One Dimensional Arrays

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Introduction to Computer Science
Unit 9

• One Dimensional Arrays

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Arrays

• An array is a sequence of variables of identical
  type
• It is used to store large amounts of data,
  without having to declare many individual
  variables

counts
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5
6
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Arrays

• Arrays bring to data what loops bring to
  actionpower.
• Arrays give us a way of defining a structured
  collection of data, without having to name each
  component (as we would have to do with individual
  variables)
• It gives us an important tool for implementing
  data abstraction

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Old Example
public static void main (String args)
int score, sumOfScores 0, numberOfScores
0 SimpleInput sinp new SimpleInput(System.in)
System.out.print(Enter score (eof ends the
data) ) score sinp.readInt(
) int maxOfScores score int minOfScores
score while ( !sinp.eof( ) )
numberOfScores //new score sumOfScores
sumOfScores score //update
sum System.out.print(Enter score (eof ends the
data) ) score sinp.readInt( )
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That was a stream-oriented approach to data

• Once a value goes past, its gonetheres only
  one variable, score, that holds input data.
• But suppose we want to read and average the input
  stream, and echo only values that are above
  average. We could
• Read the input stream twice, averaging the first
  time, printing large numbers the second (a
  file-oriented approach not practical for human
  input)
• Read and store the input values, summing them as
  theyre stored. Get average at end, then echo
  only the larger ones (this will use an array)

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Variables and Arrays
95
264
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score
sumOfScores
numberOfScores
Simple variables give us a snapshot of the data.
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2
3
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An array lets us reinspect any part of the data
stream.
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A Random Access Data Structure

• Arrays are a random access data structure.
• Values can be stored or retrieved in any order we
  want.

counts
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Things to Notice with an Array

• 1. Whats the arrays name?
• 2. What kinds of values can it hold?
• 3. How many values can it hold?
• 4. How do we refer to individual values in the
  array?
• Well now look at each of these answers for a
  Java array

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Arrays Are a Special Kind of Object

• An array contains a fixed number of variables of
  identical type
• This fixed number is called the arrays length
• Like any object, an array must first be declared,
  then allocated (created)
• Remember that an array is an object, and this
  fact will help you

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Declaring an Array Variable

• Declare a variable to be an array int
  counts double scores Time
  appointmentTimes
• The brackets tell Java that the variable (counts,
  scores, or appointmentTimes) will be an array

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Answers to some questions

• Weve answered questions 1 and 2 with our
  declarations1. What is the arrays name?2.
  What kinds of values can it hold? int
  counts double scores Time
  appointmentTimes
• First arrays name is counts, and it holds
  values of type int, etc.

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Allocating (creating) an Array in the Heap

• We create an array using the new notation, but
  with some variations
• We have to tell new how many variables will be in
  the array object (use square brackets) counts
  new int10 scores new double15 appoint
  mentTimes new Time10
• In general, new type size creates an array
  of length size with variables of type type,
  returning an array object (in the heap) Answer
  to Question 3. How many values can it hold?
  (given during allocation)

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Doing it All At Once

• Obviously, like with any variable (simple or
  object), you can write it on one line int
  counts new int10 double scores new
  double15 Time appointmentTimes new
  Time10
• Now we have array objects sitting in the heap,
  called counts (that can hold 10 ints), scores
  (that can hold 15 doubles), and appointmentTimes
  (that can hold 10 Time objects)

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Creation of an Array Object

• new type size
• size can be any integer or integer
  expression int i 3 char name new
  chariis just as good as char name new
  char3
• The memory in an array is automatically
  initialized for us (to 0 for numbers, false for
  booleans, or null for objects, as appropriate)

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Lets Look at counts
counts
0
1
2
3
4
5
6
7
8
9

• Place to hold 10 ints
• Each int is held in a separate box
• The subscript, or index, of the variables runs
  from 0 to 9 (the index always starts at 0)
• Arrays can contain any type of value simple
  value or (reference to) object

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Answer to Question 4 How do we refer to
individual values in the array?
counts
0
1
2
3
4
5
6
7
8
9

• Use subscripts to obtain any specific
  variablecounts0 is the first variable in
  countscounts1 is the second variable in
  countscounts9 is the last variable in
  countscounts10 is an error!

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They are Used LikeRegular Variables
counts0counts1counts9 These are all int
variables, and can be used anywhere any int
variable (like score or sumOfScores) is
usedcounts1 17score counts8sumOfScor
es (7 ((counts4 2)/8))
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Java ChecksArray Bounds For You

• counts10
• This does not exist, since the 10 elements in
  counts run from 0 to 9
• If you try and use this nonexistent value, Java
  throws anArrayIndexOutOfBoundsExceptionerror
• More about these kinds of errors (exceptions)
  later

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class Simple public static void main (String
args) int counts new int10
SimpleInput sinp new SimpleInput(System.in)
System.out.println(Enter ten numbers
) counts0 sinp.readInt( )
counts1 sinp.readInt( ) counts2
sinp.readInt( ) counts3 sinp.readInt(
) counts4 sinp.readInt( )
counts5 sinp.readInt( ) counts6
sinp.readInt( ) counts7 sinp.readInt(
) counts8 sinp.readInt( )
counts9 sinp.readInt( )
System.out.print(counts9 counts8
counts7 counts6
counts5 counts4
counts3 counts2
counts1 counts0 \n)
Of course we can do this with a loop
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Using a Variable as an Index

• If i is an integer variable, you can, for
  example, write countsito refer to one of
  the variables in counts
• counts2 irefers to counts0 if i is 0,
  counts2 if i is 1, etc. illegal if i is
  outside the 0 to 4 range
• countsi/2refers to counts0 if i is 0 or
  1, counts1 if i is 2 or 3, etc. illegal if i
  is outside 0 to 19

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Alternative Way to Declare and Allocate an Array
All At Once

• Arrays can be initialized by giving a list of all
  their elementsint primes 2, 3, 5, 7, 11,
  13, 17, 19, 23, 29
• The declaration is on the left side of the
  assignment
• No explicit creation of the object is necessary
  Java handles it automatically
• It must be done in a single line this is
  illegalint primesprimes 2, 3, 5, 7, 11,
  13, 17, 19, 23, 29

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Lets Use the Technique

• Method that takes an integer and prints out that
  day of the weekfinal String NAME
  , Sunday, Monday, Tuesday,
  Wednesday, Thursday, Friday, Saturday
  void printName (int day) System.out.print(N
  AMEday)

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What is the Effect of the Following?

• int a, b a new int10 a b
• Since arrays are objects, a and b are references
  to objects (in the heap, as expected)
• So now a and b refer to the same array
• An array is a mutable object changes made to the
  array are reflected in both a and b

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Passing an Array as an Argument Between Methods
public int firstMethod( ) int a new
int10 a0 9 secondMethod(a)
return a0public void
secondMethod(int b) b0 5
a is an array object variable. b receives a copy
of a, and thus points to the same object. Changes
to b are reflected in a. firstMethod( )
therefore returns 5.
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The public Instance Variable length

• Every array object has a public instance variable
  called length
• It tells how many elements are in the array
• Since it is public, it can be freely accessed if
  a is an array, a.length says how many elements it
  has
• Print all elements of the array counts for (int
  i 0 i lt counts.length i) System.out.pri
  ntln( countsi )

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A Small Note on Variable Scope

• When we write for (int i 0 i lt
  counts.length i) System.out.println(
  countsi )the variable i exists only for the
  scope of the for loop
• It cannot be reused later, unless it is
  redeclared
• This is just a convenience for writing loops

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Simple Array Processing Loops

• Copy all the elements from array b into array
  a for (int i 0 i lt a.length i) ai
  bi
• How does this differ from a b

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Another Example

• Initialize the array counts to contain the
  numbers 0, 10, 20, etc. up to 90

0
40
10
20
50
60
80
counts
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70
90
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1
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9
for (int i 0 i lt counts.length
i) countsi i 10
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Read 10 numbers into array counts and print
them in reverse order
class Simple2 public static void main (String
args) int counts new int10
SimpleInput sinp new SimpleInput(System.in)
System.out.println(Enter ten numbers
) for (int i 0 i lt 10 i)
countsi sinp.readInt( ) for (int i
0 i lt 10 i) System.out.print(counts9 -
i ) System.out.println( )
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Another Way to Do It
class Simple2 public static void main (String
args) int counts new int10
SimpleInput sinp new SimpleInput(System.in)
System.out.println(Enter ten numbers
) for (int i 0 i lt 10 i)
countsi sinp.readInt( ) for (int i
9 i gt 0 i- -) System.out.print(countsi
) System.out.println( )
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An Arrays Size is FixedWhen Created

• An array cannot grow
• Its size is determined when it is created
• One simple way of dealing with this is to make a
  large array and only partially fill it
• Lets look at an example

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Encapsulate reading and echoing an array in a
class

• Well make a class Collection, which will include
  an array and methods for handling the array
• The client uses Collection by creating a
  Collection object, passing its constructor the
  (large) size of the array desired
• Lets look at how the client would use the
  Collection class, and how the class itself is
  defined

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The client to the Collection class
class MyClient static final int
INPUT_MAX 1000 public static void main
(String arg) Collection c new
Collection(INPUT_MAX) c.readAndEcho( )
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class Collection int _item int
_size 0 public Collection (int number)
_item new intnumber public void
readAndEcho( ) SimpleInput sinp new
SimpleInput(System.in) System.out.println(
Enter first number ) int n
sinp.readInt( ) while ( !sinp.eof( ) )
_item_size n _size System.out.pri
ntln(Enter next number ) n sinp.readInt(
) System.out.println( ) for
(int i 0 i lt _size i) System.out.println(_
itemi)
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Another Example

• Create program that reads in student name and two
  scores and prints average for each student (until
  end-of-file)Enter name and two exam grades
  Bill5769Enter name and two exam grades
  Scott9486Enter name and two exam grades D
  or Z

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The Expected Output
Name AverageBill 63Scott 90
The client program class MyClient static
final int INPUT_MAX 100 public static
void main (String arg) GradeBook grades
new GradeBook(INPUT_MAX) grades.readAndAverage(
)
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class GradeBook String _names
int _exam1, _exam2 int _size
0 public GradeBook (int number) _names
new Stringnumber _exam1 new
intnumber _exam2 new intnumber publ
ic void readAndAverage( ) SimpleInput
sinp new SimpleInput(System.in) while
( true ) System.out.println(Enter name and
two exam grades ) _names_size
sinp.readString( ) if ( sinp.eof( ) )
break _exam1_size sinp.readInt(
) _exam2_size sinp.readInt(
) _size
System.out.println( ) System.out.println(
\tName\tAverage) for (int i 0 i lt
_size i) System.out.println(\t _namesi
\t\t (_exam1i _exam2i) / 2)
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Arrays Can Contain Objects

• Arrays can contain any kind of data, not just
  integers and strings
• So instead of building GradeBook the way we did,
  lets create a Student class, and fill up the
  array with Student objects
• Each object includes names and grades

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Class Student, for making Student Objects
class Student private String _name private
int _exam1, _exam2 public Student ( )
public void setName (String s) _name s
public void setExam1 (int n) _exam1 n
public void setExam2 (int n) _exam2 n
public String getName ( ) return _name
public int getExam1 ( ) return _exam1
public int getExam2 ( ) return _exam2
public int getAvg ( ) return
(_exam1_exam2)/2
can be omitted Java will provide a default
constructor that does nothing if we left this out
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class GradeBook Student _students
int _size 0 public GradeBook (int number)
_students new Studentnumber public
void readAndAverage( ) SimpleInput sinp
new SimpleInput(System.in) String
nextname while ( true )
System.out.println(Enter name and two exam
grades ) nextname sinp.readString( ) if
( sinp.eof( ) ) break _students_size new
Student( ) _students_size.setName(nextname)
_students_size.setExam1(sinp.readInt(
)) _students_size.setExam2(sinp.readInt(
)) _size
System.out.println( ) System.out.println(
\tName\tAverage) for (int i 0 i lt
_size i) System.out.println(\t
_studentsi.getName( ) \t\t
_studentsi.getAvg( ))