Chapter 6- Arrays

1
Chapter 6- Arrays
2
Overview

• What are arrays?
• Declaring/initializing arrays.
• Using arrays.
• Arrays details.
• Using arrays with classes/ in classes.
• Sorting/searching arrays.
• Multidimensional arrays.
• Review

3
Arrays, what are they?
4
What are arrays?

• An array is a large collection of variables that
  are all of the same data type.
• It allows us to have a large number of similar
  variables without having to declare each one
  individually.

5
What are arrays used for?

• Think back to the grocery purchase program that
  we did. Imagine instead of having only five items
  (and declaring five Purchase objects) we had 100
  items the user could choose. Since much of the
  code for each Purchase object is similar, we
  could just have an array and one piece of code to
  repeatedly do stuff to that array.

6
More on arrays

• Arrays are like mailboxes. They have a collective
  name (say the zip code for the University), an
  index (your box number), and a value (any mail in
  your box).

0
1
2
3
Index
Value
Value
43
105
Name
7
Arrays

• Arrays are actually objects (though they dont
  look like it). They are initialized with the new
  operator. They store a memory address in their
  variable.
• Weve already seen them, we just havent used
  them.

8
Declaring and initializing arrays
9
Declaring arrays.

• We declare an array in a very similar way to how
  we declare variables. The only difference is the
  symbols.

int blar double 3 muglets String args
10
Initializing arrays.

• There are lots of ways to initialize arrays.

blarnew int20 //blar now has 20 0s. muglets
new int2 //muglets now has 2
0s muglets02.3 muglets11.2 String
args Hello, How, are, you
11
Array indices

• Arrays are indexed from 0 to (the length of the
  array 1). Remember this. It will cause you
  immeasurable grief in programming. The index
  always starts a 0.

blar new int20 //indexed 0-19 muglets new
double4 //indexed 0-3 args Howdy,
there //indexed 0-1 int lonely 1
//indexed 0-0
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Declaring and initializing

• Can declare and initialize arrays in just one
  statement(this is the usual way).

double muglet new double4 String words
Hello, Howdy int lonely 1
13
What is happening
int blar 1,7,4,6
0
1
2
3
blar
1
7
4
6
14
Which of the following are valid
declarations/initializations of arrays?
int arg new int(5) int arg new
int10 double blar blar new
double15 String muglet Hello char
yadda Howdy int someNum 1,5,7
Not valid
Not valid
Valid
Not valid
Not valid
Valid
15
What are the indices of the following arrays?
int arr new int10 double yadda new
double1 String hello Hello, Hello,
Hello int number 1,5,10,15,20 int I
4 double sum new doubleI
0-9
0-0
0-0
0-4
0-3
16
Using Arrays
17
Accessing arrays

• To get at the information inside of an array, we
  again use the operators.
• To access the ith element of an array called
  arrayName, we say arrayNamei.
• Remember, we start at index 0, with the 0th
  element.

18
Using arrays
double someArray 1.4, 2.6,2.1,
3.4,8.4 double I someArray0//I1.4 double
j someArray1//j 2.6 double k
someArray2//k 2.1 double m
someArray3//m 3.4 double n
someArray4//n8.4 double o someArray5//o
ArrayIndexOutOfBounds Exception
19
More info on arrays.

• The length of an array is also stored in the
  array object, and we can access it through the
  public length variable.
• arrayName.length
• We can only read this variable. We cannot set it.

20
Common uses of length.
//assume arr is an array. for(int I0
Iltarr.lengthI) System.out.println(arrI)
21
Which are valid uses of arrays?
int muglets new int5 double blar new
double3 muglets(0) 3 muglets3
24 muglets5 3 blar.length
muglets.length if(muglets.length
blar.length)
Not valid
Valid
Not valid
Not valid
Valid
22
Array Details
23
Some more rules about arrays.

• All of a single array should be of the same type.
  You cant have a mixed array, part String and
  part integer. You can only store one type of data
  in a single array.
• Array names are just memory addresses. Thus they
  act more like objects than primitives. You cant
  just use and to copy an array or compare
  arrays. Have to do it element by element.

24
More rules.

• Dont go out of bounds! Stay within your array.
  You WILL make this mistake. Often. It WILL crash
  your program.
• If you dont give an initialization list to an
  array, the computer just initializes all of the
  data in the array to some default value for the
  data type. If your data type is a class, it just
  stores null in the array position(you need to
  construct a new object when before you use
  this!).

25
Array details review

• What is output to the screen?
• int someArray new int5
• someArray5 3
• System.out.println(someArray5)
• What does the name of the array store? What two
  common operations cant you do with arrays?

Nothing
26
Array Details Review

• What is output to the screen?
• int someArray new int20
• System.out.println(someArray5)
• Is the following valid?
• int someArray new int20
• someArray5 1

0
No
27
Using arrays with classes / in classes.
28
Using arrays in classes

• We can use an array in any class that we want.
  They can be private instance variables. They can
  be public static variables. They can be variables
  that are local to a method.
• We can also pass arrays to methods, return arrays
  from methods, or pass single elements from arrays
  to a method.

29
Using arrays in classes
private int blar //want to initialize
//in a constructor or //in some
method. public static double mug new
double15 private Purchase
itemsOffered
30
Passing single elements to a method

• If a method requires a variable of the same data
  type as your array base type, then you can pass a
  single element to the method.
• double blar 3.0,4.0
• Triangle a new Triangle()
• a.setBase(blar0) //a.setBase(3.0)
• b.setHeight(blar1) //a.setHeight(4.0)

31
Passing arrays to methods

• We can also pass whole arrays to methods(notice,
  every main method accepts a String array).
• public void doSomething(int someArray)
• int blar new int20
• doSomething(blar)

Indicates it requires an array
Dont use array index things . Just the name.
If you use the symbols, you will pass a
single element, not the whole array.
32
Returning arrays from methods

• We can also return arrays from methods, if we
  like. Be careful, this is just like returning a
  non-String object. If you want the array to be
  private, it is better to make a copy and return
  it.
• public int doSomething()
• int someArray new int20
• return someArray

Indicates going to return an array.
Again, want to return the whole array, so we use
the whole name, no symbols.
33
Review - Arrays in classes and methods.

• Can we use arrays in classes? Is there anything
  special that we have to do to use them in
  classes?
• How do we pass or return a single element of an
  array?
• Make a method declaration that is public, returns
  nothing, is called muglet, and accepts an
  integer array as a parameter.

34
Review- Arrays in Classes.

• Make a method declaration for a method that is
  public, returns an array of doubles, is called
  blar, and accepts an array of integers.
• Why do we have to be careful when returning an
  array from a method? What can we do to be safe?

35
Searching/Sorting
36
How to find information in arrays

• Now that we are able to store information in
  arrays, we also need to be able to find the
  information again and retrieve it.
• To find information we will perform a searching
  algorithm on the array(well look for the value
  in the array).

37
Sequential search

• The sequential search is very simple We start at
  the first element in the array, and compare it
  against what we want to find. We continue this
  till the last element in the array. If we ever
  find the value, were done. If we get to the end
  of the array, and we havent found it, then the
  value isnt in the array.

38
Sequential search in code
int toSearchFor 5, i int anArray new
int20 //anArray gets some values from the
user. for(I0 IltanArray.length
I) if(toSearchFor anArrayI)
System.out.println(Found it.)
System.out.println(At location I)
break if(IanArray.length) System.out.prin
tln(Didnt find it.)
39
Other searches

• There are other searches we can perform that are
  sometimes faster.
• A binary search can be performed if all of the
  data is in order(like a phone book)
• Look at the halfway point. If you are at too
  small of a value, disregard the left half and
  proceed searching the right half. If you are at
  too large a value, disregard the right half, and
  look at the left half.

40
Other searches continued...

• Many of these faster searches, such as a binary
  search, require that our list of information (or
  array) be sorted.
• We must find some way of sorting an array.

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Why sorting

• Organization in general can make things quicker.
  We spend less time looking for things, and more
  time using them. This is why we do sorting. It
  makes our information easier to find and use.
• The first sorting algorithm we will look at is
  the Selection Sort.

42
Selection Sort algorithm(smallest to largest).

• For each element i in the list.
• Find the ith smallest value.
• Place this value in the ith position.
• This is probably the most intuitive sort
  algorithm. We need to refine this algorithm more
  before we can use it in Java code, though.

43
Selection Sort algorithm refined.

• For each ith element in the list
• For each of the elements from I on
• compare against the smallest seen so far.
• Swap the ith value in the list with the ith
  smallest value.

44
Selection sort code

• See the code on page 426 and 427 for the full
  code of an implementation of the Selection Sort
  algorithm.

45
Swapping-Exchanging values in arrays.

• Swapping is an important step that occurs in
  every sorting algorithm.
• The idea is to exchange the value from one index
  with the value located at another index.
• For this, we will always need a temporary
  variable to store the value from one of the
  elements while it is being overwritten.

46
Other sorting algorithms.

• There are other sorting algorithms out there, and
  some even run much faster than the selection
  sort.
• If you would like to do some research, look at
  the Bubble Sort, the Insertion Sort, the Merge
  Sort, and the Quick Sort.

47
Sorting examples in action.

• View the Sorting demo in the applet demos in the
  directory you installed Java to
  (C\jdk1.3\demo\applets\SortDemo)

48
Searching/Sorting review.

• Explain the Sequential Search algorithm in your
  own words.
• Why do we want to do sorting?
• Explain the Selection Sort algorithm in your own
  words.
• Are there faster sorting algorithms than the
  Selection Sort algorithm?

49
Sorting Review

• Go through the steps to do the Selection Sort on
  the following list of numbers
• 1,5,17,6,3,12,18

50
Multidimensional Arrays
51
Multidimensional arrays

• We can have arrays with more than a single
  dimension.
• These are used for things like tables of data,
  game boards, keeping 3/4 dimensional data,
  weather forecasting, etc.
• Can have as many dimensions as you would like,
  but we usually keep it under 5 dimensions.
• Most often we will deal with 2-dimensional
  arrays.

52
Declaring multidimensional arrays

• Just add an extra set of square brackets for
  every dimension.

double excelSheet new double108 dou
ble temps3D new double202020
char ticTacToeBoard new char33
Creates a table with 10 rows, 8 columns.
Creates a 3-Dimensional cube for storing
temperatures in a three dimensional space.
Creates a tic tac toe board with 3 rows, 3 columns
53
2 dimensional tables.

• Our most common multi-dimensional array.
• By convention, listed in row-column order.

int numRows 3, numCols 2 int table new
intnumRowsnumCols for(int rows 0 rows
ltnumRows rows) for(int cols 0 cols lt
numCols cols) tablerowscols rowscols
54
What that code created
int numRows 3, numCols 2 int table new
intnumRowsnumCols for(int rows 0 rows
ltnumRows rows) for(int cols 0 cols lt
numCols cols) tablerowscols rowscols
55
Rules about arrays.

• Make sure you dont reverse the row-column
  ordering. Row-Column. Row-Column. Row-Column. RC,
  RC, RC, Row-Column, Row-Column, Row-Column. RC.
  RC. Row-Column. RC. Row-Column. Row-Column. Row
  Column. Row-Column. Row-Column.
• You can pass and return multidimensional arrays
  to and from methods like any other array. Just
  make sure you get enough brackets in the method
  declaration.

56
Implementation of arrays.

• A multi-dimensional array is actually just an
  array of arrays.

int table new int4103 table25.
length //3 table2.length //10 table.length
//4
57
Multidimensional array review.

• Write a declaration and initialization of a
  3-dimensional array of integers that is sized 3
  by 5 by 7.
• Are the following declarations legal?
• int table new int 152352413
• double anotherTable new double23

58
Multidimensional array review.
Table03 Table54 Table14 Table11
Table00 Table60
14 23 234 3 8 54
Table13 Table34 Table44 Table21
Table30 Table35
4 32 73 23 98 89
59
Review
60
Arrays Review

• What is an array?
• What data types can you use in arrays?
• Declare an array of integers called prices and
  initialize it to have size 10.
• What are the indices for the prices array?

61
Arrays Review

• What is output to the screen?
• int someArray new int20
• System.out.println(someArray5)
• What is output to the screen?
• int someArray new int5
• someArray5 3
• System.out.println(someArray5)

62
Arrays Review

• What is actually stored in an array name?
• What are the following values?

Prices2 Prices6
Prices3 Prices0
Prices.length
63
Arrays Review

• Run the selection sort on the following numbers
• 5,23,1,3,2,15,8

64
Arrays Review
Table11 Table00 Table60
Table21 Table30 Table35