cs212: data structures

CS212: DATA STRUCTURES

Lecture 2: ArraysComputer Science Department

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19 Apr 2023 Computer Science Department

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Lecture Contents

Arrays What is array? Array declaration Array initialization Array of objects Multidimensional Arrays Array Advantages and Disadvantages


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Arrays Definition

An array contains multiple objects of identical types stored sequentially in memory.

The individual objects in an array, referred to as array elements, can be addressed using a number, called index or subscript.

Index/subscript starts begins with 0 Array has fixed size and cannot be

changed after it has been declared.


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Arrays

19 Apr 2023

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Array Name

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The name of the array is a pointer to its first elementName of array

(note that all elements of this array have the same name, c)

Index (or subscript) of the element in array c

c[ 0 ]

c[ 1 ]

c[ 2]

c[ 3 ]

c[ 4 ]

c[ 5 ]

c[ 6 ]

c[ 7 ]

c[ 8 ]

c[ 9 ]

c[ 10]

c[ 11 ]

-45

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0

72

1543

-89

0

62

-3

1

6453

78


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Index Also called subscript Position number in square brackets Must be positive integer or integer

expression

a = 5;b = 6;c[ a + b ] += 2;

Adds 2 to c[ 11 ]


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Examine array c c is the array name c.length accesses array c’s length c has 12 elements ( c[0], c[1], … c[11] )

The value of c[0] is –45

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Array declaration

19 Apr 2023

Array must be declared before it is used. Array occupies a contiguous memory space. The memory spaces an array needs is

calculated by multiplying the number of elements by the memory required for their types, example if we have an array of 10 elements, this space is

10*sizeof(float) = 40 bytes


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Array declaration con..

One way to declare and initialize an array is as follows:

element_type[] array_name = {init_val_0,init_val_1,…,init_val_N−1};


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Declaring and Creating arrays Arrays are objects that occupy memory Created dynamically with keyword new

int c[] = new int[ 12 ]; Equivalent to

int c[]; // declare array variable c = new int[ 12 ]; // create array

We can create arrays of objects tooString b[] = new String[ 100 ];


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1 // Fig. 7.2: InitArray.java

2 // Creating an array.

3 import javax.swing.*;

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5 public class InitArray {

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7 public static void main( String args[] )

8 {

9 int array[]; // declare reference to an array

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11 array = new int[ 10 ]; // create array

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13 String output = "Index\tValue\n";

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15 // append each array element's value to String output

16 for ( int counter = 0; counter < array.length; counter++ )

17 output += counter + "\t" + array[ counter ] + "\n";

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19 JTextArea outputArea = new JTextArea();

20 outputArea.setText( output );

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22 JOptionPane.showMessageDialog( null, outputArea,

23 "Initializing an Array of int Values",

24 JOptionPane.INFORMATION_MESSAGE );

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26 System.exit( 0 );

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28 } // end main

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30 } // end class InitArray

Declare array as an array of ints

Create 10 ints for array; each int is initialized to 0 by default

array.length returns length of array

array[counter] returns int associated with index in array


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Each int is initialized to 0 by default


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Example

/**Adds all the numbers in an integer array. */ public static int sum(int[] a) { int total = 0; for (int i=0; i < a.length; i++) // note the use of the length variable total += a[i]; return total; }


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Arrays are Objects

Arrays in Java are special kinds of objects. In fact, this is the reason we can use the new operator to create a new instance of an array.

An array can be used just like any general object in Java, but we have a special syntax (using square brackets, "[" and "]") to refer to its members.

An array in Java can do everything that a general object can. Since an array is an object, though, the name of an array in Java is actually a reference to the place in memory where the array is stored.


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The fact that arrays in Java are objects has an important implication when it comes to using array names in assignment statements. For when we write something like

b = a; in a Java program, we really mean that b

and a now both refer to the same array. So, if we then write something like

b[3] = 5;


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Multidimensional Arrays

Can be described as "arrays of arrays". Multidimensional arrays are not limited to two

indices.


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Memory Representation of 2D Array


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Array Advantages and Disadvantages

Array Advantages: Easier to declare and use. Can be used with most of the data types.

Array Disadvantages: Fixed size data.

If not all the array elements are used. waste of memory space.

If more array elements are required if the number of elements to be stored are more

than the maximum size, the array cannot accommodate those new values.

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