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Arrays

Arrays. Chapter 7. Declaring arrays. Stepping through arrays. Passing arrays as parameters. Inspecting arrays. "All students to receive arrays!" reports Dr. Austin. scores : 85 79 92 57 68 80 . . . . 0 1 2 3 4 5 98 99.

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Arrays

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  1. Arrays Chapter 7

  2. Declaring arrays Stepping through arrays Passing arrays as parameters Inspecting arrays "All students to receive arrays!" reports Dr. Austin. scores : 85 79 92 57 68 80 . . . 0 1 2 3 4 5 98 99

  3. Note the difference in the declaration - the number of elements is not specified in the first set of square brackets. Declaring and Creating Arrays • Recall that an array is a collection of elements all of the same type • Array objects in Java must be created with the key word new • Syntaxint c[] = new int [12]; • Results: • 12 integer locations are allocated • They are initialized to 0(null for reference variables, false for boolean)

  4. Using Arrays • View example Figure 7.2 • Note • Declaration of the array • Allocation of memory with new • Initialization of array elements • Use of array attribute (length) • Printing of array contents

  5. The Array Initializer • A comma separated list of expressions • The initializer list • Enclosed in braces • View the Example • Note again, how the array knows its own lengthfor (int count = 0 ; count < array.length ; count++) . . .

  6. On Average People Are Mean Processing Elements of An Array • Finding and using the arithmetic mean • Declare the array, create • Read in values, count how many • Declare a summation variable, initialize • Sum the array, calculate average • Sort the array • Print results, accessing the array • See sample program

  7. Histogram Program • Use the value in the array to • Print the value • Print a string of asterisks for visualization of size of the value • View Figure 7.6,

  8. Using Array Elements as Counters • Recall program which tested random number generator (Figure 6.8) • Used 6 different variables • Used switch statement to increment • At the time we noted the inefficiency • Consider a new version of the program • Uses array elements to total the number of rolls of each number 1 – 6 • No switch statement needed • View Figure 7.7

  9. Enhanced for Statement • Enhanced for statement • New feature of J2SE 5.0 • Allows iterates through elements of an array or a collection without using a counter • Syntax for ( parameter : arrayName ) statement • View example, Figure 7.12

  10. 17 Object References and Reference Parameters • In Java, primitive-type variables are always passed by value • Incoming data only • Objects are not passed to methods • References to objects are passed • The reference is passed by value • With a reference to the object the method can manipulate the object directly Address of object

  11. References and Reference Parameters • Passing arrays by reference • Improves performance • Saves time • Initialization of large array not required • Saves memory • Use memory already occupied by array • No new memory allocation required • View example program, Figure 7.13

  12. Using Arrays • Sorting Arrays • Recall previous example • Searching Arrays • Linear search (see example) • Step through array until desired value located • Binary search • Array must be sorted • Look in middle, then above or below • Look in middle of sub-section then above or below • Etc.

  13. Multidimensional Arrays • Java does not support multidimensional arrays directly • Does allow declaration of arrays whose elements are arrays! int b[][];b = new int[ 2 ][ ]; // allocate rowsb[ 0 ] = new int[ 5 ]; // allocate columns for row 0b[ 1 ] = new int[ 3 ]; // allocate columns for row 1

  14. Fig. 7.16| Two-dimensional array with three rows and four columns. Multidimensional Arrays

  15. Creating Two-dimensional Arrays • Can be created dynamically • 3-by-4 array int b[][]; b = newint[ 3 ][4 ]; • Rows can have different number of columns int b[][]; b = newint[ 2 ][ ]; // create 2 rows b[ 0 ] = newint[ 5 ]; // create 5 columns for row 0 b[ 1 ] = newint[ 3 ]; // create 3 columnsfor row View ExampleFigure 7.17

  16. Variable-Length Argument Lists • New feature in J2SE 5.0 • Unspecified number of arguments • Use ellipsis (…) in method’s parameter list • Can occur only once in parameter list • Must be placed at the end of parameter list • Array whose elements are all of the same type • View example, Fig. 7.20

  17. Using Command-Line Arguments • Pass arguments from the command line • String args[] • Appear after the class name in the java command • java MyClass a b • Number of arguments passed in from command line • args.length • First command-line argument • args[ 0 ] View ExampleFigure 7.21

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