Data Structures Midterm Exam Review

1. Cpt S 122 – Data Structures Course ReviewMidterm Exam # 1 Nirmalya Roy School of Electrical Engineering and Computer Science Washington State University

2. Midterm Exam 1 • When: Friday (09/28) 12:10 -1pm • Where: In Class • Closed book, Closed notes • Comprehensive • Material for preparation: • Lecture Slides • Quizzes, Labs and Programming assignments • Deitel & Deitel book (Read and re-read Chapter 12)

3. Course Overview • Functions (Chapter 5) • Function Call Stack and Stack Frames • Pass-by-value and Pass-by-reference • Pointers (Chapter 7) • Pointer Operators • Passing Arguments to Functions by Reference • const qualifiers with Pointers • Characters and Strings (Chapter 8) • Fundamentals of Strings and Characters

4. Course Overview • Data Structures (Chapter 12) • Self Referential Structures • Dynamic Memory Allocation • Linked Lists, Stacks and Queues • Insert, delete, isEmpty, printList • Binary Trees, Binary Search Trees • Tree Traversals • inOrder, preOrder, postOrder

5. Functions Review (Chapter 5) • The data structure—working “behind the scenes”—supports the function call/return mechanism. • Stack • Supports the creation, maintenance and destruction of each called function’s automatic variables. • Stack overflow error • If more function calls occur since the amount of memory is finite.

6. Functions Review (Chapter 5) • Two ways to pass arguments • pass-by-value and pass-by-reference. • passed by value • a copy of the argument’s value is made and passed to the called function. • changes to the copy do not affect an original variable’s value in the caller. • Pass-by-value is used whenever the called function does not need to modify the value of the caller’s original variable. • passed by reference • the caller allows the called function to modify the original variable’s value.

7. Pointers Review (Chapter 7) • Enables programs to simulate pass-by-reference. • pass functions between functions. • For creating and manipulating dynamic data structures (i.e., data structures that can grow and shrink at execution times). • Linked lists, queues, stacks, trees.

8. Pointer Variable Definitions and Initialization count 7 count directly references a variable that contains the value 7. countPtr count Pointer countPtr indirectly references a variable that contains the value 7. Memory Location of 7 7 • Pointers are variables whose values are memory addresses. • Normally, a variable directly contains a specific value. • A variable directly references a value. • A pointer contains an address of a variable that contains a specific value. • A pointer indirectly references a value. • Indirection means referencing a value through a pointer.

9. Using the const Qualifier with Pointers • The const qualifier enables you to inform the compiler that the value of a particular variable should not be modified. • Four ways to pass a pointer to a function • Non-constant pointer to non-constant data • Non-constant pointer to constant data • Constant pointer to non-constant data • Constant pointer to constant data

10. Using the const Qualifier with Pointers • Non-constant pointer to non-constant data • The data can be modified through the dereferenced pointer, and the pointer can be modified to point to other data items. • Non-constant pointer to constant data • The non-constant pointer to constant data can be modified to point to any data item of the appropriate type, but the data to which it points cannot be modified.

11. Using the const Qualifier with Pointers • Constant pointer to non-constant data • A constant pointer to non-constant data always points to the same memory location, and the data at that location can be modified through the pointer. • Constant pointer to constant data • Such a pointer always points to the same memory location, and the data at that memory location cannot be modified.

12. Void, Null, Dangling Pointer • The void pointer, also known as the generic pointer, is a special type of pointer that can be pointed at objects of any data type. • A pointer to voidcannot be dereferenced. • A null pointer has a value reserved for indicating that the pointer does not refer to a valid object. • A dangling pointer is a pointer to storage that is no longer allocated.

13. Characters and Strings (Chapter 8) • A char * needs to point to some allocated memory • this memory may be allocated dynamically or via another automatic local variable • A str[] has memory associated with it • str refers to the address of the 0th element in the array • A char *str[] is an array of pointers • each pointer could refer to dynamically allocated memory or automatic local variable memory • A str[][] may be considered an array of strings or a 2-D array of characters • all required memory is allocated upfront; str[] (one index specified) refers to the start of the corresponding row

14. Data Structures (Chapter 12) • Dynamicdata structures with sizes that grow and shrink at execution time • Linked lists are collections of data items “lined up in a row” • insertions and deletions are made anywherein a linked list. • Stacks are important in compilers and operating systems • insertions and deletions are made only at one end of a stack—its top. • Queues represent waiting lines • insertions are made only at the back(also referred to as the tail) of a queue and deletions are made only from the front(also referred to as the head) of a queue. • Binary trees facilitate high-speed searching and sorting of data • efficient elimination of duplicate data items, • representing file system directories and compiling expressions into machine language.

15. Abstract Data Type • Data abstraction and abstract data types (ADT). • notion of an object (from object-oriented programming) is an attempt to combine abstractions of data and code. • ADT is a set of objects together with a set of operations • e.g., List, Operations on a list: Insert, delete, search, sort • C++ class are perfect for ADTs • Enable us to build the data structures in a dramatically different manner designed for producing software that’s much easier to maintain and reuse.

16. Self Referential Structures • A self-referential structure contains a pointer member that points to a structure of the same structure type. • For example, the definition • struct node { int data;struct node *nextPtr;}; // end struct node defines a type, structnode. • A structure of type structnode has two members • integer member data and pointer member nextPtr.

17. Dynamic Memory Allocation (Cont.) • Function malloc takes as an argument the number of bytes to be allocated • returns a pointer of type void* (pointer to void)to the allocated memory. • Function malloc is normally used with the sizeof operator. • A void* pointer may be assigned to a variable of any pointer type.

18. Dynamic Memory Allocation (Cont.) • For example, the statement newPtr = malloc( sizeof(struct node ) ); • evaluates sizeof(structnode) to determine the size in bytes of a structure of type structnode, • allocates a new area in memory of that number of bytes and stores a pointer to the allocated memory in variable newPtr. • The allocated memory is not initialized. • If no memory is available, malloc returns NULL.

19. Dynamic Memory Allocation (Cont.) • Function freedeallocates memory • the memory is returned to the system so that it can be reallocated in the future. • To free memory dynamically allocated by the preceding malloc call, use the statement • free( newPtr ); • C also provides functions calloc and realloc for creating and modifying dynamic arrays. • calloc allocates multiple blocks of storage, each of the same size. • realloc changes the already allocated memory size.

20. Memory Leak • When using malloc test for a NULL pointer return value. • Memory Leak: Not returning dynamically allocated memory when it’s no longer needed can cause system to run out of memory prematurely. This is known as “memory leak”. • Use free to return the memory to system.

21. Memory Allocation Process • C programming language manages memory statically, automatically, or dynamically. Stack Heap Permanent Storage Area Conceptual view of storage of a C program in memory

22. Linked Lists • A linked list is a linear collection of self-referential structures • known as nodes, connected by pointer links. • A linked list is accessed via a pointer to the first node of the list. • Subsequent nodes are accessed via the link pointer member stored in each node. • The link pointer in the last node of a list is set to NULL to mark the end of the list. • Data is stored in a linked list dynamically • each node is created as necessary.

23. Linked Lists Functions • The primary functions of linked lists are insert and delete. • Function isEmpty • If the list is empty, 1 is returned; otherwise, 0 is returned. • Function printList prints the list.

24. Insert Example

25. Function insert

26. Function insert

27. delete Example

28. Function delete

29. Function delete

30. Function printList

31. Linked List Practice Exercise • Printing the elements of a Linked List in reverse: Write a C program to print the linked list elements in reverse order • Either using a data structure discussed in the class • Or using recursive function calls • Inserting into an Ordered List: Write a C program to insert 25 random integers from 0 to 100 in descending order in a linked list.

32. Stacks • A stack can be implemented as a constrained version of a linked list. • New nodes can be added to a stack and removed from a stack onlyat the top. • For this reason, a stack is referred to as a last-in, first-out (LIFO) data structure. • A stack is referenced via a pointer to the top element of the stack. • The link member in the last node of the stack is set to NULL to indicate the bottom of the stack.

33. Stack Functions • The primary functions of stacks are push and pop. • Function isEmpty tests if stack is empty. • Function printList prints the stack elements.

34. push operation

35. Function push

36. pop operation

37. Function pop

38. Applications of Stacks • Balancing symbols • Compiler checks for program syntax errors • Every right brace, bracket, and parenthesis must correspond to its left counterpart • The sequence [()] is legal, but [(]) is wrong • Infix to Postfix Conversion • Postfix Expressions Evaluations

39. Stack Exercise • Think of using Stack to implement Linked List function • printListReverse(): printing linked list elements in reverse order • Think of using Stack to implement Queue function • enqueue and dequeue. • Write a C program to implement push and pop.

40. Queues • Queue is another common data structure. • A queue is similar to a checkout line in a grocery store—the first person in line is serviced first, and other customers enter the line only at the end and wait to be serviced. • Queue nodes are removed only from the headof the queue and are inserted only at the tailof the queue. • For this reason, a queue is referred to as a first-in, first-out (FIFO) data structure. • The insert and remove operations are known as enqueue and dequeue, respectively.

41. Queue Functions • The primary functions of queues are enqueue and dequeue. • Function isEmpty tests if queue is empty. • Function printList prints the queue elements.

42. Operation enqueue • The dotted arrows in part (b) illustrate Steps 2 and 3 of function enqueue that enable a new node to be added to the end of a queue that is not empty.

43. Function enqueue

44. Operation dequeue • Part (b) shows tempPtr pointing to the dequeued node, and headPtr pointing to the new first node of the queue. • Function free is used to reclaim the memory pointed to by tempPtr.

45. Function dequeue

46. Queue Exercise • Implement the level order binary tree traversal using queue data structure. • Write the pseudo code of this algorithm. • Write a C program to implement enqueue and dequeue.

47. Level Order Binary Tree Traversal • Use the Queue data structure to control the output of the level order binary tree traversal. • Algorithm • Insert/enqueue the root node in the queue • While there are nodes left in the queue, • Get/dequeue the node in the queue • Print the node’s value • If the pointer to the left child of the node is not null • Insert/enqueue the left child node in the queue • If the pointer to the right child of the node is not null • Insert/enqueue the right child node in the queue

48. Implementing linked list, stack, queue functions

49. Trees • Binary Trees • Binary Search Tree • insertNode, deleteNode • inOrder, preOrder, postOrder • Applications • Duplicate elimination, Searching etc

50. Trees • Linked lists, stacks and queues are linear data structures. • A tree is a nonlinear, two-dimensional data structure with special properties. • Tree nodes contain two or more links.