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Two Dimensional Arrays

Two Dimensional Arrays. Found in chapter 8, Section 8.9. Introduction. Where have you seen them? When do we need them? Why are they useful? How do we declare them? How do we access them?. Review. Primitive variables are designed to hold only one value at a time.

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Two Dimensional Arrays

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  1. Two Dimensional Arrays Found in chapter 8, Section 8.9

  2. Introduction • Where have you seen them? • When do we need them? • Why are they useful? • How do we declare them? • How do we access them?

  3. Review • 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.

  4. More Review • An array is an object so it needs an object reference. int[] numbers; OR int number []; //declares a reference to an array that will hold integers. • The next step creates the array and assigns its address to the numbers variable numbers = new int[6]; //creates a new array that will hold 6 integers.

  5. Accessing Array Elements • An array is accessed by: • the reference name • a subscript that identifies which element in the array to access. • Examples • ArrayDemo1.java • ArrayDemo2.java • Array Bounds Checking • Examples • InvalidSubscript.java • ArrayIndexOutOfBoundsException.

  6. Initializing Arrays • When relatively few items need to be initialized, an initialization list can be used to initialize the array. int[]days = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}; • The numbers in the list are stored in the array in order: • Example • ArrayInitialization.java

  7. 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, 10, 11}; ++score[2]; // Pre-increment operation score[4]++; // Post-increment operation • Example: • PayArray.java • Array elements can be used in relational operations: if(cost[20] < cost[0]) { //statements } • They can be used as loop conditions: while(value[count] != 0) { //statements }

  8. The Enhanced for Loop • Simplified array processing (read only) • Always goes through all elements • General: for(datatypeelementVariable : array) statement; Example: int[] numbers = {3, 6, 9}; For(int val : numbers) { System.out.println(“The next value is” + val); }

  9. Array Methods & Fields • Arrays are objects and provide a public field named length that is a constant that can be tested. double[] temperatures = new double[25]; • The length of an array can be obtained via its length constant. int size = temperatures.length; • The variable size will contain 25.

  10. Interesting Java feature • It is possible to get the size of an array from a user: ; int numTests; int[] tests; Scanner keyboard; keyboard = new Scanner (System.in); System.out.print("How many numbers do you have? "); numTests = keyboard.nextInt(); tests = new int[numTests]; • Example: • DisplayTestScores.java

  11. Understanding 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[10]; // Reassign numbers to a new array. numbers = new int[5]; • Copying arrays • Wrong way • Right way

  12. Parameters & Arrays • Passing single elements • PassElements.java • Passing entire arrays • public static void showArray(int[] array) • PassArray.java • Passing partially filled arrays • Returning arrays • public static double [ ] getArray() • ReturnArray.java

  13. Doing things with Arrays • Bad way • Good way • compare lengths (else they can’t be the same) • then compare individual items • Summing array elements • Finding the average of the array elements • Finding the minimum and maximum value • Searching arrays • Sequential search – SearchArray.java • Binary search

  14. Sorting an Array • 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.

  15. Other Arrays • String Arrays • MonthDays.java • String objects have several methods. • toUpperCase, • compareTo • equals • charAt • Each element of a String array is a String object. • String objects have a method named length() • Arrays of Objects • ObjectArray.java • Parallel arrays • Two-Dimensional arrays

  16. Two-Dimensional arrays • Is an array of arrays • Can be thought of as having rows and columns • Declaring a two-dimensional array requires two sets of brackets and two size declarators • The first one is for the number of rows • The second one is for the number of columns. double[][] scores = new double[3][4]; • Each array element has two subscripts

  17. Initializing and Accessing • Accessing two-dimensional array elements • Can access individual elements • Can use a loop to access all elements • Example • CorpSales.java • Initializing a two-dimensional array requires enclosing each row’s initialization list in its own set of braces. int[][] numbers = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}; • Java automatically creates the array and fills its elements with the initialization values. • row 0 {1, 2, 3} • row 1 {4, 5, 6} • row 2 {7, 8, 9} • Declares an array with three rows and three columns.

  18. The length Field • Two-dimensional arrays are arrays of one-dimensional arrays. • The length field of the array gives the number of rows in the array. • Each row has a length constant tells how many columns is in that row. • Each row can have a different number of columns. • Example • Lengths.java

  19. Things to do with arrays • Summing rows • Summing columns • Finding max value • Finding min value • Passing and returning arrays • There is no difference between passing a single or two-dimensional array as an argument to a method. • The method must accept a two-dimensional array as a parameter. • Example: Pass2Darray.java

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