C Programming : Dynamic memory allocation & Structures

C Programming :Dynamic memory allocation & Structures 2008/11/19 Made by Jimin Hwa (jmhwa@se.kaist.ac.kr) Edited and presented by Souneil Park (spark@nclab.kaist.ac.kr)

Contents • Dynamic memory allocation • Structure • Linked list

Motivation of Dynamic Allocation • The allocated space of an array is not flexible • What if more space is required? • Data insertion/deletion is difficult char [] “CC510” int [] 1 2 3 4 5 6 7 8 9 10

Dynamic Memory Allocation • malloc • Allocates size bytes and returns a pointer to the allocated memory. void* malloc(size_t size) /* <stdlib.h> : malloc/calloc/realloc */ int *pi; int size, i; scanf(“%d”, &size); pi = (int *)malloc(sizeof(int) * size); // (type): static type casting for (i = 0; i < size; i++) // Initialization pi[i] = -1;

Dynamic Memory Allocation (Cont’d) • calloc – initializes memory before allocation • Allocates size bytes, initialize the space with 0, and returns a pointer to the allocated memory void* calloc(size_t nmeb, size_t size) int *pi; int size, i; scanf(“%d”, &size); pi = (int *)calloc(size, sizeof(int)); // (type): static type casting

Dynamic Memory Deallocation • Synopsis • Frees the memory space pointed by ptr. • Frees(deallocates) the dynamically allocated memory space • void free(void *ptr) /* <stdlib.h> : free */ int *pi; int size, i; scanf(“%d”, &size); pi = (int *)malloc(sizeof(int) * size); if (pi != NULL) { free(pi); pi = NULL; }

Structure • A structure is a collection of variables, possibly of different types, grouped together under a single name. • Using structure, we can keep together different pieces of information as a single data record • Data packing mechanism • Example> structure student attributes: Name, student ID, and grade

Structure(Cont’d) struct student { /* struct struct_name{ */ char name[32]; /* definition of members */ int student_id; /* }; */ char grade; }; struct class { struct student member[100]; int num_of_students; int average_grade; }; int main(){ struct class CC510; /* struct struct_name var_name */ … }

Structure Operators • Operator for member access : “.” (dot) • connects the structure name and the member namestructure_name.member • Example> struct student john; john.grade = ‘A’; john.student_id = 4473; int id = john.student_id; • operator for member access (pointer) : -> • Exmple> struct student *pJohn; pJohn->grade = ‘A’; // (*pJohn).grade = ‘A’; pJohn->student_id = 4473; int id = john->student_id;

Structure Operators(Cont’d) • Assignment between structure variable: “=“ • Example> struct student John; struct student John_clone; John.name = “John”; John.grade = ‘A'; John.student_id = 945433; John_clone = John; /* John_clone.name = = “John”, John_clone.grade = = ‘A‘, John_clone.student_id = = 945433 */

Structure Comparison • We need to compare all fields one by one explicitly. • Example> if(john == john_clone){ … } /* Wrong */ if(strcmp(John.name, john_clone.name) == 0 && John.student_id == john_clone.student_id && …){…} /*Right */

Structure as a Function Argument • They are passed to functions by the usual parameter-passing mechanism • The entire structure is copied to the function • Example> int fail(struct student of_classA[], int size) { inti, count = 0; for (i=0; i < size; ++i){ if(of_class[i].grade == ‘F’) count ++; } return count }

Linked List • An alternative to array. • Data structure in which objects are arranged in a linear order • Consists of nodes, each containing arbitrary data fields and link pointing to the next nodes. • The size of a linked list would be changed in runtime. Node 8 2 1 7 7 7 HEAD X Data field Link

Linked List Implementation • An node is represented in a C structure. • malloc() is used to dynamically create node structure typedef struct node_t { int data; struct _node_t *next; } node_t; node_t head; head.next = NULL; /* Linked list is empty */ node_t* create_node(int d) { node_t *n = (node_t*)malloc( sizeof(node_t)); if (!n) return NULL; n->data = d; n->next = NULL; return n; }

LAB #7 - 1 • Implement a circular list • Get input from users (through standard input) • Input is an integer • Whenever a user enters an positive integer, • Store the input at the last • First, print the 1st, 3rd, 5th, … element • Then, print the 2nd, 4th, 6th, … element • If zero or a negative integer is entered, terminate the program.

LAB #7 - 2 • Implement a clist_copy function which copies the circular list • (struct xxx *) clist_copy (struct xxx *head) { …. } • Which returns the head of the copied circular list • In the main function, • Print the head address of the original list and the copied list • Print the elements of the copied circular list in a sequential order

The End Any Question?

C Programming : Dynamic memory allocation & Structures