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Arrays
Exam 1
1. Java source and executable files have
the following extensions?
a. .java and .class
b. .src and .class
c. .javadoc and .exe
d. .list and .exe
Exam 1
2. Which of the following is a Java
comment
a. ; This is my first program
b. # This is my first program
c. /* This is my first program */
d. ‘ This is my first program
e. none of the above
Exam 1
3. We use the following operator to create
a new object in Java
a. malloc
b. new
c. string
d. newObject
Exam 1
4. Commands to compile and execute a
java program are
a. run and java
b. execute and javac
c. javac and java
d. compile and run
Exam 1
5. Identifiers may start with
a. $
b. @
c. a number
d. &
Exam 1
6. It is necessary to declare an object
before you can use it
a. True
b. False
Exam 1
7. Which of the following is a valid
identifier?
a. Bank Account
b. bank account
c. bank$account
d. bank-account
Exam 1
8. A series of characters that appear in
double quote is a char data type
a. True
b. False
Exam 1
9. boolean isDone == false; is a valid
assignment statement in Java
a. True
b. False
Exam 1
10. Which of the following is a correct
variable declaration statement?
a. int x - float y;
b. int x: float y;
c. int x,y;
d. Long int x;
Exam 1
11. Boolean expressions are built by use of
_relational______operators and
_boolean_______operators
12. A data value that can not be changed is
called __constant____________
13. $Bad-Variable is __bad/invalid/not _ Java
identifier.
Exam 1
14. These two data types: __float__, and
___double___are used for real numbers
15. _An object____is an instance of a
class
Exam 1
16. double
loanPeriod;
if (loanPeriod < 0 || >1000 ); {
System.out.println(“Loan period is invalid”);
System.exit(1);
}
1.
loanPeriod is not initialized
2.
loanPeriod or a variable name is missing
3.
; is not needed.
Exam 1
17. double s;
s = 1.0;
switch (s) {
case 1.0: System.out.println(“ March madness”); break;
case 2.0: System.out.println(“ November rain”); break;
case 3.0: System.out.println(“White Christmas”); break;
default: System.out.println(“No Special name”); break;
}
1. Switch doesn’t support double data type
Exam 1
18. char aChar = ”NFL Championship”;
1. Char datatype can only contain 1
character
2. Char data type needs single quotes
instead of double ones
Exam 1
19. int i, j;
double x,y;
i=1;
j=2;
0=1.0
½
=
0,
3
x= Math.pow(3, (i/j));
y = x % 2;
if (y ==0)
y = 1.0 % 2 = 1.0
System.out.println(x +" is an even number ");
else
System.out.println(x +" is an odd number");
1.0 is an odd number
Exam 1
20. int count=1, sum=0;
while (count < 9) {
if (count % 2 != 0) {
sum += count;
}
count++;
}
System.out.println(" Sum ="+ sum);
Exam 1
count =1, sum =0, 1 %2 = 1 (!= 0), sum = 0+1=1
count =2, sum=1, 2 % 2 = 0
count =3, sum=1, 3 % 2 = 1 (!=0), sum = 1+3 = 4
count =4, sum=4, 4 % 2 = 0
count =5, sum=4, 5 % 2 = 1 (!=0), sum = 4+5 = 9
count =6, sum=9, 6 % 2 = 0
count =7, sum=7, 7 % 2 = 1 (!=0), sum = 9+7 = 16
count =8, sum=16, 8 % 2 = 0
count =9, exit
Sum = 16
Exam 1
21. int sum =0;
for (int i=1; i<=2; i++) {
for (int j=1; j<=3; j++) {
sum = sum + (i+j);
}
}
System.out.println(" Sum ="+ sum);
Exam 1
sum =0
i=1;
j=1; sum=0+(1+1)
j=2; sum =(1+1)+(1+2)
j=3; sum = (1+1)+(1+2)+(1+3) = 2+3+4=9
i=2
j=1; sum=9+(2+1)
j=2; sum=9+(2+1)+(2+2)
j=3; sum=9+(2+1)+(2+2)+(2+3)=
9+(3+4+5)=9+12=21
Sum =21
Lab 4
Lesson plan

Arrays
Arrays

We often need to group together related
items of data.
Cards in a pack.
 Ships in a port.


Java provides two distinct facilities:
Traditional array.
 Flexible-size collection classes
(java.util).

Problems That Arrays Solve
……
minValue = firstNumber;
if (secondNumber < minValue)
minValue = secondNumber;
if (thirdNumber < minValue)
minValue = thirdNumber;
if (fourthNumber < minValue)
minValue = fourthNumber;
if (fifthNumber < minValue)
minValue = fifNumber;
What is this code doing?
……
Finding minimum value from a set of 5 values
Arrays for Numerical data type
(primitive data types)
int[] number = new int [5];
int minValue;
String inputStr;
Array
for (int i=0; i<5; i++) {
declaration &
inputStr = JOptionPane.showInputDialog(null,"Please
allocationenter the
value for element "+i);
memory
number[i] = Integer.parseInt(inputStr);
}
minValue = number[0];
for (int i=1; i<5; i++) {
if (minValue > number[i])
minValue = number[i];
}
System.out.println(" minValue ="+ minValue);
Arrays for Numerical data type
(primitive data types)
int[] number = new int [5];
int minValue;
Getting values
for all elements
in the array
String inputStr;
for (int i=0; i<5; i++) {
inputStr = JOptionPane.showInputDialog(null,"Please enter the
value for element "+i);
number[i] = Integer.parseInt(inputStr);
}
minValue = number[0];
for (int i=1; i<5; i++) {
if (minValue > number[i])
minValue = number[i];
}
System.out.println(" minValue ="+ minValue);
Arrays for Numerical data type
(primitive data types)
int[] number = new int [5];
int minValue;
String inputStr;
for (int i=0; i<5; i++) {
inputStr = JOptionPane.showInputDialog(null,"Please enter the
value for element "+i);
number[i] = Integer.parseInt(inputStr);
Access each
}
element of an
array
minValue = number[0];
for (int i=1; i<5; i++) {
if (minValue > number[i])
minValue = number[i];
}
System.out.println(" minValue ="+ minValue);
Arrays
Array is a collection of data values of the
same data type.
 Example:
int[ ] number; // This is an array of
integers
double[] gpa; // This an array of double

Array Declaration
Format:
<data type>[] <array_name>;
OR
<data type> <array_name>[];
data type: double, int, float, string
Example:
double[] gpa;
Or
double gpa[];
Arrays

The amount of memory allocated to store an array
depends on the size and type of values in the array



Size: number of elements in the array
Type of values: data type of each element in the array
An individual value in an array is called array element
Example:
int[ ] number = new int[5];
data type: integer (4 bytes)
size: 5
memory: 20 bytes
Array is a reference data type. Array is NOT an object
Arrays
Example:
int[ ] number = new int[5];
number
number[0]
6
0
1
2
3
4
Arrays

Elements of an array are indexed from zero to
size -1
 Size: the number of elements in an array
 length: a public constant represents the size of
an array.
Example:
sum =0;
for (int i=0; i<number.length; i++)
sum += number[i];
Fixed –size array declaration
Size of an array is pre-determined.
Example:
int[] number= new int[5];
Problems:
• What if we have more than predetermined size of an array?
• Underutilization of space.
Variable-size array declaration


In Java, we can declare an array of different size every
time we run a program
Example:
int size;
int[] number;
inputStr = JOptionPane.showInputDialog(null,"Please
enter the number of elements ");
size = Integer.parseInt(inputStr);
number = new int[size];
Practice (in class and/or at home)
Modify the class ComputeMin (week 4) to
print out the minimum of an array with
n integers.
Arrays for Objects
// An array for the names of the distinct
private Loan[] loanArray = new Loan[5];
• Note: No Loan objects are created. Only a
container for Loan.
• Each location is initialized to null.
loanArray
NULL
Arrays of Objects
private Loan[] loanArray = new Loan[5];
for (i=0; i<5; i++) {
loanArray[i] = new Loan();
}
loanArray
Indexing an Array
int[] number = new int [5];
number[0] =2;
number[1]=3;
number[2]=4;
number[3]= 6;
number[4]=-1;
Initializing Arrays
...
// Differentially number the assignments.
private int[] number = {1, 2, 1, 5, 1,};
private final int totalNumber =
number.length;
• No new required.
• Terminating semicolon.
• Optional final comma.
Further Initializer Examples
String[] suitNames = {
"Spades", "Hearts", "Diamonds", "Clubs"
};
Point[]
new
new
new
new
};
vertices = {
Point(0,0),
Point(0,1),
Point(1,1),
Point(1,0),
YearlyRainfall y2k = new YearlyRainfall(
new int[]{10,10,8,8,6,4,4,0,4,4,7,10,});
Note for project 2
a.
For each class (Loan and
Amortization), list:
data members:
data type, name
methods:
return data type, name, parameters
constructor
parameters
Note for project 2
b. For Amorization main list
methods:
return data type, name, parameters
Draw a diagram
c. Submit your code
Test case
Interest rate = 0.4% (0.004)
Number of months = 12
Loan amount = 100
Amortization payment = $8.55
Iterating over an Array in
Reverse
int size = 5;
minValue = number[size-1];
for (int i=size-2; i>=0; i--) {
if (minValue > number[i])
minValue = number[i];
}
Copy an array
Use arraycopy method from System
class.
public static void arraycopy(Object src,
int srcPos, Object dest, int destPos,
int length)
Copying Arrays
int size=5;
int number[] = new int[size];
// Assuming that we get values for number array here
// Declare another array called number1
int number1[] = new int[size];
// Copy the values of array number to array number1
System.arraycopy(number,0,number1,0,size);
Passing Array to Methods
When an array is passed to a method,
only its reference is passed. A copy of
the array is not created in the method.
That means:
we pass the identifier for that array which
in fact is a reference to a start address of
the array.

Passing Array to Methods
Assuming changeArrayValue is one method of a class
named ArrayClass
public void changeArrayValue(int[] arrayChanged)
for (int i=0; i< arrayChanged.length; i++)
arrayChanged[i] += 1;
}
We call this method as:
{
Passing Array to Methods
ArrayClass anObj = new ArrayClass();
int number[5] = new int[5];
for(int i=0; i<number.length; i++)
number[i] = i;
anObj.changeArrayValue(number);
Passing Array to Methods
for(int i=0; i<number.length; i++)
number[0] = i;
number
0
1
2
3
4
anObj.changeArrayValue(number);
number
1
2
3
4
5
“Just in time” overview
1.
It is acceptable to have an array
containing values of both type BYTE
and type INT, as long as all of the
values are integers:
a.
b.
True
False
“Just in time” overview
1.
It is acceptable to have an array
containing values of both type BYTE
and type INT, as long as all of the
values are integers:
a.
b.
True
False
“Just in time” overview
2. A code fragment that declares an array
of type long named accounts is
a. long accounts;
b. long[ ] accounts;
c. long{ } accounts;
“Just in time” overview
2. A code fragment that declares an array
of type long named accounts is
a. long accounts;
b. long[ ] accounts;
c. long{ } accounts;
Matching game (again)
3. A code fragment that declares an array
named grades and allocates the memory
to store 15 values of type double:
=
A.[15]
B. new
C. grades
D. double E. double[ ]
;
Matching game (again)
4. A code fragment that declares an array
named grades and allocates the memory
to store 15 values of type double:
E
C
=
B
D
A.[15]
B. new
C. grades
D. double E. double[ ]
A
;
Searching an Unsorted Array
We often need to search an array for a
particular item of data.
 The data is often unsorted.
 The item might or might not be present.

Care must be taken not to search beyond
the end of the array.
 We need to decide how to return a found
item.

Search with Multiple Returns
public int indexOf(int[] numbers,int value){
final int notPresent = -1;
for(int index = 0; index < numbers.length;
index++){
if(numbers[index] == value){
return index;
}
}
// We did not find it.
return notPresent;
}
The Arrays Class

Defined in the java.util package.

Contains static methods for manipulating
arrays:
binarySearch: search for a value.
 equals: compare the contents of two
arrays.
 fill: fill an array with a particular value.
 sort: sort the contents of an array.

Multi-Dimensional Arrays

Arrays of multiple dimensions are
possible.
2D grid, for a board game such as chess.
 3D cube structure, etc.

Multi-dimensional arrays are regarded as
being arrays of arrays.
 Non-rectangular structures are possible.

2D Array Construction
final int numRows = 10, numCols = 5;
double[][] matrix = new double[numRows][numCols];
char[][] hiddenWord = {
{ 'd', 'g', 'i', 'b' },
{ 'e', 'i', 'u', 'm' },
{ 't', 'a', 's', 'a' },
};
Review
Arrays make it possible to group related
items in a single object.
 An array's length is fixed on construction.
 Arrays may have multiple dimensions.
