Lecture06: Complex Types
4/18/2012
Slides modified from Yin Lou, Cornell CS2022: Introduction to C
1
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
• Example
struct 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);
}
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;
}
6
Structure Syntax
To define a structure
To declare a variable of the
new structure type
Definition
struct structure_name
{
/* list of variable declarations */
};
Example
void main()
{
struct structure_name variable_name;
}
The struct keyword is required
in both places
7
Structures and typedef
• To avoid the struct keyword, use a typedef
• Create alias for struct student called student_t
Example (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
Example (revised from example 2)
// a shortcut to define structure
typedef 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);
}
void main()
{
point a = {12.0, 42.0};
point b = {15.0, 36.0};
printf(“The distance is %lf”, distance(a, b) );
return 0;
}
9
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
Example
void 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, 42.0
reset(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
Example
void 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, 42.0
b.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
void main()
{
int a[] = {3, 7, -1, 4, 6};
int i;
double mean = 0;
// compute mean of values in a
for (i = 0; i < 5; ++i)
{
mean += *(a + i);
}
mean /= 5;
printf("Mean = %.2f\n", mean);
}
12
Pointers and Arrays
• Access array elements using pointers
void main()
{
int a[5] = {3, 7, -1, 4, 6};
int i;
double mean = 0.0;
// compute mean of values in a
for (i = 0; i < 5; ++i)
{
mean += a[i];
}
mean /= 5.0;
printf("Mean = %.2f\n", mean);
}
13
Pointers and Arrays
• What is *(a + i)?
for (i = 0; i < 5; ++i)
{
mean += *(a + i);
}
• Try this:
int a[5];
// print the address of array a
printf("%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);
14
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 ?
• Easy to map a number into a meaningful word
• Common usage 1:
#define SIZE 1000
int main()
{
int array[SIZE];
for (int i = 0; i < SIZE; i++)
array[i] = 0;
return 0;
}
19
What is #define ?
• Easy to map a number into a meaningful word
• Common usage 2:
#define sun 0
#define mon 1
#define tue 2
#define wed 3
#define thu 4
#define fri 5
#define sat 6
int main() {
int week;
scanf("%d", &week);
if (week == wed) {
// do something
}
return 0;
}
20
What is #define ?
• #define can also be dangerous!!
• Common error:
#define FIVE 4 + 1
#define SIX 5 + 1
int main() {
printf("%d\n", FIVE * SIX);
return 0;
}
21
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
Example
union 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. ptr
if (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?
• Program:
– enter how many stores you met: 5
store 1: seven
store 2: watsons
store 3: starbucks
store 4: chanel
store 5: coach
from the way you went back, you will met:
coach
chanel
starbucks
watsons
seven
32
Exercise 2
• Problem:
– Given a list of student grading information, can you tell me who is the first place?
• Program:
– enter how many students: 3
s1 name: Jonn
s1 english: 75
s1 chinese: 88
s2 name: Many
s2 english: 69
s2 chinese 62
s3 name: MrTop
s3 english: 100
s3 chinese: 100
The first place is MrTop who got 200 points in total!
33
Explaination about Assignment #6
Pointer array
void main()
{
struct card_t* deck[52];
fill_deck(deck);
print_deck(deck);
}
♤A
♤2
♤3
♤4
♤5
♤6
♤7
♤8
♤9
34
Explaination about Assignment #6
Pointer array
void main()
{
struct card_t* deck[52];
fill_deck(deck)
shuffle(deck);
print_deck(deck);
}
♤A
♤2
♤3
♤4
♤5
♤6
♤7
♤8
♤9
35
Explaination about Assignment #6
Pointer array
void main()
{
struct card_t* deck[52];
fill_deck(deck)
shuffle(deck);
print_deck(deck);
}
♤A
♤2
♤3
♤4
♤5
♤6
♤7
♤8
♤9
36