[] = true; Array Index Out of Bounds Exception nums[2] = true; Type mismatch error 2 2

3 9/28/8 Array Initialization nums[] = 4; nums[] = ; nums[2] = ; nums[] = 2; Number nums[] = new Float[2]; nums[] = new Integer(); Array Store Exception 4 Array Initialization nums[] = 4; nums[] = ; nums[2] = ; nums[] = 2; int nums[]; int nums[] = 4,,, 2; 5 nums = 4,,, 2; Array constants can only be used in initializers 6

4 9/28/8 Arrays of Objects int nums[]; nums = 4,,, 2; Car garage[] = new Car[4]; Array constants can only be used in initializers void f(float[] input)... void f(double[] input)... void g() garage[] = new Car(); garage[].color = white; f(, 2,, 4); Car car4 = new Car(); car4.color = black; cars[] = car4; Which function to call? 8 Multi-dimensional Arrays Multi-dimensional Arrays (2D) Multidimensional arrays are arrays of arrays with each element of the array holding the reference of other array. int matrix[][] = new int[][]; 2-D column position -D row position datatype name []..[] = new datatype[x]..[z]; Number of dimensions Size of each dimension -D 2-D [][] Examples: [][2] Board games, Spreadsheets, 9 2 4

5 9/28/8 Array Traversal Array Traversal (2D) j String row[] = new String[2]; for (int i = ; i < row.length; i++) // assign or read row[i] for (String s : row) // read s int table[][] = new int[2][]; // some code i int sum = ; for (int i = ; i < table.length; i++) for (int j = ; j < table[i].length; j++) int value = table[i][j]; sum += value; 2 22 Ragged Arrays Array Copy int part[][] =,2,, (4,5,6; int[] list_a =,, 5, ; int[] list_b = new int[2]; list_a list_b int part[][] = new int[][]; part[] = new int[2]; part[] = new int[]; part[2] = new int[]; // list_b = list_a; // System.out.println(list_B[]); // list_b[] = -; // System.out.println(list_A[]);

6 9/28/8 Array Copy Array Comparison int[] list_a =,, 5, ; int[] list_b = new int[2]; list_a list_b int[] list_a =,, 5, ; int[] list_b =,, 5, ; list_b = list_a; System.out.println(list_B[]); // 5 if(list_a == list_b) or if(list_a.equals(list_b)) Are they referring to the same array? list_b[] = -; System.out.println(list_A[]); // - To determine whether two arrays contain the same elements, compare them element by element Passing and returning arrays java.util.arrays int numofinputs = scanner.nextint(); int[] input = getinput(numofinputs); int sum = sum(input); double average = sum/numofinputs; public int[] getinput(int num) int[] input = new int[num]; for(int i=; i< num; i++) Returns an array input[i] = scanner.nextint(); return input; public int sum(int[] nums) int result = ; for(int i : nums) Receives an array result += i; return result; fill sort binarysearch copyof equals deepequals

7 9/28/8 What you should know What an array is Array declaration, instantiation, and initialization Arrays of primitives and arrays of objects Multi-dimensional arrays Traversing arrays Passing arrays to methods and returning arrays from methods Exercise Given a text corpus, compute bi-grams and record their frequencies. For example: this is some text this is is some some text Exercise 2 A x Sudoku puzzle: The user should place the numbers to9 such that each row, column, and diagonal adds up to the same number N.B. eachnumber should be used exactly once. 2