Introduction to Arrays
CSCI 142
Object Oriented Programming
Tina Ostrander
An array…

A group of variables
that…
• Have a name
• Have a single type
• Primitive types
• Object types
• Can hold many values
Creating an Array
Declare the array variable
1.
•
Specifies the type and the name
Allocate memory for the array
2.
•
Specifies the size
Initialize the array elements
3.
•
Fills the array with values
Creating an Array
1.
Declare the array variable

2.
Create a double array
called salary
Allocate memory for the array

3.
double[] salary;
salary = new double[5];
Allocate enough memory
to store 5 elements
Initialize the array elements

salary[0] = 32500;
Assigns a value to the
1st array element
Note: Steps 1 and 2 may be combined:
double[] salary = new double[5];
Creating an Array
1.
Declare the array variable

2.
Create a GOval array
called circles
Allocate memory for the array

3.
GOval[] circles;
circles = new GOval[5];
Allocate enough memory
to store 5 elements
Initialize the array elements

circles[0] = new GOval(50, 50);
Assigns an object to
the 1st array element
Note: Steps 1 and 2 may be combined:
GOval[] circles = new GOval[5];
Practice
Q: Write a statement to declare an
int array called age.
A: int[] age;
Q: Write a statement to declare a
GRect array called squares.
A: GRect[] squares;
Practice
Q: Allocate memory for 5 elements
in the age array.
A: age = new int[5];
Q: Allocate memory for 10 elements
in the squares array.
A: squares = new GRect[10];
Memory allocation

Memory is allocated for each
element in the array.
• Try to accurately estimate the number of array elements.
• A value does not need to be stored in each array element.
• The length field contains the size of the array.

Elements are initialized
• Numbers to zero
• Objects to null
• Booleans to false
Initializing an Array

Array elements can be initialized one at a time…
double[] salary = new double[5];
salary[0] = 30000.00;
salary[1] = 45000.00;
salary[2] = 52500.00;
salary[3] = 62250.00;
salary[4] = 37500.00;
GOval[] circles = new GOval[5];
circles[0] = new GOval(50, 50);
circles[1] = new GOval(75, 75);
circles[2] = new GOval(100, 100);
circles[3] = new GOval(125, 125);
circles[4] = new GOval(150, 150);
Practice
Q: Assign the value of 20 to the first
element of the age array.
A: age[0] = 20;
Q: Assign the value of 45 to the last
element of the age array.
A: age[4] = 45; or
age[age.length - 1] = 45;
Initializing an Array

An array can be initialized with an “initializer list”…
double[] salary = {30000, 45000, 52500, 62250, 37500};
String[] name = {“Branco”, “Tierney”, “Matt”};
• Array is declared, memory is allocated, and elements are initialized all in one
statement
• Size is automatically assigned
• Semicolon is required after closing brace
• Keyword new is not required
• This only works if you know the array values when the array is declared
Initializing an Array
 An
array can be initialized with a loop…
double[] salary = new double[10];
double amount = 40000.00;
for (int i = 0; i < salary.length; i++)
{
salary[i] = amount;
amount += 10000;
}
Note that this only
works when there is
a pattern to the values.
Initializing an Array
 An
array can be initialized with a loop
This returns the number
of elements in the array.
GOval[] circles = new GOval[10];
int size = 25;
for (int i = 0; i < circles.length; i++)
{
circles[i] = new GOval(size, size);
size += 25;
}
Initializing an Array

Example 2
String months =
"JanFebMarAprMayJunJulAugSepOctNovDec";
String[] month = new String[12];
for (int i = 0; i < 12; i++)
month[i] = months.substring(i * 3, i * 3 + 3);

Example 3
int count = 0;
double[] cost = new double[5];
while (count < cost.length) {
cost[count] = readDouble(“Enter cost”);
count++;
}
Practice
Q: Assign a GRect object to the first
element of the squares array.
A: squares[0] = new GRect(50, 50);
Q: Assign a GRect to each element of
the squares array.
A: for (int i = 0; i<squares.length; i++)
squares[i] = new GRect(50, 50);
Practice
Q: Assign even numbers 2 through 10 to
the age array.
A: for (int i = 0; i < age.length; i++)
age[i] = i * 2 + 2;
A: int val = 2;
for (int i = 0; i < age.length; i++) {
age[i] = val;
val += 2;
}
Practice
Q: Assign the numbers 5 through 1 to
the age array.
A: for (int i = 0; i < age.length; i++)
age[i] = 5-i;
A: int val = 5;
for (int i = 0; i < age.length; i++) {
age[i] = val;
val--;
}
Practice
Q: Assign a GRect to each element of the
squares array, where each square is 10
pixels larger than the previous square .
A: int size = 10;
for (int i = 0; i < squares.length; i++)
{
squares[i] = new GRect(size, size);
size += 10;
}
Try It





Create a ConsoleProgram
Declare an array called grades
Allocate enough memory for 5
grades
Initialize the array by prompting
the user to enter each grade
Calculate the average grade
Searching an Array



Loop through the array
Compare search value to
each value in the array
Use a variable to keep
track of if/where the
search value was found
public class Search extends ConsoleProgram
{
public void run()
{
int[] nums = {15, 9, 6, 2, 4, 7, 23, 18, 29, 18};
int findNum = 7, position = -1;
for (int i = 0; i < nums.length; i++)
if (nums[i] == findNum)
{
position = i;
break;
//Quit looking
}
if (position != -1)
println (findNum + " found at position " + position);
else
println (findNum + " was not found.");
}
}
Try It

Modify the Search program so
that it displays every instance
of findNum.
Passing Arrays


An array variable or an entire array
may be passed to a method.
Array values are passed by value.
myMethod(salary[3])
• Copy of the value is passed

Arrays are passed by reference.
myMethod(salary)
• Array address is passed
public class Salary extends ConsoleProgram
{
public void run()
{
double[] salary = {29000, 32500, 56000, 48500, 42950};
println ("The average salary is " + average(salary));
}
public double average(double[] array)
{
double sum = 0.0;
for(int i=0; i<array.length; i++)
{
sum += array[i];
}
return sum / array.length;
}
}
Try It


Add a maximum method to the
Salary program.
The method should take an
array and return the largest
value in the array.
Ordered Arrays


An array is ordered if its values
are in ascending or descending
order.
Ordered arrays can be searched
more efficiently than unordered
arrays.
Sorting Arrays

Bubble sort
• Compare adjacent items
if (someArray[i] > someArray[i + 1])
• If they are out of order, swap them
temp = someArray[i];
someArray[i] = someArray[i + 1];
someArray[i + 1] = temp;
• For n items, make n-1 passes through the list
int[] salary = {32500, 45950, 24675, 23000, 29800};
int temp = 0;
for (int i = 0; i < salary.length-1; i++)
for (int j = 0; j < salary.length – i - 1; j++)
if (salary[j] > salary [j + 1])
{ //swap
temp = salary[j];
salary[j] = salary[j + 1];
salary[j + 1] = temp;
This
}
sorts
ints
for (int i = 0; i < salary.length; i++)
print (salary[i] + "\t");
println();
Why use Arrays?




Arrays allow for faster data access
(RAM vs. storage)
Data entered into an array once can
be used multiple times
Arrays simplify programming
Arrays allow fewer variable names to
be defined and used