1. Arrays ICS 111: Introduction to Computer Science I
William Albritton
Information and Computer Sciences Department at the University of Hawai‘i at Mānoa
"Heavier-than-air flying machines are impossible."
Lord Kelvin, president, Royal Society, 1895
11/16/2018
© 2007 William Albritton

2. Terminology Array Array index (subscript) Array element
A list of data of the same type
Array index (subscript)
A number corresponding to the position of an element in an array
Numbered from 0, 1, 2, N-1, where N is the size of the array
Array element
The value that is stored in an array
11/16/2018
© 2007 William Albritton

3. Array Examples Array of Integers Array of Doubles finalGrades
examScores
index address element
Array of Doubles finalGrades
indices addresses elements
11/16/2018
© 2007 William Albritton

4. Example Code See InitializingArrays.java This program shows how to:
Declare arrays
Instantiate arrays
Initialize arrays
Display arrays
11/16/2018
© 2007 William Albritton

5. Declaring Arrays Syntax for an array declaration Example Java code
DataType [] arrayName;
Example Java code
Integer [] examScores;
Double [] finalGrades;
A declaration creates a variable with the address to an array, but not the array itself
Since arrays are objects in Java, you must declare an array variable (which will later contain the address to the location of the array object)
11/16/2018
© 2007 William Albritton

6. Declaring Arrays
These declarations create a reference variable with a null value (does not point to an object)
Integer [] examScores; examScores
null
Double [] finalGrades; finalGrades null
11/16/2018
© 2007 William Albritton

7. Variables & Objects
A variable stores a reference (address) to an object
In the below example, name is a variable that will store the address to a String object
String name; null
In the below example, examScores is a variable that will store the address to an array of Integers
Integer [] examScores; null
11/16/2018
© 2007 William Albritton

8. Instantiating Arrays Syntax for array instantiation (creation)
arrayName = new DataType[SIZE];
SIZE is the number of elements in the array
Example Java code
final Integer SIZE = 6; //constant
examScores = new Integer[SIZE];
finalGrades = new Double[SIZE];
This instantiates (creates) an array object & stores its address in a variable
The array object will contain no addresses (null)
11/16/2018
© 2007 William Albritton

9. Example Array Instantiation
Array of integers
examScores null
1 null
2 null
3 null
index null address
5 null
Since no addresses have been assigned, the values stored in the array are “null”
Array of doubles finalGrades null
1 null
2 null
3 null
4 null
5 null
indices addresses
11/16/2018
© 2007 William Albritton

10. Class Exercise 1 Declare an array that is used to store
Student ID numbers
The letters of the alphabet
Declare and instantiate an array that is used to store
All the high temperatures of this week
5 boolean values
Draw the “box & arrow” representation of what this looks like in memory
In your drawing, include variable labels, addresses (arrows), element values, and index values
11/16/2018
© 2007 William Albritton

11. Initializing Arrays Syntax for initializing arrays Example Java code
arrayName[i] = value;
Where i is the index of an element in the array
Example Java code
examScores[0] = 80;
examScores[1] = 77;
examScores[2] = 92;
examScores[3] = 80;
examScores[4] = 63;
examScores[5] = 95;
11/16/2018
© 2007 William Albritton

12. Initializer List
You can also declare, instantiate, & initialize an array with one statement (initializer list)
Syntax
DataType [] arrayName = {value0, value1, value2, . . ., valueN-1};
The size of the array is automatically calculated
Example Java code
Double [] finalGrades = {83.33, 73.56, 91.29, 55.55, 67.02,83.33};
11/16/2018
© 2007 William Albritton

13. Bug for Initializer List
Can only use initializer list when the array is declared
Incorrect Java code
Double [] finalGrades; finalGrades = {83.33, 73.56, 91.29, 55.55, 67.02,83.33}; //bug!
11/16/2018
© 2007 William Albritton

14. Array Initialization Array of Integers Array of Doubles finalGrades
examScores
index address element
Array of Doubles finalGrades
indices addresses elements
11/16/2018
© 2007 William Albritton

15. Class Exercise 2
Declare and instantiate and initialize an array that is used to store
Vowels
Even integers from 2 to 100
Draw the “box & arrow” representation of what this looks like in memory
In your drawing, include variable labels, addresses (arrows), element values, and index values
11/16/2018
© 2007 William Albritton

16. Array Size
Size of an array is stored in a public variable called length
length is not a method!
Bug: array.length()
Useful in loops
for(Integer i=0;i<examScores.length;i++){ System.out.print(examScores[i] + ", "); }
11/16/2018
© 2007 William Albritton

17. Out of Bounds Error
What if an array index is greater or less than range of the array?
An exception will be thrown & the program will end
Example Java code
for(Integer i=0;i<=examScores.length; i++){ System.out.print(examScores[i] + ", ");}
Program will end and display a message about an ArrayIndexOutOfBoundsException
Since arrays start their index numbering at 0, a common bug is to be “off by one”
Range for arrays is 0 to SIZE-1 (length-1)
11/16/2018
© 2007 William Albritton

18. for-each Loop
A for-each loop loops through all elements of an array from lowest to highest index
Will loop array.length times
Syntax
for(DataType element: arrayName){ //use element in code }
Java code example
for(Double grade: finalGrades){ System.out.print(grade + ", "); }
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© 2007 William Albritton

19. Class Exercise 3 What is the output of the Java program?
See Exercise3.java
11/16/2018
© 2007 William Albritton