An Introduction to Programming in Java: Arrays

Modified by
Martin Chapman
4CCS1PRP, Programming Practice 2012
Lecture 6: Arrays - Part 1
Java for Everyone by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.
Slides by Donald W. Smith
TechNeTrain.com
Martin Chapman

Contents
 Arrays
 The Enhanced for Loop
 Common Array Algorithms
 Using Arrays with Methods
 Two-Dimensional Arrays
 Array Lists

Chapter Goals
 To become familiar with using arrays and
array lists to collect values
 To use the enhanced for loop for traversing
arrays and array lists
 To learn about common algorithms for
processing arrays and array lists
 To learn how to use two-dimensional arrays
In this chapter, you will learn about
arrays, array lists, and common
algorithms for processing them.

Sequences of Values
‣ A Computer Program often needs to
store a list of values and then process
them.
‣ For example, to find the highest.
‣ Say you had this list of values, how
many variables would you need?
‣ double input1, input2, input3….
‣ Impractical for manipulation;
unnecessary code replication.

An Example
beer Carlsberg, Guinness, Budweiser

An Example
if ( Carlsberg.alcoholPercentage is >= 5 )
then
My head hurts
otherwise
In for work at 9am
end
if ( Guinness.alcoholPercentage is >= 5 )
then
My head hurts
otherwise
In for work at 9am
end
if ( Budweiser.alcoholPercentage is >= 5 )
then
...
Only a ‘pseudocode’ example
beer Carlsberg, Guinness, Budweiser

Arrays
‣ Are the answer!
An array collects sequences of values of the
same type.

Declaring an Array
 Declaring an array is a two step process.
1) double[] data; // declare array variable
2) data = new double[10]; // initialise size
You cannot use the array
until you tell the compiler
the size of the array in
step 2.

Declaring an Array (Step 1)
 Make a named ‘list’ with the following parts:
Type Square Braces Array name semicolon
double
[ ] data ;
 You are declaring that
• There is an array named data
• The elements inside are of type double
• You have not (YET) declared how many elements
are in inside
 Other Rules:
 Arrays can be declared anywhere you can declare a
variable
 Do not use ‘reserved’ words or already used names

Declaring an Array (Step 2)
data
[0] [1] [2] [3] [4] … [9]
double double double double double double
 Reserve memory for all of the elements:
Array name Keyword Type Size semicolon
data = new double [10] ;
 You are reserving memory for:
• The array named data
• Needs storage for [10]
• Elements the size of type double
 You are also setting up the array variable
 Now the compiler knows how many elements there are

Common Error 1
double[] data;
...
data[0] = 29.95; // Error—data not initialised
 Uninitialised Arrays
 Don’t forget to allocate the array contents!
 The compiler will catch this error data
0
1
2
3
4
5
6
7
8
9
double[] data;
data = new double[10];
data[0] = 29.95; // No error
Error: D:JavaUnitialised.java:7:
variable data might not have been initialized

One Step Array Declaration
 Declare and Create at the same time:
Type Braces Array name Keyword Type Size semi
double [] data = new double [10] ;
 You are declaring that
• There is an array named data
• The elements inside are of type double
 You are reserving memory for the array
• Needs storage for [10]
• Elements the size of type double
 You are also setting up the array variable

Declaring and Initialising an Array
 You can declare and set the initial contents of all
elements by:
Type Braces Array name contents list semi
int
[ ] primes = { 2, 3, 5, 7} ;
 You are declaring that
 There is an array named primes
 The elements inside are of type int
 Reserve space for four elements
• No need for ‘new’ keyword, the compiler counts
them for you.
 Set initial values to 2, 3, 5, and 7
 Note the curly braces around the contents list

Accessing Array Elements
 Each element is numbered
 We call this the ‘index’
 Access an element by:
• Name of the array
• Index number
data[i]
public static void main(String[] args)
{
double data[];
data[4] = 35;
System.out.println(data[4]);
}
Elements in the array data are
accessed by an integer index i,
using the notation data[i].

The last element is at index 9:
Array Index Numbers
 Array index numbers start at 0
 The rest are positive integers
 A 10 element array has indexes 0 through 9
 There is NO element 10!
{
double data[];
}
The first element is at index 0:

Problem: Common Error 2
 Array Bounds Errors
 Accessing a nonexistent element is very common error
 Array indexing starts at 0
 Your program will stop at run time
public class OutOfBounds
{
{
double data[];
data[10] = 100;
}
}
java.lang.ArrayIndexOutOfBoundsException: 10

at OutOfBounds.main(OutOfBounds.java:7)
The is no element 10:

Solution: Array Bounds Checking
 An array knows how many elements it can hold
 data.length is the size of the array named data
 It is an integer value (index of the last element + 1)
 Use this to range check and prevent bounds errors
{
int i = 10, value = 34;
double data[];
if (0 <= i && i < data.length) // value is 10
{
data[i] = value;
}
}

Visiting Array Elements
 An array knows how many elements it can hold
 data.length is the size of the array named data
 It is an integer value (index of the last element + 1)
 Similarly, use this to visit all the elements in an array.
{
int[ ] data = { 1 , 2 , 3 , 4 , 5 };
for ( int i = 0; i < data.length; i++ )
{
System.out.println( data[ i ] );
}
}

 The benefits of using an array:
Back to Beer...
beer array = { Carlsberg, Guinness, Budweiser }
Set variable i = 0;
Loop until i == length of beer array
if element at i alcoholContent is > 5
then
My head hurts
otherwise
In for work at 9am
end
increase i by 1.
Next Loop.
A significant decrease in code.

Array Syntax Review
 To declare an array, specify the:
 Array variable name
 Element Type
 Length (number of elements)

Problems?
double[] inputs = new double[ ? ];
for (i = 0; i < inputs.length; i++)
{
inputs[i] = in.nextDouble();
}

Problems?
double[] inputs = new double[ ? ];
{
}
double[] inputs = new double[100];
while (in.hasNextDouble())
{
inputs[ ? ] = in.nextDouble();
}

Problems?
for (int i = 0; i < ? ; i++)
{
System.out.println(data[i]);
}

Partially-Filled Arrays
 An array cannot change size at run time
 The programmer may need to guess at the maximum
number of elements required
 It is a good idea to use a constant for the size chosen
 A partial solution?
 Use a variable to track how many elements are filled

Partially-Filled Arrays
ﬁnal int LENGTH = 100;
double[] data = new double[LENGTH];
int currentSize = 0;
Scanner in = new Scanner(System.in);
{
if (currentSize < data.length)
{
data[currentSize] = in.nextDouble();
currentSize++;
}
}
Use the .length value
and currentSize
variable to prevent
over-filling the array

‘Walking’ a Partially Filled Array
for (int i = 0; i < currentSize; i++)
{
System.out.println(data[i]);
}
A for loop is a natural choice
to walk through an array
‣ Use currentSize, not data.length for the last element

 Using for loops to ‘walk’ arrays is very common
 The enhanced for loop simplifies the process
 Also called the “for each” loop
 Read this code as:
• “For each element in data”
double[] data = . . .;
double sum = 0;
for (double element : data)
{
sum = sum + element;
}
A shortcut: The Enhanced for Loop

 As the loop proceeds, it will:
 Access each element in order (0 to length -1)
 Copy it to the element variable
 Execute loop body
 Not possible to:
 Change elements
 Get bounds error
double[] data = . . .;
double sum = 0;
{
sum = sum + element;
}
A shortcut: The Enhanced for Loop

Syntax: The “for each” loop
 Use the “for each” loop when:
 You need to access every element in the array
 You do not need to change any elements of the array

Common Array Algorithms
 Practical array implementations:
 Filling an Array
 Sum and Average Values
 Maximum and Minimum
 Output Elements with Separators
 Linear Search
 Removing an Element
 Inserting an Element
 Copying Arrays
 Reading Input

Common Algorithms 1:
1) Filling an Array
 Initialise an array to a set of calculated values
 Example: Fill an array with squares of 0 through 10
int[] squares = new int[11];
for (int i = 0; i < squares.length; i++)
{
squares[i] = i * i;
}

2) Sum and Average
 Use ‘for each’ loop, and make sure not to divide by zero
double total = 0, average = 0;
{
total = total + element;
}
if (data.length > 0) { average = total / data.length; }

double largest = data[0];
for (int i = 1; i < data.length; i++)
{
if (data[i] > largest)
{
largest = data[i];
}
}
double largest = data[0];
{
if (element > largest)
largest = element;
}
 Maximum and Minimum
 Set largest to first element
 Use for or a ‘for each’ loop
 Use the same logic for minimum
double smallest = data[0];
{
if (element < smallest)
smallest = element;
}
Typical for loop to find maximum
‘for each’ to find maximum ‘for each’ to find minimum

 Element Separators
 Output all elements with separators between them
 No separator before the first or after the last element
 Handy Array method: Arrays.toString()
 Useful for debugging!
for (int i = 0; i < data.length; i++)
{
if (i > 0)
{
System.out.print(" | ");
}
System.out.print(data[i]);
}
import java.util.*;
System.out.println(Arrays.toString(data));

 Linear Search
 Search for a specific value in an array
 Start from the beginning (left), stop if/when it is found
 Uses a boolean ‘found’ flag to stop loop if found
int searchedValue = 100;
int pos = 0;
boolean found = false;
while (pos < data.length && !found)
{
if (data[pos] == searchedValue) {
found = true; }
else {
pos++; }
if (found) {
System.out.println(“Found at position: ” + pos); }
else {
System.out.println(“Not found”); }
}
Compound condition to prevent
bounds error if value not found.

 Removing an element (at a given position)
 Requires tracking the ‘current size’ (# of valid elements)
 But don’t leave a ‘hole’ in the array!
 Solution depends on if you have to maintain ‘order’
• If not, find the last valid element, copy over position, update size
data[pos] = data[currentSize – 1];
currentSize--;

 Removing an element (at a given position)
 Requires tracking the ‘current size’ (# of valid elements)
 But don’t leave a ‘hole’ in the array!
• If so, move all of the valid elements after ‘pos’ up one spot,
update size
for (int i = pos; i < currentSize - 1; i++)
{
data[i] = data[i + 1];
}
currentSize--;

 Inserting an Element
• If so, find the right spot for the new element, move all of the
valid elements after ‘pos’ down one spot, insert the new
element, and update size
• If not, just add it to the end and update the size
if (currentSize < data.length)
{
for (int i = currentSize; i > pos; i--)
{
data[i] = data[i - 1];
}
data[pos] = newElement;
currentSize++;
}

Note: Array References
 Make sure you see the difference between the:
 Array variable: The named ‘handle’ to the array
 Array contents: Memory where the values are stored
int[] scores = { 10, 9, 7, 4, 5 };
An array variable contains a reference to the
array contents. The reference is the location
of the array contents (in memory).
Array variable Array contents
Reference
Values

Note: Array Aliases
 You can make one array reference refer to the
same contents of another:
int[] scores = { 10, 9, 7, 4, 5 };
Int[] values = scores; // Copying the array reference
An array variable specifies the location of
an array. Copying the reference yields a
second reference to the same array.
Array contents
References

 Copying Arrays
 Not the same as copying only the reference
• Only have one actual set of contents!

double[] data = new double[6];
. . . // Fill array
double[] prices = data; // Only a reference so far
double[] prices = Arrays.copyOf(data, data.length);
// copyOf creates the new copy, returns a reference
 Want to copy the contents of one array to another
 Use the Arrays.copyOf method

 Growing an array
 Copy the contents of one array to a larger one
 Change the reference of the original to the larger one
 Example: Double the size of an existing array
 Use the Arrays.copyOf method
 Use ‘2 *’ in the second parameter
double[] values = new double[6];
. . . // Fill array
double[] newValues = Arrays.copyOf(values, 2 * values.length);
values = newValues;
Arrays.copyOf second parameter is
the length of the new array

Increasing the Size of an Array - Step 1
 Copy all elements of values to newValues
double[] newValues = Arrays.copyOf(values, 2 * values.length);

Increasing the Size of an Array - Step 2
 Then copy newValues reference over values
reference
values = newValues;

 Reading Input
 Solution depends on if know how many values to expect
• If so, make an array of that size and fill it one-by-one
•
double[] inputs = new double[NUMBER_OF_INPUTS];
{
}

 Reading Input
 Solution depends on if know how many values to expect
• If so, make an array of that size and fill it one-by-one
• If not, make an array large enough for the maximum, and add
values to the end until no more are available.
double[] inputs = new double[NUMBER_OF_INPUTS];
{
}
double[] inputs = new double[MAX_INPUTS];
while (in.hasNextDouble() && currentSize < inputs.length)
{
inputs[currentSize] = in.nextDouble();
currentSize++;
}

double[] inputs = new double[INITIAL_SIZE];
{
if (currentSize >= inputs.length)
{
inputs = Arrays.copyOf(inputs, 2 * inputs.length);
}
inputs[currentSize] = in.nextDouble();
currentSize++;
}
 Reading Input
 A better solution: double the array when it is full.
 Able to handle input of arbitrary length

LargestInArray.java (1)
Input values and store in next
available index of the array

LargestInArray.java (2)
Use a for loop and the
‘Find the largest’ algorithm

Common Error 3
 Underestimating the Size of the Data Set
 The programmer cannot know how someone might want
to use a program!
 Make sure that you write code that will politely reject
excess input if you used fixed size limits
Sorry, the number of lines of text is
higher than expected, and some could not be
processed. Please break your input into
smaller size segments (1000 lines maximum)
and run the program again.

Think you’re ahead?
Friday 2pm - 4pm, Lab S4.01
Objects, classes, data structures, GUIs...
martin.chapman@kcl.ac.uk

Feel like you’re behind?
Friday 2pm - 4pm, Lab S4.01
Cover all basic concepts.
martin.chapman@kcl.ac.uk

An Introduction to Programming in Java: Arrays

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