Topics. Introduction to Arrays. Topics. Creating Arrays. Creating Arrays CS 146. Introduction to Programming and Algorithms Module 7

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1 Introduction to Programming and Algorithms Module 7 CS 146 Sam Houston State University Dr. Tim McGuire Topics Module 7 discusses the following main topics: Introduction to Arrays Processing Array Contents Passing Arrays as Arguments Some Useful Array Algorithms and Operations Returning Arrays from Methods String Arrays Arrays of Objects The Sequential Search Algorithm 2 Topics Module 7 discusses the following main topics: The Selection Sort and Binary Search algorithms Two-Dimensional Arrays Arrays with Three or More Dimensions Command Line Arguments and Variable Length Argument Lists The Vector Class Introduction to Arrays Primitive variables are designed to hold only one value at a time. Arrays allow us to create a collection of like values that are indexed. An array can store any type of data but only one type of data at a time. An array is a list of data elements. 3 4 Creating Arrays An array is an object so it needs an object reference. int[] numbers; //declares a reference to an array that will hold integers. The next step, typically performed in the class constructor: numbers = new int[6]; //creates a new array that will hold 6 integers. index index 1 index 2 index 3 index 4 Array element values are initialized to. Array indexes always start at. index 5 Creating Arrays It is possible to declare an array reference and create it in the same statement. int[] numbers = new int[6]; Arrays may be of any type, not just int. float[] temperatures = new float[1]; char[] letters = new char[41]; long[] units = new long[5]; double[] sizes = new double[12]; 5 6 1

2 Creating Arrays The array size must be a non-negative number. It may be a literal value or be derived from a constant or variable. final int ARRAY_SIZE = 6; int[] numbers = new int[array_size]; Once created, an array size is fixed and cannot be changed. Accessing the Elements of an Array 2 numbers[] numbers[1] numbers[2] An array is accessed by: the reference name a subscript that identifies which element in the array to access. numbers[] = 2; //pronounced numbers sub zero numbers[3] numbers[4] numbers[5] 7 8 Inputing and Outputing Array Elements Array elements can be treated as any other variable. They are simply accessed by the same name and a subscript. Example: ArrayDemo1.java Array subscripts can be accessed using variables (such as for loop counters). Example: ArrayDemo2.java 9 Bounds Checking Array indexes always start at zero and continue to (array length - 1). int values = new int[1]; This array would have indexes through 9. Example: InvalidSubscript.java In for loops, it is typical to use i, j, and k as counting variables. It might help to think of i as representing the word index. 1 Off-by-1 Errors Array Initialization It is very easy to be off-by-one when accessing arrays. // This code has an off-by-one error. int[] numbers = new int[1]; for (int i = 1; i <= 1; i++) numbers[i] = 99; Here, the equal sign allows the loop to continue on to index 1, where 99 is the last index in the array. This code would throw an ArrayIndexOutOfBoundsException. Intitializing the data in an array could mean a lot of typing. int[] days = new int[12]; days[] = 31; // January days[1] = 28; // February

3 Array Initialization When relatively few items need to be initialized, an initialization list can be used to initialize the array. int[]days = 31, 28, 31, 3, 31, 3, 31, 31, 3, 31, 3, 31; The numbers in the list are stored in the array in order: days[] is assigned 31, days[1] is assigned 28, days[2] is assigned 31, days[3] is assigned 3, etc. Example: ArrayInitialization.java 13 Alternate Array Declaration Previously we showed arrays being declared: int[] numbers; However, the brackets can also go here: int numbers[]; These are equivalent but the first style is typical. Multiple arrays can be declared on the same line. int[] numbers, codes, scores; With the alternate notation each variable must have brackets. int numbers[], codes[], scores; The scores variable in this instance is simply an int variable. 14 Processing Array Contents Processing data in an array is the same as any other variable. grosspay = hours[3] * payrate; Pre and post increment works the same: int[] score = 7, 8, 9, 1, 11; ++score[2]; // Pre-increment operation score[4]++; // Post-increment operation Example: PayArray.java Processing Array Contents Array elements can be used in relational operations: if(cost[2] < cost[]) They can be used as loop conditions: while(value[count]!= ) Example: Overtime.java Array Size Arrays are objects and provide a length constant that can be tested. double[] temperatures = new double[25]; The length of this array is 25. The length of an array can be obtained via its length constant. int size = temperatures.length; The variable size will contain 25. Array Size The length constant can be used in a loop to provide automatic bounding. for(int i = ; i < temperatures.length; i++) System.out.println( Temperature + i : + temperatures[i]); // Be careful of off-by-one errors. It is possible to get the size of an array from a user: int size; int[] numbers; System.out.print("How many numbers do you have? "); size = Keyboard.readInt(); numbers = new int[size]; Example: DisplayTestScores.java

4 Reassigning Array References Reassigning Array References An array reference can be assigned to another array of the same type. // Create an array referenced by the numbers variable. int[] numbers = new int[1]; // Reassign numbers to a new array. numbers = new int[5]; If the first (ten element) array no longer has a reference to it, it will be garbage collected. The numbers variable holds the address of an int array. int[] numbers = new int[1]; 19 2 Reassigning Array References Copying Arrays The numbers variable holds the address of an int array. This array gets marked for garbage collection numbers = new int[5]; This is not the way to copy an array. int[] array1 = 2, 4, 6, 8, 1 ; int[] array2 = array1; // This does not copy array1. array1 holds an address to the array array2 holds an address to the array Example: SameArray.java Copying Arrays You cannot copy an array by merely assigning one reference variable to another. You need to copy the individual elements of one array to another. int[] firstarray = 5, 1, 15, 2, 25 ; int[] secondarray = new int[5]; for (int i = ; i < firstarray.length; i++) secondarray[i] = firstarray[i]; This code copies each element of firstarray to the corresponding element of Passing Array Elements as Arguments Array elements are handles like an independent variable. They are passed by value like any variable. showarray(numbers[]); Value Example: PassElements.java public static void showvalue(int element) System.out.print(element + " "); secondarray

5 Passing Arrays as Arguments Comparing Arrays Arrays are objects. Their references can be passed to methods like any other object reference variable. showarray(numbers); Example: PassArray.java public static void showarray(int[] array) for (int i = ; i < array.length; i++) System.out.print(array[i] + " "); 25 The == operator only compares if the array references point to the same array object. To compare the contents of an array: int[] firstarray = 2, 4, 6, 8, 1 ; int[] secondarray = 2, 4, 6, 8, 1 ; boolean arraysequal = true; int i = ; if (firstarray.length!= secondarray.length) arraysequal = false; while (arraysequal && i < firstarray.length) if (firstarray[i]!= secondarray[i]) arraysequal = false; i++; if (arraysequal) System.out.println("The arrays are equal."); else System.out.println("The arrays are not equal."); 26 Useful Array Operations Partially Filled Arrays Summing Array Elements: int total = ; // Initialize accumulator for (int i = ; i < units.length; i++) total += units[i]; Averaging Array Elements: double total = ; // Initialize accumulator double average; // Will hold the average for (int i = ; i < scores.length; i++) total += scores[i]; average = total / scores.length; Example: SalesData.java, Sales.java 27 Typically, if it is unknown how much data an array will be holding: size the array to the largest expected number of elements. use a counting variable to keep track of how much valid data is in the array. int[] array = new int[1]; int count = ; System.out.print("Enter a number or -1 to quit: "); number = Keyboard.readInt(); while (number!= -1 && count <= 99) count++; array[count - 1] = number; 28 Arrays and Files Arrays and Files Saving the contents of an array to a file: int[] numbers = 1, 2, 3, 4, 5; Reading the contents of a file into an array: int[] numbers = new int[5]; //assuming we know the size String str; int i = ; FileWriter fwriter = new FileWriter("Values.txt"); PrintWriter outputfile = new PrintWriter(fwriter); FileReader freader = new FileReader("Values.txt"); BufferedReader inputfile = new BufferedReader(freader); for (int i = ; i < numbers.length; i++) outputfile.println(numbers[i]); outputfile.close(); str = inputfile.readline(); while (str!= && i < numbers.length) numbers[i] = Integer.parseInt(str); i++; str = inputfile.readline(); inputfile.close();

6 Returning an Array Reference A method can return a reference to an array. The return type of the method must be declared as an array of the right type. public static double[] getarray() double[] array = 1.2, 2.3, 4.5, 6.7, 8.9 ; return array; The getarray method is a public static method that returns an array of doubles. Example: ReturnArray.java 31 String Arrays Arrays are not limited to primitive data. An array of String objects can be created: String[] names = "Bill", "Susan", "Steven", "Jean" ; The names variable holds the address to the array. names[] address names[1] address names[2] address names[3] address A String array is an array of references to String objects. Bill Susan Steven Jean Example: MonthDays.java 32 String Arrays If an initialization list is not provided, the new keyword must be used to create the array: String[] names = new String[4]; The names variable holds the address to the array. String Arrays When an array is created in this manner, each element of the array must be initialized. The names variable holds the address to the array. names[] = "Bill"; names[1] = "Susan"; names[2] = "Steven"; names[3] = "Jean"; names[] names[1] names[2] names[3] names[] names[1] names[2] names[3] Bill Susan Steven Jean Calling String Methods On Array Elements String objects have several methods. touppercase, tolowercase, compare, etc. Each element of a String array is a String object. Methods can be used by using the array name and index as before. System.out.println(names[].toUpperCase()); Example: StringArrayMethods.java The length Field & The length method Arrays have a final field named length. String objects have a method named length. To display the length of each string held in a String array: for (int i = ; i < names.length; i++) System.out.println(names[i].length()); An array s length member is a field You do not write a set of parentheses after its name. You do write the parentheses after the name of the String class s length method

7 Arrays of Objects Arrays of Objects Since Strings are objects, we know that arrays can contain objects. InventoryItem[] inventory = new InventoryItem[5]; The inventory variable holds the address of an InventoryItem array. Each element needs to be initialized. for (int i = ; i < inventory.length; i++) inventory[i] = new InventoryItem(); Example: ObjectArray.java The inventory variable holds the address of an InventoryItem array. units: units: inventory[] inventory[1] inventory[2] inventory[3] inventory[4] 37 inventory[] inventory[1] inventory[2] inventory[3] inventory[4] units: units: units: 38 Sorting an Array Using Array.Sort() Java provides a class named Array that simplifies some array operations. The Array class has a static method named sort that will sort a numeric array in ascending order. Array.sort(numbers); To use the class, the import statement, import java.util.array; must be used. Example: SortDemo.java The Sequential Search Algorithm A search algorithm is a method of locating a specific item in a larger collection of data. The sequential search algorithm uses a loop to: sequentially step through an array, comparing each element with the search value, and stopping when the value is found or the end of the array is encountered. Example: SearchArray.java, TestSearch.java 39 4 The Selection Sort Algorithm A sort algorithm is a method of arranging data in a chosen order The Selection Sort algorithm uses a nested loops to: sequentially step through an array multiple times, comparing elements to a chosen minimum value and swapping when Smaller value is found Selection Sort Pseudocode For startscan is each subscript in array from through the next-to-last subscript Set index variable to startscan. Set minindex variable to startscan. Set minvalue variable to array[startscan]. For index is each subscript in array from (startscan + 1) through the next-to-last subscript If array[index] is less than minvalue Set minvalue to array[index]. Set minindex to index. End If. Increment index. End For. Set array[minindex] to array[startscan]. Set array[startscan] to minvalue. End For. Example: SelectionSortDemo.java

8 Selection Sort Method public static void selectionsort(int[] array) int startscan, index, minindex, minvalue; for (startscan = ; startscan < (array.length-1); startscan++) minindex = startscan; minvalue = array[startscan]; for(index = startscan + 1; index < array.length; index++) if (array[index] < minvalue) minvalue = array[index]; minindex = index; array[minindex] = array[startscan]; array[startscan] = minvalue; 43 The Binary Search Algorithm Binary search is far more efficient that sequential search, especially for large data collections Often referred to as Divide and Conquer The Binary Search algorithm uses a loop to: Select the middle array element, Compare the search value to the value found End if values match Eliminate top half of array if search value < middle Eliminate bottom half of array if search value > middle Example: BinarySearchDemo.java 44 Binary Search Pseudocode Set first to. Set last to the last subscript in the array. Set found to false. Set position to -1. While found is not true and first is less than or equal to last Set middle to the subscript half-way between array[first]and array[last]. If array[middle] equals the desired value Set found to true. Set position to middle. Else If array[middle] is greater than the desired value Set last to middle - 1. Else Set first to middle + 1. End If. End While. Return position. 45 Binary Search Method public static int binarysearch(int[] array, int value) int first, last; // First and Last array elements int middle, position; // Middle position & position of search value boolean found; // Flag // Set the inital values. first = ; last = array.length - 1; position = -1; found = false; // Search for the value. while (!found && first <= last) middle = (first + last) / 2; // Calculate mid point if (array[middle] == value) // If value is found at mid found = true; position = middle; else if (array[middle] > value) // If value is in lower half last = middle - 1; else first = middle + 1; // If value is in upper half // Return the position of the item, or -1 // if it was not found. return position; 46 Parallel Arrays By using the same subscript, you can build relationships between data stored in two or more arrays. String[] names = new String[5]; String[] addresses = new String[5]; The names array stores the names of five persons The addresses array stores the addresses of the same five persons. The data for one person is stored at the same index in each array. 47 names[] Person #1 Parallel Arrays Relationship between names and addresses array elements. names[1] Person #2 names[2] Person #3 names[3] Person #4 names[4] Person #5 addresses[] addresses[1] addresses[2] addresses[3] addresses[4] Parallel arrays are useful when storing data of unlike types. Example: ParallelArrays.java 48 8

9 Command-Line Arguments A Java program can receive arguments from the operating system command-line. The main method with a header that looks like this: public static void main(string[] args) The main method receives a String array as a parameter. The array that is passed into the args parameter comes from the operating system command-line. Example: CommandLine.java Command-Line Arguments To run the example: java CommandLine How does this work? args[] is assigned How args[] is assigned does args[] is assigned this args[] is assigned work? It is not required that the name of main s parameter array be args The Random Class Some applications, such as games and simulations, require the use of randomly generated numbers. The Java API has a class, Random, for this purpose. The Random class uses an algorithm to generate a sequence of random numbers. 51 9