1. Data Structures Chapter 1 Programming Principles Andreas Savva

2. 2 Data Structures Structure of data Type of data

3. 3 Data Type Consists of two parts: Consists of two parts: Set of values Set of values Set of operations that act on the values Set of operations that act on the values Example : Integer data type Example : Integer data type Values - whole numbers in some defined range Values - whole numbers in some defined range Operations - add, subtract, multiply, divide, etc. Operations - add, subtract, multiply, divide, etc.

4. 4 Data Structure Consists of two parts: Consists of two parts: Collection of elements each of which is either a data type or another data structure. Collection of elements each of which is either a data type or another data structure. A set of associations or relationships (structure) involving the collection of elements. A set of associations or relationships (structure) involving the collection of elements.

5. 5 Examples – Data Structure Array Array Sequence of elements (data types) Sequence of elements (data types) Position association among the elements Position association among the elements Record Record Collection of data into a single structure Collection of data into a single structure No association No association 481220324760 pos  0123456 Surname:EastwoodName:Clint Nationality:USA Telephone:+15678900 Age:75

6. 6 Classification - Data Structures Linear Structure Linear Structure Hierarchical (Tree) structure Hierarchical (Tree) structure Graph structure Graph structure Set structure Set structure Unique predecessor Unique predecessor Unique successor Unique successor Unique predecessor Unique predecessor Many successors Many successors Many predecessors Many predecessors Many successors Many successors No predecessor No predecessor No successor No successor

7. 7 Examples Linear Structure Linear Structure Hierarchical (Tree) structure Hierarchical (Tree) structure Graph structure Graph structure Set structure Set structure Stack Stack Queue Queue Family Tree Family Tree Computer Directories Computer Directories Computer Network Computer Network London Underground London Underground You - Students in this class You - Students in this class

8. 8 Why do we need data structures? Example – Towns Data Structure You need to visit all houses in two towns once only. You need to visit all houses in two towns once only. In each town you start and finish at house number 1. In each town you start and finish at house number 1. All roads are the same length. All roads are the same length. Same number of houses in each town. Same number of houses in each town.

9. 9 123 4 567 8 Town 2Town 1 1 2 3 4 5 6 7 8 In town 1 you walk twice as far as in town 2. Why? Because the structures are different.

10. 10 Choosing a data structure The process involves: The process involves: analyzing the problem, analyzing the problem, determining basic operations needed, determining basic operations needed, selecting the data structure. selecting the data structure.

11. 11 Choosing the right data structure The right data structure will make your operations simple and efficient. The right data structure will make your operations simple and efficient. The wrong data structure will make your operations complex and inefficient. The wrong data structure will make your operations complex and inefficient.

12. 12 Abstract Data Types (ADT) A module (object) containing: A module (object) containing: the data structure, the data structure, the associated operations (subprograms). the associated operations (subprograms). Details of the module are hidden from the user (encapsulation). Details of the module are hidden from the user (encapsulation). This is called data abstraction. This is called data abstraction. Modules are stored as Unit in Pascal, Class in C++ and Java

13. 13 Abstract Data Types (ADT) Data cannot be accessed directly. Data cannot be accessed directly. Data can only be accessed indirectly via the subprograms (methods) associated with the data. Data can only be accessed indirectly via the subprograms (methods) associated with the data. Thus, the module contains: The data structure. Subprograms to access and/or modify the data structure.

14. 14 C++ Classes, Objects, and Methods A class collects data and the methods used to access or change the data. A class collects data and the methods used to access or change the data. Such a collection of data and methods is called an object belonging to the given class. Such a collection of data and methods is called an object belonging to the given class. Every class consists of members that represent either variables (called data members) or functions (called methods or member functions). The member functions of a class are normally used to access or alter the data members. Every class consists of members that represent either variables (called data members) or functions (called methods or member functions). The member functions of a class are normally used to access or alter the data members. Clients (user programs) can declare and manipulate objects of that class. Clients (user programs) can declare and manipulate objects of that class.

15. 15 Information Hiding A client does not need to know how the data are actually stored or how the methods are programmed. This important programming strategy is called information hiding. A client does not need to know how the data are actually stored or how the methods are programmed. This important programming strategy is called information hiding. Data members and methods available to a client are called public. Data members and methods available to a client are called public. Private variables and functions may be used in the implementation of the class, but are not available to a client. Private variables and functions may be used in the implementation of the class, but are not available to a client. Methods of a class are public Functions in a class are private

16. 16 Comparison Abstract Data Types Abstract Data Types High level description – logical picture of the data, and operations that manipulate the data. High level description – logical picture of the data, and operations that manipulate the data. Implementation of program is independent to the data structure. Implementation of program is independent to the data structure. Implementation of program is concerned with what it can do. Implementation of program is concerned with what it can do. NOT Abstract Data Types NOT Abstract Data Types Concrete description – collection of data types & operations that store and retrieve individual elements. Concrete description – collection of data types & operations that store and retrieve individual elements. Implementation of program is dependant to the data structure. Implementation of program is dependant to the data structure. Implementation of program is concerned with how a task is done. Implementation of program is concerned with how a task is done.

17. 17 Example – Abstract Data Type Main activities for operating a car are: Main activities for operating a car are: steering, accelerating, braking steering, accelerating, braking The design of a car can be viewed as an ADT with operations: The design of a car can be viewed as an ADT with operations: steer, accelerate, brake steer, accelerate, brake Two cars may implement these operations in different ways. Two cars may implement these operations in different ways. Most drivers can operate any car since the ADT presents a uniform method of operation. Most drivers can operate any car since the ADT presents a uniform method of operation.

18. 18 Programming Assumption: Assumption: You can design and write programs. You can design and write programs. This Course: This Course: Uses C++ as a tool. Uses C++ as a tool. Will not teach you how to program. Will not teach you how to program.

19. 19 Programming Principles We must learn to observe important principles of program design. We must learn to observe important principles of program design. We usually ignore them when we write small programs. We usually ignore them when we write small programs. BUT ignoring them when writing large projects could be disastrous. BUT ignoring them when writing large projects could be disastrous.

20. 20 Problems of large programs More difficult to maintain them than to write them. More difficult to maintain them than to write them. In time there will be new requests on the program, and if it is not well-designed or if the data is not well structured it will be impossible to restructure it and the program will become unusable. In time there will be new requests on the program, and if it is not well-designed or if the data is not well structured it will be impossible to restructure it and the program will become unusable. It will cost less to write another program from scratch than maintaining the existing one. It will cost less to write another program from scratch than maintaining the existing one. The approach The approach “First make your program work and then make it pretty” may be effective for small programs but not for large ones. may be effective for small programs but not for large ones.

21. 21 Program Design Divide the problem into smaller problems until they become of manageable size. Divide the problem into smaller problems until they become of manageable size. Project Sub-problem Data Structure Common and compatible Each part of a program must be well organized, clearly written or else its structure will have been forgotten at some time later or will not be understood by other programmers. Each part of a program must be well organized, clearly written or else its structure will have been forgotten at some time later or will not be understood by other programmers. You must form good programming habits from the beginning.

22. 22 Choice of Data Structures The most important aspect in algorithm design is the way in which the data of the program is stored: The most important aspect in algorithm design is the way in which the data of the program is stored: How they are arranged in relation to each other. How they are arranged in relation to each other. Which data are kept in memory. Which data are kept in memory. Which are calculated when needed. Which are calculated when needed. Which are kept in files, and how this files are arranged. Which are kept in files, and how this files are arranged. Program Testing The difficulty of debugging a program increases much faster than its size. A program twice the size of another will likely to not take twice as long to debug, but most probably, four times as long. The difficulty of debugging a program increases much faster than its size. A program twice the size of another will likely to not take twice as long to debug, but most probably, four times as long. Many large programs (such as operating systems) are put into use still containing errors that the programmers have not spotted. Many large programs (such as operating systems) are put into use still containing errors that the programmers have not spotted. Sometimes projects that have consumed years of effort must be discarded because it is impossible to discover why they will not work. Sometimes projects that have consumed years of effort must be discarded because it is impossible to discover why they will not work.

23. 23 Program Correctness If we do not wish such a fate to our projects, then we must use methods that: If we do not wish such a fate to our projects, then we must use methods that: Reduce the number of errors, making it easier to spot those that remain. Reduce the number of errors, making it easier to spot those that remain. Enable us to verify in advance that our algorithms are correct. Enable us to verify in advance that our algorithms are correct. Provide us with ways to test our programs so that we can be reasonably confident that they will not misbehave. Provide us with ways to test our programs so that we can be reasonably confident that they will not misbehave. Maintenance Since new demands will arise in the future it is important that a large project is written to make it as easy as possible to be understood and modified. Since new demands will arise in the future it is important that a large project is written to make it as easy as possible to be understood and modified.

24. 24 Case Study The Game of Life The Game of Life Introduced by the British mathematician J.H. Conway in 1970. Introduced by the British mathematician J.H. Conway in 1970. It is a simulation, not a game with players. It is a simulation, not a game with players.

25. 25 Rules for the Game of Life It takes place on an unbounded rectangular grid in which each cell can either be occupied by an organism or not. Occupied cells are called alive; unoccupied cells are called dead. Which cells are alive changes from generation to generation according to the number of neighboring cells that are alive, as follows: It takes place on an unbounded rectangular grid in which each cell can either be occupied by an organism or not. Occupied cells are called alive; unoccupied cells are called dead. Which cells are alive changes from generation to generation according to the number of neighboring cells that are alive, as follows: The neighbors of a given cell are the eight cells that touch it vertically, horizontally, or diagonally. The neighbors of a given cell are the eight cells that touch it vertically, horizontally, or diagonally. If a cell is alive but either has no neighboring cells alive or only one alive, then in the next generation the cells dies of loneliness. If a cell is alive but either has no neighboring cells alive or only one alive, then in the next generation the cells dies of loneliness. If a cell is alive and has four or more neighboring cells also alive, then in the next generation the cell dies of overcrowding. If a cell is alive and has four or more neighboring cells also alive, then in the next generation the cell dies of overcrowding. A living cell with either two or three living neighbors remains alive in the next generation. A living cell with either two or three living neighbors remains alive in the next generation. If a cell is dead, then it will become alive if it has exactly three neighboring cells that are alive. All other dead cells remain dead in the next generation. If a cell is dead, then it will become alive if it has exactly three neighboring cells that are alive. All other dead cells remain dead in the next generation. All births and deaths take place at exactly the same time. All births and deaths take place at exactly the same time.

26. 26 Configuration Example 1 ** Living neighbors 000000 012210 011 * 10 012210 000000 By rule 2 both the living cells will die in the next generation and rule 5 shows that no cells will become alive, so that configuration dies out.

27. 27 Example 2 000000 012210 023 * 20 02 20 012210 000000 Each of the living cells has a neighbor count of three, and hence remain alive, but the dead cells all have neighbor countsof two or less, and none of them becomes alive. Thus all new configurations will be identical to this one.

28. 28 Working Example * ** ** ** * ** ** ** ** ** ** *** **** *** **

29. 29 Another Example * * * *** The two configuration continue to alternate from generation to generation.

30. 30 The Algorithm Set up an initial Life configuration. Set up an initial Life configuration. Print the Life configuration. Print the Life configuration. While the user wants to see further configurations: While the user wants to see further configurations: Update the configuration by applying the rules of the Life game. Update the configuration by applying the rules of the Life game. Print the current configuration. Print the current configuration.

31. 31 The Class Life enum status {dead,alive}; class Life { public: void initialize(); void print() const; void update(); private: int neighbor_count(int, int) const; status grid[maxrow][maxcol]; };

32. 32 The Main Program int main() { Life configuration; welcome(); configuration.initialize(); configuration.print(); cout << "Continue viewing new generations?" << endl; while (user_says_yes()) { configuration.update(); configuration.print(); cout << "Continue viewing new generations?" << endl; } return 0; }

33. 33 Initialize void Life::initialize() { int row,col; for (row=0;row<maxrow;row++) for (col=0;col<maxcol;col++) grid[row][col] = dead; cout << "Enter row and column of living cells and finish with -1 -1" << endl; while ((row!=-1) || (col!=-1)) { cin >> row >> col; if (row==-1 && col==-1) cout << "Done" << endl; else if ((row =maxrow)) cout << "Row out of range" << endl; else if ((col =maxcol)) cout << "Column out of range" << endl; else grid[row][col] = alive; }

34. 34 Print void Life::print() const { }

35. 35 Count Neighbors int Life::neighbor_count (int row, int col) const { }

36. 36 Update void Life::update () { }

37. 37 User Reply bool user_says_yes() { char c; bool initial_response = true; do { if (initial_response) cout << "(y/n)?"; else cout << "Respond with either y or n: "; initial_response = false; do { cin >> c; } while (c=='\n' || c==' ' || c=='\t'); } while (c!='y' && c!='Y' && c!='n' && c!='N'); return (c=='y' || c=='Y'); }

38. 38 Programming Guidelines Choose meaningful names. Choose meaningful names. Use common prefixes or suffixes to associate names of the same general category, i.e. Use common prefixes or suffixes to associate names of the same general category, i.e. input_file out_file total_file Be careful with the use of the letters “I” and “O”, i.e. Be careful with the use of the letters “I” and “O”, i.e. I = 1; x = 1; x = I; x = O; x = 0; Avoid global variables when possible. Avoid global variables when possible. Avoid side-effects (changing the values of global variables). Avoid side-effects (changing the values of global variables). Keep the functions short – less than a page. Keep the functions short – less than a page.

39. 39 Documentation Guidelines The reading time for programs is much more than the writing time. “Make your program readable”. The reading time for programs is much more than the writing time. “Make your program readable”. Place a prologue at the beginning of your program. Place a prologue at the beginning of your program. When each variable, constant, or type is declared explain what it is and how it is used. Better still, make this information evident from the name. When each variable, constant, or type is declared explain what it is and how it is used. Better still, make this information evident from the name. Introduce each significant part of your program (paragraph or function) and indicate where it ends if it is not obvious. Introduce each significant part of your program (paragraph or function) and indicate where it ends if it is not obvious. Avoid comments that parrot what the code does, i.e. Avoid comments that parrot what the code does, i.e. count++ // Increase counter by 1 The code itself should explain how the program works. The documentation should explain why it works and what it does. The code itself should explain how the program works. The documentation should explain why it works and what it does. Modify the comments along with the code. Modify the comments along with the code. Add pre-conditions and post-conditions to your functions. Add pre-conditions and post-conditions to your functions.

40. 40 Programming Questions Does the program solve the problem that is requested? Does the program solve the problem that is requested? Does the program work correctly under all conditions? Does the program work correctly under all conditions? Does the program have a good user interface? Does the program have a good user interface? Is the program logically and clearly written? Is the program logically and clearly written? Is the program well documented? Is the program well documented? Does the program make efficient use of time and space? Does the program make efficient use of time and space? Does the program run on the right hardware? Does the program run on the right hardware? Can the program handle the maximum size of input? Can the program handle the maximum size of input?