1 Structures

2 User-Defined Types C provides facilities to define one’s own types. These may be a composite of basic types ( int, double, etc) and other user-defined types. The most common user-defined type is a structure, defined by the keyword struct.

3 Structures A structure is a collection of one or more variables, possibly of different types, grouped together under a single name Structures are user-defined aggregate types. They assist program organisation by –Grouping logically related data, and giving this set of variables a higher-level name and more abstract representation. –Reducing the number of parameters that need to be passed between functions. –Providing another means to return multiple values from a function.

4 Structure Syntax A structure is defined by the keyword struct followed by a set of variables enclosed in braces. Consider the following structure to represent a person’s details. struct Personnel { char name[100]; int age; double height; }; The variables name, age and height are called members of the structure type Personnel.

5 Declaring Structure Variables There are two ways to define variables of a particular structure type. 1.Declare them at the structure definition. struct Personnel { char name[100]; int age; double height; } p1, p2, p3; /* Define 3 variables */ 2.Define the variables at some point after the structure definition. struct Personnel p1, p2, p3; /* Define 3 variables */

6 Initialising Structure Variables A structure may be initialised when it is defined using brace notation. struct Personnel captain = {“Fred”, 37, 1.83}; The order of values in the initialise list matches the order of declarations in the structure.

7 Accessing Members Members of a structure type may be accessed via the “. ” member operator. struct Personnel captain; strcpy(captain.name, “Fred”); captain.age = 37; captain.height = 1.83; printf(“%s is %d years old.”, captain.name, captain.age);

8 Nested Structures Structures may be defined inside other structures. struct first { struct second { int a; }s; double amount; }f; To access lower-level members, need to use member operator multiple times. f.s.a = 2.1; f.amount = 75.4;

9 Operations on Structures Structure types only support one of the operations that are permitted on primitive types. –Assignment (ie., copying) is permitted –All other operations (ie., arithmetic, relational, logical) are not allowed struct Personnel p1 = {“Fred”, 37, 1.83}; struct Personnel p2; p2 = p1; /* Valid. */ if (p1 == p2) /* Invalid. Won’t compile. */ printf(“People are equal\n"); if (strcmp(p1.name, p2.name) == 0 && /* Valid. */ p1.age == p2.age && p1.height == p2.height) printf("People are equal\n");

10 Structures and Functions Structures may be passed to functions and returned from functions. –Like all variables, they are passed by value

11 Pointers to Structures Defining pointers is the same as for variables of primitive types struct Personnel captain = {“Fred”, 37, 1.83}; struct Personnel *pp; pp = &captain; pp->age = 38; /* captain.age is now 38. */

12 Arrays of Structures The definition of arrays of structure types is the same as for arrays of primitive types. struct Personnel pa[10];

13 Self-Referential Structures A structure may not contain a variable of its own type. struct Node { int item; struct Node n; /* Invalid */ }; However, a structure may contain a pointer, self referential structure struct Node { int item; struct Node *pn; /* Valid */ };

14

15 Example: A Linked List Linked lists come in two basic varieties: singly linked and doubly linked. We describe here a simple version of a singly linked list. List consists of a set of nodes, where each node contains an item and a pointer to another list node. struct List { int item; struct List *next; }; (Here we have chosen an int as the contained item. Any other type(s) may be used.)

16 Singly Linked List List is formed by connecting the pointer of one node to the address of the next. We keep a pointer to the head of the list. This permits traversal. The end of the list is marked by a NULL pointer. Example, to start at the head of the list and traverse to the end node: struct List *node = head; while (node->next != NULL) node = node->next; printf(“Last node item: %d“, node->item);

17 Linked-List Properties Linked-Lists are useful because they can be grown (or shrunk) very easily. Unlike arrays, there are no issues of reallocating memory and copying data. Nodes can even be inserted (or removed) from midway along the list without difficulty (and efficiently).

18 Adding Nodes to a List Show example code for adding a node to the end of the list. Show example code for adding a node midway through the list.

19 Splicing in a New Node Remember that everything is manipulated as an address. Consider the pointer variables, eg., –node is address 0x4D –node->next is address 0xA1 –newnode is address 0xB6 0x4D0xA1 Splicing: newnode->next = node->next; assign to 0xA1 node->next = newnode; assign to 0xB6 0xB6 0x4D 0xA1