TRACING AND EVALUATING ALGORITHMS IN JAVA

Steven Fox – Chapter 2 Sections 2.1.4 & 2.1.5

Chapter 2 Section 2.1.4

TRACE ALGORITHMS IN JAVA

TRACE ALGORITHM EXAMPLE SPECS.

As an example, we will consider the useful process of maintaining a full index to a data file (or another array).

The Class BookIndex will be described on the next slide.

BOOKINDEX CLASS

A Class has the following data members:Public class BookIndex{

// instance variables or data membersprivate String bookNum; // book reference numberprivate int pos; // book details location in main file

// This class also has accessor and mutator methods.}

A Class then declares an array of these objects:BookIndex[] bookIndexes = new BookIndex[500];

STRUCTURE

This structure is used in a library containing fiction and non-fiction books. There are several shelf units labeled A-H. Each shelf unit has four shelves in it.

BOOKNUM FIELD STRUCTURE

The BOOKNUM field has the following structure: The first character indicates a fiction or non-fiction book The second character indicates the shelf unit letter (A to

H) The third indicates the number of the shelf (1 is bottom, 4

is top) The last three numbers indicate a subject code Example a book number: NG2023 The POS field indicates where the rest of the details of

the book are located in a direct access data file. When a new book is added, an entry is made at the end

of the data file. The book number is then inserted in the correct position in the array.

SAMPLE DATA

Element 0 1 2 3 4 5 6 7

BOOKNUM

FA1021

FA1122

FA1233

FA2099

FA2103

FA2145

FA2238

…

POS 3 6 2 5 0 4 1 …

A new book, with reference number FA2139 is added to the end of the data file at a position contained in an identifier newPos. This book now needs to be added to the array. Consider the following algorithm.

INSERT METHOD

private void insert(String bookNum, int newPos)

{int i = newPos; //pointer to the array// Shuffle up array entries from top end to entry position.// This will find the next smallest to bookNum (or first element).while ( (i>0) && (bookNum.compareTo(bookIndexes[i].getBookNum()) < 0)){

bookIndexes[i] = bookIndexes[i-1];i = i-1;

}//Check boundary position at i = 0if (i != 0){i = i + 1; //insert above smaller entry}bookIndexes[i] = new BookIndex(bookNum, newPos);

}

TRACING THE INSERT METHOD

Assuming that the array is as shown in the table and the next vacant location in the file is 7, let’s trace the insert method.newP

osbookNu

mi book <compare

condition>

Indexes[i]

7 FA2139 7 FA2238 TRUE

7 6 FA2145 TRUE

7 5 FA2103 FALSEAt this point, the array looks like:Element 0 1 2 3 4 5 6 7

BOOKNUM

FA1021

FA1122

FA1233

FA2099

FA2103

FA2145

FA2238

FA2238

POS 3 6 2 5 0 4 1 2

EXECUTING

The array looks like:

and the value of i is 4. Therefore, we execute:

if (i != 0){

i = i + 1; // insert above smaller entry}bookIndexes[i] = new BookIndex(bookNum, newPos);

Thus, bookNum FA2139 and the value 7 are inserted into the correct position (5) in the array.

Element 0 1 2 3 4 5 6 7

BOOKNUM

FA1021

FA1122

FA1233

FA2099

FA2103

FA2145

FA2238

FA2238

POS 3 6 2 5 0 4 1 2

EVALUATE ALGORITHMS IN JAVA

When you evaluate algorithms in Java, you are essentially tracing them with virtual data. An example is an algorithm that takes an array NAMES and sorts it so that names are at the front (Bob) and other values are at the end (XXXX). To evaluate this, take sample arrays with this type of data and run through the algorithm. After multiple traces, you have evaluated the algorithm.