Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods...

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Arrays Module 6

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Transcript of Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods...

Page 1: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Arrays

Module 6

Page 2: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Objectives

• Nature and purpose of an array

• Using arrays in Java programs

• Methods with array parameter

• Methods that return an array

• Array as an instance variable

• Use an array not filled completely

Page 3: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Creating and Accessing Arrays

• An array is a special kind of object• Think of as collection of variables of same type• Creating an array with 7 variables of type double

• To access an element use The name of the array An index number enclosed in braces

• Array indices begin at zero

Page 4: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Creating and Accessing Arrays

• Figure 7.1 A common way to visualize an array

• Note sample program, listing 7.1class ArrayOfTemperatures

Page 5: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Creating and Accessing Arrays

Sample screen output

Sample screen output

Page 6: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Array Details

• Syntax for declaring an array with new

• The number of elements in an array is its length

• The type of the array elements is the array's base type

Page 7: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Square Brackets with Arrays

• With a data type when declaring an arrayint [ ] pressure;

• To enclose an integer expression to declare the length of the array

pressure = new int [100];

• To name an indexed value of the arraypressure[3] =

keyboard.nextInt();

Page 8: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Array Details

• Figure 7.2 Array terminology

Page 9: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

The Instance Variable length

• As an object an array has only one public instance variable Variable length Contains number of elements in the array It is final, value cannot be changed

• Note revised code, listing 7.2class ArrayOfTemperatures2

Page 10: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

The Instance Variable length

Sample screen output

Sample screen output

Page 11: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

More About Array Indices

• Index of first array element is 0• Last valid Index is arrayName.length – 1• Array indices must be within bounds to be

valid When program tries to access outside bounds,

run time error occurs

• OK to "waste" element 0 Program easier to manage and understand Yet, get used to using index 0

Page 12: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Initializing Arrays

• Possible to initialize at declaration time

• Also may use normal assignment statements One at a time In a loop

Page 13: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Indexed Variables as Method Arguments

• Indexed variable of an array Example … a[i] Can be used anywhere variable of array base

type can be used

• View program using indexed variable as an argument, listing 7.5class ArgumentDemo

Page 14: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Entire Arrays as Arguments

• Declaration of array parameter similar to how an array is declared

• Example:

Page 15: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Entire Arrays as Arguments

• Note – array parameter in a method heading does not specify the length An array of any length can be passed to the

method Inside the method, elements of the array can

be changed

• When you pass the entire array, do not use square brackets in the actual parameter

Page 16: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Arguments for Method main

• Recall heading of method mainpublic static void main (String[] args)

• This declares an array Formal parameter named args Its base type is String

• Thus possible to pass to the run of a program multiple strings These can then be used by the program

Page 17: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Array Assignment and Equality

• Arrays are objects Assignment and equality operators behave

(misbehave) as specified in previous chapter

• Variable for the array object contains memory address of the object Assignment operator = copies this address Equality operator == tests whether two

arrays are stored in same place in memory

Page 18: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Array Assignment and Equality

• Two kinds of equality

• View example program, listing 7.6class TestEquals

Sample screen output

Sample screen output

Page 19: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Array Assignment and Equality

• Note results of ==• Note definition and use of method equals

Receives two array parameters Checks length and each individual pair of

array elements

• Remember array types are reference types

Page 20: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Methods that Return Arrays

• A Java method may return an array

• View example program, listing 7.7class ReturnArrayDemo

• Note definition of return type as an array

• To return the array value Declare a local array Use that identifier in the return statement

Page 21: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Sorting, Searching Arrays: Outline

• Selection Sort

• Other Sorting Algorithms

• Searching an Array

Page 22: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Selection Sort

• Consider arranging all elements of an array so they are ascending order

• Algorithm is to step through the array Place smallest element in index 0 Swap elements as needed to accomplish this

• Called an interchange sorting algorithm

Page 23: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Selection Sort

• Figure 7.5a

Page 24: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Selection Sort

• Figure 7.5b

. . .

Page 25: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Selection Sort

• Algorithm for selection sort of an array

Page 26: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Selection Sort

• View implementation of selection sort, listing 7.10 class ArraySorter

• View demo program, listing 7.11class SelectionSortDemo

Sample screen output

Sample screen output

Page 27: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Other Sorting Algorithms

• Selection sort is simplest But it is very inefficient

• Java Class Library provides for efficient sorting Has a class called Arrays Class has multiple versions of a sort method

Page 28: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Searching an Array

• Method used in OneWayNoRepeatsList is sequential search Looks in order from first to last Good for unsorted arrays

• Search ends when Item is found … or … End of list is reached

• If list is sorted, use more efficient searches

Page 29: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Multidimensional Arrays: Outline

• Multidimensional-Array Basics

• Multidimensional-Array Parameters and Returned Values

• Java's Representation of Multidimensional

• Ragged Arrays

Page 30: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Multidimensional-Array Basics

• Consider Figure 7.6, a table of values

Page 31: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Multidimensional-Array Basics

• Figure 7.7 Row and column indices for an array named table

table[3][2] has a value of 1262

table[3][2] has a value of 1262

Page 32: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Multidimensional-Array Basics

• We can access elements of the table with a nested for loop

• Example:

• View sample program, listing 7.12class InterestTable

Page 33: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Multidimensional-Array Basics

Sample screen output

Sample screen output

Page 34: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Multidimensional-Array Parameters and Returned Values

• Methods can have Parameters that are multidimensional-arrays Return values that are multidimensional-

arrays

• View sample code, listing 7.13class InterestTable2

Page 35: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Java's Representation of Multidimensional Arrays

• Multidimensional array represented as several one-dimensional arrays

• Given int [][] table = new int [10][6];

• Array table is actually 1 dimensional of type int[] It is an array of arrays

• Important when sequencing through multidimensional array

Page 36: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Ragged Arrays

• Not necessary for all rows to be of the same length

• Example:

Page 37: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Summary

• An array is a collection of variables all of the same type

• Arrays are objects, created with operator new

• Elements numbered starting with 0, ending with 1 less than length

• Indexed variable can be used as a parameter – treated like variable of base type

Page 38: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Summary

• Entire array can be passed as parameter to a method

• Method return value can be an array

• Partially filled array usually stores values in initial segment, use an int to track how many are used

• Privacy leak caused by returning array corresponding to private instance variable

Page 39: Arrays Module 6. Objectives Nature and purpose of an array Using arrays in Java programs Methods with array parameter Methods that return an array Array.

Summary• Selection sort orders an array into

ascending or descending order

• Multidimensional arrays are implemented as an array of arrays

• Treat two-dimensional array as a table with rows and columns


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