Arrays Chapter 7

2 "All students to receive arrays!" reports Dr. Austin. Declaring arrays scores : Inspecting arrays Stepping through arrays Passing arrays as parameters

3 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 Syntax int c[] = new int [12]; Results: 12 integer locations are allocated They are initialized to 0 (null for reference variables, false for boolean) Note the difference in the declaration - the number of elements is not specified in the first set of square brackets.

4 Using Arrays View example Figure 7.2Figure 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 ExampleExample Note again, how the array knows its own length for (int count = 0 ; count < array.length ; count++)...

6 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 programsample program On Average People Are Mean

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,Figure 7.6

8 Using Array Elements as Counters Recall program which tested random number generator (Figure 6.8)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 7.7Figure 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 7.12Figure 7.12

10 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 17 Object 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 7.13Figure 7.13

12 Using Arrays Sorting Arrays Recall previous exampleprevious example Searching Arrays Linear search (see example)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 rows b[ 0 ] = new int[ 5 ]; // allocate columns for row 0 b[ 1 ] = new int[ 3 ]; // allocate columns for row 1

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

15 Can be created dynamically 3 -by- 4 array int b[][]; b = new int[ 3 ][ 4 ]; Rows can have different number of columns int b[][]; b = new int[ 2 ][ ]; // create 2 rows b[ 0 ] = new int[ 5 ]; // create 5 columns for row 0 b[ 1 ] = new int[ 3 ]; // create 3 columns for row Creating Two-dimensional Arrays View Example Figure 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. 7.20Fig. 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 Example Figure 7.21