Data Structure Midterm Lesson Arrays

2
What are 1-D arrays?

 An array is an object that is used to store a list of values of the same type.
 It is made out of a contiguous block of memory that is divided into a number of "slots“ or varia
 Each slot can be accessed by using its index (or subscript).
 If there are N slots in an array, the indexes will be 0 through N-1.
 The diagram on the right shows a 5-element array. The indexes are from 0 to 4
 An array that uses a single subscript is called a one dimensional array.
 Two dimensional arrays, three dimensional arrays, and higher dimensional arrays also
exist:

3
1-D Array Declaration
 For any type T, T[ ] is a class, whose instances are arrays of type T.
 Thus, the following statement declares a reference variable, b, of type T array:
T[] b;
 For any positive integer n, the following expression creates a new T[ ] object of size n and
stores its reference in b:
b = new T[n] ;
 As usual, the two expressions can be combined together as:
T[] b = new T[n] ;
 For example, the following declares an int[] , grades, of size 10:
int[ ] grades = new int[10];

4
1-D Array Declaration (cont’d)
 The declaration of an array of size n creates n variables of the base type T.
 These variables are indexed starting from 0 to n-1. They are called subscripted variables.
 A subscripted variable can be used anywhere an ordinary variable of the same type can
be used.
 The slots of an array are initialized to the default value for their type.
 Each slot of a numeric array is initialized to zero.
 Each array object has a public instance variable, length, that stores the size of the array.
 Thus, the following statement prints 10, the size of grades:
System.out.println(grades.length);
 Once an array has been constructed, its length does not change.
 Other examples of 1D-array declaration are:
double[ ] price = new double[500]; // each element is initialized to 0.0
boolean[ ] flag = new boolean[20]; // each element is initialized to false

5
Accessing elements of a 1-D Array
 A particular variable is accessed by indexing the array reference with the index
(subscript) of the variable in square brackets:
grades[4] = 20;
 The following example, re-initializes each variable with twice its index:
int[] grades = new int[10];
for(int i = 0; i < grades.length; i++)
grades[i] = 2*i;
 The use of grades.length instead of 10 makes the code more general.

6
Accessing elements of a 1-D Array (cont'd )
 The following prints the values of the array re-initialized by the example in the previous
slide.
for(int i = 0; i < grades.length; i++)
System.out.print(grades[i] + “ “);
 Output:
0 2 4 6 8 10 14 16
18
 Note: Trying to access an element with an invalid index causes a run-time error:
ArrayIndexOutOfBoundsException:
int x = grades[10]; // causes run-time error

7
Accessing elements of a 1-D Array (cont'd )
 The indexes (subscripts) of an array must always be of integer type except long
(int, char, short, byte).
 An array index can be any expression that evaluates to an integer within the allowed
index range:
double[] val = new double[20]; int k = 3;
val[0] = 1.5;
val[1] = 10.0;
val[2] = 15.5;
val[k] = val[k-1] + val[k - 2]; // same as val[3] = val[2] + val[1]
val[ k*2 + 3 ] = 24 + val[13]; // same as: val[ 9 ] = 24 + 0.0
System.out.println( "val[" + k + "] == " + val[k] ); // output: val[3] = 25.5
Note: the following are invalid because the index is not of type int, char, short, or byte:
long k = 5L;
val[k] = 33.2;
val[8.0] = 75.5;

8
Using Subscripts with an Array
Enhanced for loop
» Cycle through array
» Without specifying starting and ending points for loop control variable
for(int val : scoreArray)
System.out.println(val);

9
Initializer List
 Initializer list can be used to instantiate and initialize an array in one step:
int[ ] prime = {2, 3, 5, 7, 11, 13, 17, 19, 23, 29};
char[ ] letterGrade = {’
A’
, ‘
B’
, ‘C’, ‘
D’
, ‘
F’
};
 It is actually the compiler that fills the gap. Thus, in the first example, the compiler
would
add the following:
int[] prime = new int[10];
prime[0] = 2; prime[1] = 3; ... prime[9] = 29;
 Observe that when an initializer list is used:
 The new operator is not required.
 The size is not required; it is computed by the compiler.

10
Copying one array to another
 Consider the following declarations:
int[ ] array1 = {22, 3, 50, 7, 11, 13, 17};
int[ ] array2 = {3, 5, 8, 20, 0, 100, 40};
 The assignment statement :
array1 = array2;
does not copy the contents of array2 to array1; it makes the reference array1 to refer
to the array object referenced by array2. The object that was referenced by array1
becomes garbage.
 To copy the contents of array2 to array1, code like the following is used:
for(int k = 0; k < array2.length; k++)
array1[k] = array2[k];

11
Array used as a Parameter
Remember that a parameter is data that is supplied to a method just before it starts
running.
The following method prints the content of an int array passed to it as parameter:
class MyArray {
public void print (int[] x) {
for(int i = 0; i < x.length; i++)
System.out.print(x[i]+ " ");
System.out.println();
}
}
The method is written using the parameter x to refer to the actual data that it will be
supplied with.
The method is written without referring to any particular data. When the print()
method is running it has a reference to an array object in its parameter x.

12
Example(1)
class ArrayDemo {
public static void main ( String[] args ) {
MyArray operate = new MyArray();
int[] ar1 = { -20, 19, 1, 5, -1, 27 } ;
System.out.print ("The array is: " );
operate.print( ar1 );
}
}
MyArray
print(x)
operate
-20
19
1
5
-1
27
ar1
public static void print (int[] x) {
for(int i = 0; i < x.length; i++)
System.out.print(a[x]+ " ");
System.out.println();
}

13
Parameter Connected to New Data
Could the main() method create a second array and use the print()
method with it?
Example (2): What is the output?
class ArrayDemo {
public static void main ( String[] args ) {
MyArray operate = new MyArray();
int[] ar1 = { -20, 19, 1, 5, -1, 27 } ;
int[] ar2 = {1, 2, 6, 3, 9, 5};
System.out.print("The array is: " );
System.out.print("The second array is: " );
}
}
The array is: -20 19 1 5 -1 27
The second array is: 1 2 6 3 9 5

14
Using Two-Dimensional and
Multidimensional Arrays
One-dimensional or single-dimensional array
» Picture as column of values
» Elements accessed using single subscript
Two-dimensional arrays
» Two or more columns of values
» Rows and columns
» Use two subscripts
» Matrix or table
int[][] someNumbers = new int[3][4];

15
View of a Two-Dimensional Array in Memory

16
Multidimensional Arrays (continued)
int[][] rents = { {400, 450, 510},
{500, 560, 630},
{625, 676, 740},
{1000, 1250, 1600} };
public static void displayScores(int[]
[]scoresArray)

17
Multidimensional Arrays (continued)
Multidimensional arrays
» More than two dimensions
Create arrays of any size
» Keep track of order of variables needed as subscripts
» Don’t exhaust computer’s memory

Data Structure Midterm Lesson Arrays

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