Linked Lists in C: Understanding the Data Structure and Implementation, Quizzes of Computer Science

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Quiz

 Wednesday?

 Closed book

 Based on homeworks and programs gone

through in class

 No need to memorize syntax

 If I want you to use a function/ technique, I will

provide a small example

Arrays of strings

 Arrays of strings are just 2-D arrays of

characters

 Because strings are just arrays of characters

char *array2[] = {“Test.”, “Vacation!”, “”};

array2 (char **)

T E S T. \

V a c a t i o n! \

\

(char *)

(char *)

(char *)

Creating your own types

 C lets you name your own types using typedef

 Example:

 typedef syntax

 As if you’re declaring a variable of the type you

want to redefine

 Give the variable the name of the new type

 Put typedef in front

// create a new type called “scalar” typedef double scalar; scalar add(scalar a, scalar b) { return a + b; }

typedef example

 Use typedef to create alias for complex types

// create new types called “scalar” and “vector” typedef double scalar; scalar add_scalars(scalar a, scalar b) { return a + b; }

add_vectors(scalar result[10], scalar a[10], scalar b[10]) { int j; for(j=0; j<10; j++) result[j] = a[j] + b[j]; }

Structures

 Structures aggregate variables of different types

 Like classes in Java, but they can only store data

 Defined with the keyword struct

// define new structure struct Student { int id, year; char grade; };

int main() { struct Student s; s.id = 10001; s.year = 2010; s.grade = ‘B’; printf(“%d %d %c\n”, s.id, s.year, s.grade); }

(int) id

(int) year

(char) grade

struct student

struct student requires approximately 9 bytes

Structure syntax

 To define a structure

 To declare a variable of the new structure type

 The struct keyword is required in both places

struct structure_name { /* list of variable declarations */ };

int main() { struct structure_name variable_name; /* code */ }

Another example: 2-D points

// define new structure typedef struct { double x, y; } Point;

double distance(Point p1, Point p2) { return (p1.x - p2.x) * (p1.x - p2.x) + (p1.y - p2.y) * (p1.y - p2.y); }

int main() { Point a = { 12.0, 42.0 }; Point b = { 15.0, 36.0 }; printf(“Distance: %lf\n”, distance(a, b)); }

Structures in C

 C treats structures as if they were primitive data types

 (^) I.e. not like reference types in Java  (^) Passing a structure as an argument to a function means that a temporary copy is made of the entire structure  (^) Assignments do a deep copy of the entire structure

void reset(Point p) { p.x = p.y = 0; }

int main() { Point a = { 12.0, 42.0 }; Point b = a;

reset(a); b.x = 0; printf(“a: %d,%d\n”, a.x, a.y); printf(“b: %d,%d\n”, b.x, b.y); }

Pointers to structures

 Function calls with structure arguments can be

costly

 Solution: Use pointers to structures. Only the

address of the structure is copied, not the entire

structure.

 Use &s to get the address of a structure s

 Use *p to get the structure pointed to by p

 Use (*p).field to access a field

 Or the cleaner syntax: p->fieldU

Pointers to structures

 Function calls with structure arguments can be costly

 (^) Solution: Use pointers to structures. Only the address of the structure is copied, not the entire structure.

typedef struct { char name[10000]; int id; } Person;

void print_person(Person *p) { printf(“%d %s\n”, p->id, p->name); }

int main() { Person a = { “Mike Smith”, 1234 }; print( &a ); }

Dynamically allocating structures

 Example

typedef struct { char name[10000]; int id; } Person;

int main() { Person *p; p = (Person *) malloc( sizeof(Person) );

strcpy(p->name, “George Washington”); p->id = 1776;

free(p); return 0; }

Practical example: Linked Lists

 A linked list is a very common data structure in CS

 (^) The list is not (necessarily) stored in contiguous memory

 Instead, each item is stored individually

 Each item includes a reference to the next item in the list

 The last item has a null reference

 (^) Advantage:

 (^) List can grow and shrink dynamically

 (^) Disadvantage:

 No random access to the data

 (^) Memory overhead from next pointers

Linked lists in C

 Function to add a cell to the head of the list

 returns a pointer to the new head of the list

 Function to print the contents of a list

ListCell *add_front(ListCell *head, int data) { return make_ListCell(data, head); }

void print_list(ListCell *head) { for( ; head ; head = head->next) printf(“%d\n”, head->data); }

Linked lists in C

 Function to destroy a ListCell

 like a destructor for ListCell in C++

 Function to destroy an entire list

void destroy_ListCell(ListCell *cell) { free(cell); }

void destroy_list(ListCell *head) { ListCell *next; for( ; head ; head = next) { next = head->next; destroy_ListCell(head); } }