1

Lecture06: Complex Types4/18/2012

Slides modified from Yin Lou, Cornell CS2022: Introduction to C

Administrative Items

• Midterm exam in class on 4/25– Cover from the start to Pointers and Arrays

• Assignment #6 due on 5/2• r, long double, etc.

2

Complex Types

• Structures– User-defined combinations of other types

• Enumerations– User-defined weekday: sunday, monday, ...

• Unions– Same data, multiple interpretations

• Arrays and pointers of the above• Function pointers

3

Preliminary types we have learned

• Integer– short– int– long long int

• Real number– float– double

• Character– char

• String– char[]

4

Structure example 1: student

• Structures aggregate variables of different types– Defined with the keyword struct

• Examplestruct student{

int id;int year;char grade;

};

void main(){

struct student s;s.id = 10001;s.year = 2011;s.grade = 'A';

}5

Structure example 2: 2-D Points

struct point{

double x;double y;

};

double distance(struct point p1, struct point p2){

double dx = p1.x - p2.x;double dy = p1.y - p2.y;return sqrt(dx * dx + dy * dy);

}

6

void main(){

struct point a;struct point b;a.x = 12.0;a.y = 42.0;b.x = 15.0;b.y = 36.0;printf(“The distance is %lf”, distance(a, b) );return 0;

}

Structure Syntax

7

To define a structure

Definitionstruct structure_name{

/* list of variable declarations */};

To declare a variable of the new structure type

Examplevoid main(){ struct structure_name variable_name;}

The struct keyword is required in both places

Structures and typedef

• To avoid the struct keyword, use a typedef• Create alias for struct student called student_tExample (revised from example 1)

struct student{

int id;int year;char grade;

};typedef struct student student_t;void main(){

student_t s;s.id = 10001;s.year = 2011;s.grade = 'A';

}

8

Structures and typedef

// a shortcut to define structuretypedef struct point{

double x;double y;

};

double distance(point p1, point p2){

double dx = p1.x - p2.x;double dy = p1.y - p2.y;return sqrt(dx * dx + dy * dy);

}

9

void main(){

point a = {12.0, 42.0};point b = {15.0, 36.0};printf(“The distance is %lf”, distance(a, b) );return 0;

}

Example (revised from example 2)

Structures in C• C treats structures as if they were primitive data types

– i.e. not like reference types (e.g. 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

Examplevoid reset(point p){

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

point a;a.x = 12.0;a.y = 42.0;printf(“before: [%.1lf, %.1lf]\n", a.x, a.y); // a = 12.0, 42.0reset(a);printf(“after: [%.1lf, %.1lf]\n", a.x, a.y); // a = ?, ?

}

10

Structures in C• C treats structures as if they were primitive data types

– i.e. not like reference types (e.g. 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

Examplevoid main(){

point a;a.x = 12.0;a.y = 42.0;

point b = a;printf(“before: [%.1lf, %.1lf]\n", b.x, b.y); // b = 12.0, 42.0b.x = 0;printf(“after: [%.1lf, %.1lf]\n", b.x, b.y); // a = 0.0, 42.0

}

11

Pointers and Arrays

• Access array elements using pointers

12

void main(){

int a[] = {3, 7, -1, 4, 6};int i;double mean = 0;

// compute mean of values in afor (i = 0; i < 5; ++i){

mean += *(a + i);}mean /= 5;printf("Mean = %.2f\n", mean);

}

Pointers and Arrays

• Access array elements using pointers

13

void main(){

int a[5] = {3, 7, -1, 4, 6};int i;double mean = 0.0;

// compute mean of values in afor (i = 0; i < 5; ++i){

mean += a[i];}mean /= 5.0;printf("Mean = %.2f\n", mean);

}

Pointers and Arrays

• What is *(a + i)?

14

for (i = 0; i < 5; ++i){

mean += *(a + i);}

• Try this:int a[5];

// print the address of array aprintf("%d\n", a);

// print the address of a[0]printf("%d\n", &a[0]);

// print the address of a[1]printf("%d\n", &a[1]);

// test what is “a + 1”printf("%d\n", a + 1);

Structures and Function Calls

• Function calls with structure arguments can be expensive.10000+ bytes must be copied

typedef struct{

char name[10000];int id;

} person;

void print_person(person p){

printf("%d %s\n", p.id, p.name);}void main(){

person a = {"Mike Smith", 1234};print_person(a);

}

15

Solution

• Use pointers. 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 by p• Use (*p).field to access a field of the structure• Or a cleaner syntax:

– p->field

16

Pointers to Structures

typedef struct{

char name[10000];int id;

} person;

void print_person(person *p){

printf("%d %s\n", p->id, p->name);}

void main(){

person a = {"Mike Smith", 1234};print_person(&a);

}

17

Pointers to Structure

typedef struct{

char name[10000];int id;

} person;

void print_person(person *p){

printf("%d %s\n", p->id, p->name);}

void main(){

person *p;p = (person *) malloc(sizeof(person));strcpy(p->name, "Mike Smith");p->id = 1234;print_person(&a);free(p);

} 18

What is #define ?

#define SIZE 1000

int main(){

int array[SIZE];for (int i = 0; i < SIZE; i++)

array[i] = 0;return 0;

}

19

• Easy to map a number into a meaningful word• Common usage 1:

What is #define ?

#define sun 0#define mon 1#define tue 2#define wed 3#define thu 4#define fri 5#define sat 6

20

• Easy to map a number into a meaningful word• Common usage 2:

int main() {int week;scanf("%d", &week);if (week == wed) {

// do something}return 0;

}

What is #define ?

#define FIVE 4 + 1#define SIX 5 + 1

int main() {printf("%d\n", FIVE * SIX);return 0;

}

21

• #define can also be dangerous!!• Common error:

Enumerations

enum days {mon, tue, wed, thu, fri, sat, sun};// Same as:// #define mon 0// #define tue 1// ...// #define sun 6

enum days {mon = 3, tue = 8, wed, thu, fri, sat, sun};// Same as:// #define mon 3// #define tue 8// #define wed 9// ...// #define sun 13

22

Enumerations

enum days day;// Same as: int day;

for (day = mon; day <= sun; ++day){

if (day == sun){

printf("Sun\n");}else{

printf("day = %d\n", day);}

}

23

Enumerations

• Basically integers• Can use in expressions like this• Makes code easier to read• Cannot get string equiv.• Caution: day++ will always add 1 even if enum values aren't

contiguous.

24

Creating Your Own Types

• C lets you name your own types using typedef• Example

typedef double scalar;scalar add(scalar a, scalar b){

return a + b;}

• 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

25

More on typedef

typedef double scalar;typedef scalar vector[10];

scalar add_scalars(scalar a, scalar b){

return a + b;}

void add_vectors(vector result, vector a, vector b){

int i;for (i = 0; i < 10; ++i){

result[i] = a[i] + b[i];}

}

26

Structures and the Heap

• So far we've been storing structures on the stack• Structures can also be stored on the heap

– Call malloc() to request a memory region of the correct size (Use sizeof to determine the correct size)

– Cast the pointer returned by malloc to a pointer to the structure– Free the region when the structure is no longer needed

27

Union

• A union is a variable that stores objects of dierent types– But it can only store one object at a time– Like a structure, but all fields start at offset 0

• e.g. You have a floating point number– Sometimes you want to treat it as a oat– Sometimes you want to treat it as an int– Sometimes you want to treat it as a char array

Exampleunion new_float{

float float_view;int int_view;char array_view[4];

};

28

Example

union new_float{

float float_view;int int_view;char array_view[4];

};

void main(){

union new_float f;f.float_view = 3.7;printf("%f %d %c %c %c %c\n", f.float_view, f.int_view,f.array_view[0], f.array_view[1], f.array_view[2],f.array_view[3]);

}

29

Function Pointers

int min(int a, int b);int max(int a, int b);

int foo(int do_min){

int (*func)(int, int); // declaring func. ptrif (do_min){

func = min;}else{

func = max;}return func(10, 20); // indirect call

}

30

Function Pointers

• Points to a function• Has a *-type of the function it points to

31

Exercise 1

• Problem:– I walked down the street one day and found n stores. I wrote down the names of these

stores. Can you tell me when I went back, what store would I meet first during the way? Can you list these n stores in order?

32

• Program:– enter how many stores you met: 5

store 1: sevenstore 2: watsonsstore 3: starbucksstore 4: chanelstore 5: coachfrom the way you went back, you will met:coachchanelstarbuckswatsonsseven

Exercise 2

• Problem:– Given a list of student grading information, can you tell me who is the first place?

33

• Program:– enter how many students: 3

s1 name: Jonns1 english: 75s1 chinese: 88s2 name: Manys2 english: 69s2 chinese 62s3 name: MrTops3 english: 100s3 chinese: 100The first place is MrTop who got 200 points in total!

Explaination about Assignment #6

34

Pointer arrayvoid main(){

struct card_t* deck[52];fill_deck(deck);print_deck(deck);

}

♤A

♤2

♤3♤4

♤5

♤6♤7

♤8

♤9

Explaination about Assignment #6

35

Pointer arrayvoid main(){

struct card_t* deck[52];fill_deck(deck)shuffle(deck);print_deck(deck);

}

♤A

♤2

♤3♤4

♤5

♤6♤7

♤8

♤9

Explaination about Assignment #6

36

Pointer arrayvoid main(){

struct card_t* deck[52];fill_deck(deck)shuffle(deck);print_deck(deck);

}

♤A

♤2

♤3♤4

♤5

♤6♤7

♤8

♤9