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Cs212: Data Structures. Lecture 2: Arrays. Lecture Contents. Arrays What is array? Array declaration Array initialization Array of objects Multidimensional Arrays Array Advantages and Disadvantages. Arrays Definition.

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cs212 data structures

Cs212: Data Structures

Computer Science Department

Lecture 2: Arrays

lecture contents
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
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
Arrays

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array name

-45

6

0

72

1543

-89

0

62

-3

1

6453

78

Array Name
  • The name of the array is a pointer to its first element

c[ 0 ]

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

c[ 1 ]

c[ 2]

c[ 3 ]

c[ 4 ]

c[ 5 ]

c[ 6 ]

c[ 7 ]

c[ 8 ]

c[ 9 ]

c[ 10]

Index (or subscript) of the element in array c

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c[ 11 ]

Fig. 7.1 A 12-element array.

slide6

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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slide7

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
Array declaration
  • 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
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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slide10

Declaring and Creating arrays

    • Arrays are objects that occupy memory
    • Created dynamically with keyword new

int c[] = newint[ 12 ];

        • Equivalent toint c[]; // declare array variable c = newint[ 12 ]; // create array
      • We can create arrays of objects too

String b[] = new String[ 100 ];

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slide11

Declare array as an array of ints

Create 10ints 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

  • 1 // Fig. 7.2: InitArray.java
  • 2 // Creating an array.
  • 3 importjavax.swing.*;
  • 4
  • 5 public classInitArray {
  • 6
  • 7 public static void main( String args[] )
  • 8 {
  • 9 int array[]; // declare reference to an array
  • 10
  • 11 array = new int[ 10 ]; // create array
  • 12
  • 13 String output = "Index\tValue\n";
  • 14
  • 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";
  • 18
  • 19 JTextAreaoutputArea = newJTextArea();
  • 20 outputArea.setText( output );
  • 21
  • 22 JOptionPane.showMessageDialog( null, outputArea,
  • 23 "Initializing an Array of int Values",
  • 24 JOptionPane.INFORMATION_MESSAGE );
  • 25
  • 26 System.exit( 0 );
  • 27
  • 28 } // end main
  • 29
  • 30 } // end class InitArray

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slide12

Each int is initialized to 0 by default

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example
Example

/**Adds all the numbers in an integer array. */

public static intsum(int[] a) {

inttotal = 0;

for (inti=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 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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slide15

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
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
Memory Representation of 2D Array

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array advantages and disadvantages
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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