Declaring Array Variables Chapter 6 Arrays datatype[] arrayRefVar; - PDF document

Declaring Array Variables Chapter 6 Arrays  datatype[] arrayRefVar; Preferred! Example: double[] myList;  datatype arrayRefVar[]; Example: double myList[]; 1 4 Motivations Creating Arrays  Suppose we want to write a program that arrayRefVar = new datatype[arraySize]; computes the average midterm score of all the students and find out how many scored above the Example: average myList = new double[10];  How many variables do we need for storing the scores? (30 students!)  Solution: Arrays 2 5 Arrays Declaring and Creating an Array in One Step  Array is a data structure that represents a Datatype[] arrayRefVar; collection of the same types of data. arrayRefVar = new datatype[arraySize];  How to use arrays?  Declaring array variables datatype[] arrayRefVar =  Creating arrays  Initializing arrays new datatype[arraySize];  Accessing and using array elements  Traversing arrays using for loops Example: double[] myList = new double[10]; 6

Array Initializers - Declaring, creating, Default Values initializing an array using one statement  When an array is created, its elements are assigned the default value: double[] myList = {1.9, 2.9, 3.4, 3.5}; This shorthand notation is equivalent to the following  0 for the numeric primitive data types statements:  (char)0 for char types double[] myList = new double[4];  false for boolean types myList[0] = 1.9; myList[1] = 2.9; myList[2] = 3.4; myList[3] = 3.5; Note: this shorthand syntax must be in one statement 7 10 Assigning Values to Array Elements The Length of an Array arrayRefVar[index] = expression; Once an array is created, its size is fixed. It cannot be changed. You can find its size using Example: myList[0] = 5.6; arrayRefVar.length myList[1] = 4.5; For example: … myList[9] = 11123; int[] numbers = new int[10]; int len = numbers.length; // 10 8 11 Inside Arrays Accessing Array Elements  The array elements are accessed through the double[] myList = new double[10]; index. myList reference 5.6  Each element in the array is represented using: myList[0] myList[1] 4.5 arrayRefVar[index] 3.3 Array reference myList[2] variable 13.2 myList[3]  The array indices starts from 0 to array.length-1 4 myList[4]  Example: Array element at myList[5] 34.33 Element value index 5 34 myList[6] 45.45 myList[7] double[] myList = new double[10]; 99.993 myList[8] myList[0] = 5.6; 11123 myList[9] myList[1] = 4.5; myList[10] = 3.33; // ArrayIndexOutOfBoundsException 9 12

Using Array Elements Example  Use elements in arrays as usual variables!!!  Array is a data structure that represents a collection of the same types of data. myList double[] myList = new double[5]; 3.0 int[] tmp = new int[10]; 0 myList[0] = 3.0; 0 . . . myList[1] = 4.5; myList[2] = 2.0; 0 System.out.println(“tmp[2]: ” + tmp[2]); 0 System.out.println(myList[2]); System.out.println(myList[4]); tmp[9] = tmp[0] + tmp[2] + tmp[4]; System.out.println(“tmp[9]: ” + tmp[9]); 13 16 Example Example myList myList double[] myList = new double[5]; double[] myList = new double[5]; 0 3.0 0 4.5 myList[0] = 3.0; myList[0] = 3.0; myList[1] = 4.5; 0 myList[1] = 4.5; 0 myList[2] = 2.0; myList[2] = 2.0; 0 0 0 0 System.out.println(myList[2]); System.out.println(myList[2]); System.out.println(myList[4]); System.out.println(myList[4]); 14 17 Example Example myArray myList double[] myList = new double[5]; double[] myList = new double[5]; 3.0 3.0 0 4.5 myList[0] = 3.0; myList[0] = 3.0; myList[1] = 4.5; 0 myList[1] = 4.5; 0 myList[2] = 2.0; myList[2] = 2.0; 0 0 0 0 System.out.println(myList[2]); System.out.println(myList[2]); System.out.println(myList[4]); System.out.println(myList[4]); 15 18

Example Using for loops to process arrays  Using for loops to traverse or process array elements  Elements are of the same type and processed repeatedly myList  The size of the array is known double[] myList = new double[5]; 3.0 4.5 myList[0] = 3.0; for (int i=0; i<myList.length; i++) { 2.0 myList[1] = 4.5; // process ith element myList[i] myList[2] = 2.0; 0 System.out.println(myList[i]); 0 System.out.println(myList[2]); } System.out.println(myList[4]); 19 Example Enhanced for Loop (for-each loop)  Traditional for loop int[] array = new int[5]; myList for (int i = 0; i < array.length; i++){ double value = array[i]; double[] myList = new double[5]; 3.0 System.out.println(value); 4.5 myList[0] = 3.0; }  Generalized for loop myList[1] = 4.5; 2.0 myList[2] = 2.0; 0 for (double value: array) // for each value in array 0 System.out.println(value); System.out.println(myList[2]); System.out.println(myList[4]); Syntax: for (elementType value: array) { // Process the value } 20 23 Example Common Array Processing  Initializing array elements  Printing arrays myList  Summing all elements double[] myList = new double[5]; 3.0  Finding largest elements 4.5 myList[0] = 3.0;  Finding smallest index of the largest element myList[1] = 4.5; 2.0 myList[2] = 2.0; 0  Shifting elements 0  Flipping elements System.out.println(myList[2]); System.out.println(myList[4]);  Counting occurrences 21

Trace Program with Arrays Initialize array elements using for loop i=1, value[1] is 1 Initialize an array with random values public class Test { for (int i=0; i<myList.length; i++) { public static void main(String[] args) After the first iteration { myList[i] = Math.random() * 100; int[] values = new int[5]; 0 0 for (int i = 1; i < 5; i++) { 1 1 } values[i] = i + values[i-1]; 0 2 } 3 0 values[0] = values[1] + values[4]; 0 4 } } 28 Trace Program with Arrays Another example of initializing arrays i=2, value[2] is 3 public class Test { public static void main(String[] args) public class Test { After the second iteration { public static void main(String[] args) int[] values = new int[5]; { 0 0 for (int i = 1; i < 5; i++) { int[] values = new int[5]; 1 1 values[i] = i + values[i-1]; for (int i = 1; i < 5; i++) { 2 3 } values[i] = i + values[i-1]; 0 3 values[0] = values[1] + values[4]; } 4 0 } values[0] = values[1] + values[4]; } } } 26 29 Trace Program with Arrays Trace Program with Arrays i=3, values[3] = 6 (3 + 3) Declare array variable, create an array, and assign its reference to values public class Test { public static void main(String[] args) public class Test { After the third iteration { public static void main(String[] args) After the array is created int[] values = new int[5]; { 0 0 for (int i = 1; i < 5; i++) { int[] values = new int[5]; 1 1 0 0 values[i] = i + values[i-1]; for (int i = 1; i < 5; i++) { 2 3 1 0 } values[i] = i + values[i-1]; 6 0 3 2 values[0] = values[1] + values[4]; } 4 0 3 0 } values[0] = values[1] + values[4]; 0 4 } } } 27 30

Trace Program with Arrays Simple Array Algorithms: Finding the Maximum or Minimum i=4, values[4] = 10 (4 + 6)  Problem: finding the maximum/minimum  Algorithm: public class Test { public static void main(String[] args)  Initialize the current maximum/minimum with the starting After the fourth iteration { element int[] values = new int[5]; 0 0 for (int i = 1; i < 5; i++) {  For each of the remaining elements 1 1 values[i] = i + values[i-1]; 3 2  Update the current maximum/minimum with the element if it is } 3 6 values[0] = values[1] + values[4]; larger or smaller 10 4 } int max = myList[0]; } for (int i=1; i<myList.length; i++) { if (max < myList[i]) max = myList[i]; } 31 Trace Program with Arrays Simple Array Algorithms: Finding the smallest index of largest item After this line, values[0] is 11 (1 + 10)  Algorithm:  Initialize current maximum with the starting element and the public class Test { index of current maximum with 0 public static void main(String[] args) {  For each of the remaining elements int[] values = new int[5]; for (int i = 1; i < 5; i++) { 0 11  Update current maximum and index if the element is larger values[i] = i + values[i-1]; 1 1 } 3 2 int max = myList[0]; values[0] = values[1] + values[4]; 3 6 int indexOfMax = 0; } 10 4 for (int i=1; i<myList.length; i++) { } if (max < myList[i]) { max = myList[i]; indexOfMax = i; } } What happens if we use max <= myList[i] ? 32 Simple Array Algorithms: Simple Array Algorithms: Counting Matches computing the sum  Problem: computing the sum of all elements in an array  Problem: counting number of a particular item in an array  Algorithm:  Algorithm:  Initialize the current sum with 0  Initialize a counter  For each of the elements  For each of the elements in the array  Add the element to the sum  Increase the counter when there is a match  Example: counting number of As in a hand of cards int sum = 0; int count = 0; for (int i=0; i<myList.length; i++) { for (int i=0; i<myList.length; i++) { sum += myList[i]; if (myList[i] == 1) } count ++; }  How to compute the average?