Lecture 6 - Arrays

 Arrays
 Declaring and constructing arrays
 Using and returning arrays
 Aliasing
2

 ArrayList is used when the size of a collection
is not known in advance
 But in some situations the maximum collection size
can be pre-determined
 A special fixed-size collection type is available: the array
 Arrays can store object references or primitive values
 Arrays use a special syntax
3

 An array is an indexed sequence of variables
of the same type
 The array is called a
 Its elements are called a[0], a[1], a[2], etc.
◦ Each element is a separate variable
 Notice that (as with ArrayList) indexing starts from 0
a[0] a[1] a[2] a[3] a[4]
a
4
a[5]

 Arrays are used when we have large numbers of identical objects that
we want to operate on as a collection
◦ A collection of student marks that we want to analyse
◦ A collection of temperatures that we want to average
◦ A collection of names that we want to sort
 e.g. Bureau of Meteorology data
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ALBANY
Max 25.1 25.1 24.1 21.5 18.7 16.6 15.7 15.9 17.4 18.8 20.8 23.4
Min 13.5 14.3 13.3 11.6 9.8 8.1 7.4 7.4 7.9 9.0 10.6 12.3
Rain 28 25 29 66 102 104 126 104 81 80 46 24
PERTH AIRPORT
Max 31.4 31.7 29.5 25.2 21.4 18.7 17.6 18.3 20.0 22.3 25.4 28.5
Min 16.7 17.4 15.7 12.7 10.2 9.0 8.0 7.9 8.8 10.1 12.4 14.6
Rain 8 14 15 46 108 175 164 117 68 48 25 12

 An array is declared using similar syntax to any other variable
int[] a;
 Declares a to be a variable representing an array of ints
double[] temps;
 Declares temps to be a variable representing an array of
doubles
String[] names;
 Declares names to be a variable representing an array of
Strings
Student[] marks;
 Declares marks to be a variable representing an array of
Student objects
6

 An array is an object in a Java program
 Therefore the declaration simply creates a reference to “point to” the
array, but does not create the array itself
 Hence the declaration
int[] a;
creates a space called a, big enough to hold an object
reference, and initially set to the special value null
a
null Only space for the array
reference has been
created, not for the array
itself
7

 In order to actually create the array, we must use the keyword new
(just like creating any other object)
int[] a;
a = new int[7];
0 0 0 0 00 0
a
An object is created that
contains seven variables of type
int
8
The expression a.length can be
used to refer to the size of a

• The seven variables do not have individual names
• They are referred to by the array name, and an index
0 0 0 0 00 0
a
9

• Array elements can be used in the same ways and
in the same contexts as any other variable of that
type
a[0] a[1]
0
a[2]
7
a[3]
-1
a[5]
0
a[4]
15
a[6]
17
a[4] = 15;
a[2] = 7;
a[3] = 2*a[2] – a[4];
a[6] = a[0] + 17;
10

 A lot of the power of arrays comes from the fact that
the index can be a variable or an expression
int x = 3;
a[x] = 5;
a[7-x] = 44;
a[0]
0
a[1]
0
a[2]
7
a[3]
5
a[5]
0
a[4]
44
a[6]
17
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private int sum(int[] a)
{
int sum = 0;
for (int i : a)
{
sum = sum + i;
}
return sum;
}
12
• Here i is an element of the array a

private int sum(int[] a)
{
int sum = 0;
for (int i = 0; i < a.length; i++)
{
sum = sum + a[i];
}
return sum;
}
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• Here i is being used as an index into the array a

• Note that this is only well-defined for non-empty
arrays!
• Again i is an element of the array a
private int max(int[] a)
{
int max = a[0];
for (int i : a)
{
if (i > max) max = i;
}
return max;
}
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{
int max = a[0];
{
if (a[i] > max) max = a[i];
}
return max;
}
15
• Again i is being used as an index into the array a

{
int k = 0;
{
if (a[i] > a[k]) k = i;
}
return k;
}
16
• A for-each loop will not work for this example

 An array literal is written using {}
 Array literals in this form
can only be used in declarations
 Related uses require new
private int[] numbers = {3, 15, 4, 5};
declaration,
creation, and
initialization
numbers = new int[] {3, 15, 4, 5};
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private int[] sums(int[] a)
{
int[] sums = new int[a.length];
sums[0] = a[0];
{
sums[i] = sums[i–1] + a[i];
}
return sums;
} 18
• Suppose we want to find the running sums of a
sums({2,5,–8,3}) = {2,7,–1,2}

private int[] avs(int[] a)
{
int[] avs = new int[a.length / 2];
for (int i = 0; i < a.length; i = i+2)
{
avs[i / 2] = (a[i] + a[i+1]) / 2;
}
return avs;
}
19
• Suppose we want to average each pair of elements of a
avs({2,6,–8,2}) = {4,–3}

private void sums(int[] a)
{
{
a[i] = a[i–1] + a[i];
}
}
20
• Sometimes instead of creating a new
array, we want to update the old one
• But this method doesn’t return
anything…

22
int[] a, b;
a = new int[3];
a[0]
0
a[1]
0
a[2]
0
a b

23
int[] a, b;
a = new int[3];
b = a;
a[0]
0
a[1]
0
a[2]
0
a b

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int[] a, b;
a = new int[3];
b = a;
System.out.println(b[2]);
a[0]
0
a[1]
0
a[2]
0
a b
0

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int[] a, b;
a = new int[3];
b = a;
a[2] = 9;
a[0]
0
a[1]
0
a[2]
9
a b
0

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int[] a, b;
a = new int[3];
b = a;
a[2] = 9;
a[0]
0
a[1]
0
a[2]
9
a b
0
9

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• This is called aliasing
• Basically one array with two names
• It applies in the same way with any
other object-types too
• Be careful with equality over arrays
• In some applications we might want to
use aliasing deliberately

 Arrays were discussed.
 Declaration and construction of arrays
 Different methods for using and returning arrays discussed.
 Aliasing with an example was discussed.
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Lecture 6 - Arrays

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