Instructor: Craig Duckett
ARRAYS
Lecture 12
Announcements
Assignment 3
GRADED! RETURNED! Woot!
Assignment 3 Revision
Due Wednesday, August 17th, by midnight
Assignment 4 (and Final Exam)
Due Wednesday, August 24th, by midnight
Extra Credit 01 (I'll go over this Monday, August 22)
Due Wednesday, August 24th, by midnight
Today’s Topics
•
•
•
•
The "Official" Introduction to Arrays
Chapter 10.2: Creating Arrays
Chapter 10.5: Arrays of Primitives
Chapter 10.1.1-10.1.7: Arrays of Objects [If Time]
And now ...
The Quiz
Introduction to Arrays
What is an Array?
 Primitive variables are designed to hold only one value at a time.
 Arrays allow us to create a collection of like values that are
indexed.
 An array can store any type of data but only one type of data at a
time.
 An array is a list of data elements.
Let’s see what this all means...
Introduction to Arrays
What is an Array?
So far, you have been working with variables that hold only one value. The integer
variables you have set up have held only one number (and next week we will see how
string variables will hold one long string of text).
An array is a collection to hold more than one value of the same data type at a time.
It's like a list of items—a list of integers, or a list of doubles, or a list of chars, or a list
of strings, etc
Think of an array as like the columns in a spreadsheet. You can have a spreadsheet
with only one column, or several columns.
index grades
The data held in a single-list (one-dimensional) array
might look like this:
0
100
1
89
2
96
3
100
4
98
Introduction to Arrays
index grades
0
100
1
89
2
96
3
100
4
98
Now, the way we might have declared data like this
up until now is to do something along these lines:
int
int
int
int
int
value1
value2
value3
value4
value5
=
=
=
=
=
100;
89;
96;
100;
98;
However, if we knew before hand that we were going to be declaring five int integers (or
ten, or fifteen, etc), we could accomplish the same type of declaration by using an array.
To set up an array of numbers like that
in the table above, you have to tell
Java what type of data is going into the
array, then how many positions the
array has. You’d set it up like this:
int[ ] grades;
NOTE
Arrays must be of the same data type,
i.e., all integers (whole numbers) or all
doubles (floating-point numbers) or all
strings (text characters)—you cannot
“mix-and-match” data types in an array.
Introduction to Arrays
index
grades
0
100
1
89
2
96
3
100
4
98
int[ ] grades;
The only difference between setting up a primitive integer
variable and an array is a pair of square brackets [ ] after the
data type.
The square brackets are enough to tell Java that you want to set
up an array. The name of the declared array above is grades.
Just like primitive variables, you can call them almost anything you like (except Java
defined keywords). While the square brackets tells Java you want to set up an array, it
doesn't say how many positions the array should hold. To do that, you have to set up a new
array object:
int[ ] grades;
grades = new int[5]; // <-- New array object
In between the square brackets you need the pre-defined size of the array. The size is how
many slots (elements) the array should hold. If you prefer, you can put all that on one line:
int[ ] grades = new int[5]; // Done at same time
How an Array is “Set Up” with the Code
int [] grades;
grades = new int[5];
int [] grades = new int[5];
// This lets compiler know a
// collection is coming, but not
// its length
// This sets up the number
// of slots (elements) based
// on the length
// This does both at same time
How an Array is “Set Up” with the Code
int [] grades = new int[5];
grades
0
0
grades[0]
grades[1]
grades[2]
grades[3]
grades[4]
=
=
=
=
=
1
2
3
4
0
0
0
0
100;
89;
96;
100;
98;
grades
0
1
2
3
4
100
89
96
100
98
How an Array is “Set Up” with the Code
// You can accomplish the same thing using an array literal
int [] grades = {100, 89, 96, 100, 98};
grades
0
1
2
3
4
100
89
96
100
98
Introduction to Arrays: Example
import java.util.*;
public class Array_Demo extends Object
{
public static void main(String[] args)
{
int [] grades = new int[5];
grades[0]
grades[1]
grades[2]
grades[3]
grades[4]
=
=
=
=
=
100;
89;
96;
100;
98;
// <-- Something especially groovy happens here!
}
}
Introduction to Arrays
index
grades
0
100
1
89
2
96
3
100
4
98
When you declare an array with a given data type, name and number,
like
grades = new int[5];
you are reserving a collection space in memory by that name, sized
according to data type, and large enough to separately contain enough
data for the declared size.
The number inside the brackets is the array’s size
declarator
or length. It indicates the number of
or values, the array can hold. In the declaration above, grades references an array with
enough memory being reserved for five (5) integer values.
elements,
STEP 1: Declare Variable
STEP 2: Allocate Memory
STEP 3: Initialize Elements
grades variable
Element1
Element2
Element3
Element4
Element5
Introduction to Arrays: Example
import java.util.*;
public class Array_Demo extends Object
{
public static void main(String[] args)
{
int [] grades = new int[5];
grades[0]
grades[1]
grades[2]
grades[3]
grades[4]
=
=
=
=
=
100;
89;
96;
100;
98;
// <-- Something especially groovy happens here!
}
}
grades
= new int[5];
grades is a named reserved space set aside to hold exactly five [5] 32-bit elements all
initializing to a value of zero 0. As we have learned about programming languages, the
“index” always starts at 0, not 1, and procedes until the size of the array is reached. In our
example, since we declared [5] the array element index starts with 0 and ends at 4.
array element index 
space reserved for data 
value initialized in element 
0
1
2
3
4
32-bits
32-bits
32-bit
32-bits
32-bits
0
0
0
0
0
grades
grades
0
100
1
89
2
96
3
100
4
98
grades[0]
grades[1]
grades[2]
grades[3]
grades[4]
index
= new int[5]; // <-- Steps 1 & 2
=
=
=
=
=
100; // Steps 3
89;
STEP 1: Declare Variable
96;
STEP 2: Allocate Memory
100;
STEP 3: Initialize Elements
98;
5
0
1
2
3
4
grades
100
89
96
100
98
Introduction to Arrays: Example
import java.util.*;
public class Array_Demo extends Object
{
public static void main(String[] args)
{
int [] grades = new int[5];
grades[0]
grades[1]
grades[2]
grades[3]
grades[4]
=
=
=
=
=
100;
89;
96;
100;
98;
// <-- Something especially groovy happens here!
}
}
Introduction to Arrays
index
grades
0
100
1
89
2
96
3
100
4
98
You can also declare the int separately and call it by its given name, like
this:
int testScores = 5;
int[ ] grades = new int[testScores];
(Whether you call the number inside the brackets or a named variable is
up to your particular style of coding and preference.)
So we are telling Java to set up an array with 5 positions in it. After this line is executed, Java will assign
default values for the array. Because we've set up an integer array, the default values for all 5 positions
will be zero ( 0 ). To assign values to the various positions in an array, you do it in the normal way:
grades[0]
grades[1]
grades[2]
grades[3]
grades[4]
=
=
=
=
=
100;
89;
96;
100;
98;
grades
grades
grades
grades
grades
[0]
[1]
[2]
[3]
[4]
=
=
=
=
=
{
100;
89;
96;
100;
98;
Length of the
array is equal to
the number of
slots declared
This is called the index
If you know what values are going to go in the array, you can also set them up like this:
int[ ] grades = { 100, 89, 96, 100, 98 }; // Java treats as new instance
int [] grades = new int[5];
grades[0]
grades[1]
grades[2]
grades[3]
grades[4]
=
=
=
=
=
Length
100;
89;
96;
100;
98;
int [] grades = new int[5];
int [] grades = new int[5];
grades[0]
grades[1]
grades[2]
grades[3]
grades[4]
grades[0]
grades[1]
grades[2]
grades[3]
grades[4]
=
=
=
=
=
Index
100;
89;
96;
100;
98;
=
=
=
=
=
Subscripts
100;
89;
96;
100;
98;
import java.util.*;
public class Array_Demo extends Object
{
public static void main(String[] args)
{
// Setting up the integer 5-element array:
int [] grades = new int[5];
grades[0]
grades[1]
grades[2]
grades[3]
grades[4]
=
=
=
=
=
100;
89;
96;
100;
98;
// Of course you could have done it this way:
// int [] grades = {100, 89, 96, 100, 98};
int i;
for(i = 0; i < grades.length; i++)
{
System.out.println("Grade " + (i + 1) + " is: " + grades[i]);
}
}
}
Arrays of Primitives: Overview
 An array is an object so it needs an object reference.
// Declare a reference to an array that will hold integers.
int[] numbers;
 The next step creates the array and assigns its address to
the numbers variable
// Create a new array that will hold 6 integers.
numbers = new int[6];
0
0
0
0
0
0
subscript 0 subscript 1 subscript 2 subscript 3 subscript 4 subscript 5
• Array element values are initialized to 0 by default.
• Array indexes always start at 0.
• The first element in an array index is always subscript 0.
Arrays of Primitives: Overview
 It is possible to declare an array reference and create it in the same
statement.
int[] numbers = new int[6];
 Arrays may be of any type.
float[] temperatures = new float[100];
char[] letters = new char[41];
long[] units = new long[50];
double[] sizes = new double[1200];
•
long - 8 bytes signed. Ranges from -9,223,372,036,854,775,808 to
+9,223,372,036,854,775,807.
•
float - 4 bytes. Covers a range from 1.40129846432481707e-45 to
3.40282346638528860e+38 (positive or negative).
•
double - 8 bytes. Covers a range from 4.94065645841246544e-324d
to 1.79769313486231570e+308d (positive or negative).
•
char - 2 bytes, unsigned, Unicode, 0 to 65,535. Chars are not the same
as bytes, ints, shorts or Strings.
Arrays of Primitives: Overview
 The array length must be a non-negative number.
 It may be a literal value, a constant, or variable.
int arraySize = 6; ( or final int ARRAY_SIZE = 6; )
int[] numbers = new int[arraySize];
 Once created, an array length is fixed and cannot be
changed.
Accessing the Elements of an Array
20
0
0
0
0
0
numbers[0]
numbers[1]
numbers[2]
numbers[3]
numbers[4]
numbers[5]
 An array is accessed by:
 the reference name (e.g., numbers)
 a subscript that identifies which element in the array to access.
numbers[0] = 20; //pronounced "numbers sub zero"
Name
Subscript
Inputting and Outputting Array Elements
 Array elements can be treated as any other variable.
 They are simply accessed by the same name and a
subscript.
 See example: ArrayDemo1.java
 Array subscripts can be accessed using variables (such
as for loop counters).
 See example: ArrayDemo2.java
Bounds Checking
 Array indexes always start at zero and continue to (array
length - 1)
int values = new int[10];
 This array would have indexes 0 through 9
 See example: InvalidSubscript.java
 In for loops, it is typical to use i, j, and k as counting
variables
 It might help to think of i as representing the word index
Watch for “Off-by-One” Errors
 It is very easy to be “off-by-one” when accessing arrays
// This code has an off-by-one error.
int[] numbers = new int[100];
for (int i = 1; i <= 100; i++) // Would work with < only
{
numbers[i] = 99;
}
 Here, the equal sign allows the loop to continue on to index 100, but 99 is
the last index in the array
 This code would throw an ArrayIndexOutOfBoundsException
Array Initialization
 When relatively few items need to be initialized, an
initialization list can be used to initialize the array
int[]days = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
 The numbers in the list are stored in the array in order:
 days[0] is assigned 31,
 days[1] is assigned 28,
 days[2] is assigned 31,
 days[3] is assigned 30,
 etc.
 See example: ArrayInitialization.java
Alternate Array Declaration
 Previously we showed arrays being declared:
int[] numbers;
 However, the brackets can also go here:
int numbers[];
 These are equivalent but the first style is typical (and preferred by most
developers/coders).
 Multiple arrays can be declared on the same line.
int[] numbers, codes, scores;
 With the alternate notation each variable must have brackets.
int numbers[], codes[], scores;
 The scores variable in this instance is simply an int variable.
Processing Array Contents
 Processing data in an array is the same as any other variable
grossPay = hours[3] * payRate;
 Pre and post increment works the same:
int[] score = {7, 8, 9, 10, 11};
++score[2]; // Pre-increment operation
score[4]++; // Post-increment operation
 See example: PayArray.java
Processing Array Contents
 Array elements can be used in relational operations
if(cost[20] < cost[0])
{
//statements
}
 They can be used as loop conditions:
while(value[count] != 0)
{
//statements
}
Array Length
 Arrays are objects and provide a public field named length that
is a constant that can be tested
double[] temperatures = new double[25];
 The length of this array is 25.
 The length of an array can be obtained via its length
constant
int size = temperatures.length;
 The variable size will contain 25.
The Enhanced for Loop
 Simplified array processing (read only)
 Always goes through all elements
 General:
for(datatype elementVariable : array)
statement;
Example:
int[] numbers = {3, 6, 9};
for(int val : numbers) // <-- Only two parts. You can read the line as
// "iterate on elements from the collection named numbers. The current
// element will be referenced by the int val."
{
System.out.println("The next value is " + val);
}
The Enhanced for Loop
int[] numbers = {3, 6, 9};
for(int val : numbers)
{
System.out.println("The next value is " + val);
}
Java knows an Enhanced for loop when it sees one (the colon gives it away), so
instead of using a counter as with the first part of a typical loop, it just looks for
the length of the named array.
As it loops it grabs the data from each subscript (part 1 of the enhanced for) that
belongs to the named array (part 2 of the enhanced for) and walks (or autoincrements) the length of the array one element at a time.
Array Size
 The length constant can be used in a loop to provide
automatic bounding.
Index subscripts start at 0 and end at one less than the array length.
for(int i = 0; i < temperatures.length; i++)
{
System.out.println("Temperature " + i ": "
+ temperatures[i]);
}
LECTURE 12:
ICE Arrays:
35