C Language Part 3. Digital Computer Concept and Practice Copyright 2012 by Jaejin Lee

Transcription

1 C Language Part 3

2 Pointers (revisited) int i = 4, j = 6, *p = &i, *q = &j, *r; if (p == &i)...; if (p == (& i))...;... = **&p;... = *(*(& p));... = 9 * *p / *q + 8;... = (((9*(*p)))/(*q)) + 8; *(r = &i) *= *p; (* (r = (& j))) *= (* p);

3 Pointers as Function Arguments Suppose we want to make a function that returns the maximum and the minimum of three integers a, b, and c We cannot pass the results with the return mechanism of the function because we need to return two values Use pointers The code in the next slide shows such a function We can call it by maxmin(a, b, c, &max, &min);

4 Pointers as Function Arguments (contd.) void maxmin(int a, int b, int c, int *pmax, int *pmin) if (a >= b) if (a >= c) /* a is maximum */ *pmax = a; if (b >= c) *pmin = c; else *pmin = b; else /* c > a >= b */ *pmax = c; *pmin = b; else if (b >= c) /* b is maximum */ *pmax = b; if (a >= c) *pmin = c; else *pmin = a; else /* c > b > a */ *pmax = c; *pmin = a;

5 Swap Function #include <stdio.h> void swap(int*, int*); int main(void) int x = 4, y = 5; swap( &x, &y ); printf( %d %d\n, x, y); return 0; void swap( int *p, int *q ) int tmp; tmp = *p; *p = *q; *q = tmp; #include <stdio.h> void swap(int, int); int main(void) int x = 4, y = 5; swap( x, y ); printf( %d %d\n, x, y); return 0; void swap( int p, int q ) int tmp; tmp = p; p = q; q = tmp;

6 Pointers and Arrays An array name is actually a constant pointer Its value cannot be changed When x is an array, x[i] is the same as *(x + i) When p is a pointer, *(p + i) is the same as p[i] #define N 100 int a[n], i, *p, sum = 0; for(p = a; p < &a[n]; p++) sum += *p; for(i = 0; i < N; i++) sum += *(a + i); for(p = a, i = 0; i < N; i++) sum += p[i];

7 Pointer Arithmetic If p is a pointer to a particular type, the expression p + 1 gives the address for the storage of the next variable of that type double a[10], *p, *q; p = a; /* p points to the first element of a */ q = p + 2; /* q points to the third element of a */ printf( %d\n, q p) /* q - p is 2 */ printf( %d\n, (int) q (int) p) /* 16 */

8 Arrays as Function Arguments The base address of the array is passed to the function double sum(double x[], int n) /* double sum(double *x, int n) */ int i; double sum = 0.0; for (i = 0; i < n; i++) sum += x[i]; return sum; double y[ 100 ]; sum(y, 100); sum(y, 20); sum(&y[10], 20); sum(y + 10, 20);

9 Arrays and Strings A character array can be initialized with a string constant when it is declared char str[12] = programming ; The length of str should be the number of characters in the string constant plus 1 The last byte contains the null character The length of the array in the example above may be omitted char str[] = programming ; 12 bytes are assigned to str by the compiler

10 Arrays and Strings (contd.) If there is no room for the null character as in the example below, carr cannot have a terminating null character and it is not a string char carr[11] = programming ; It is an array of characters The conversion specification for a string in both printf and scanf is %s

11 String Constants String constants are written between double quotes Treated as a pointer The value is the base address of the string char *p = abc ; printf( %s %s\n, p, p+1); abc [2] *( abc + 2) char s[] = abc ; char s[] = a, b, c, \0 ;

12 Pointer Arrays Suppose we want to store an array of strings such as country names The best way is to use an array of pointers char *pcountry[] = Korea, China, Japan, U.S.A., Russia ; Then pcountry[0] is a pointer that points to Korea, pcountry[1] is a pointer to China, etc.

13 Simple Cryptosystem A cryptosystem consists of an encryption function and a decryption function The encryption function gets a plaintext and a key, and produces a ciphertext The decryption function gets a ciphertext and a key, and produces a plaintext If the decryption key is the same as the encryption key, the decryption function should produce the original plaintext

14 Simple Cryptosystem (contd.) The encryption function shifts each letter of the plaintext to the right by k positions (modulo 26) For example, if the key is 3, each letter of the plaintext is changed to a ciphertext letter as shown in the table below plaintext a b c d e f g h i j k l m n o p q r s t u v w x y z ciphertext d e f g h i j k l m n o p q r s t u v w x y z a b c

15 Encryption Function #include <stdio.h> #define SMAX 100 #define KMOD 26 void EncShift(char ptext[], char ctext[], int key) int i=0; while (ptext[i]!= '\0') ctext[i] = (ptext[i]-'a'+key) % KMOD + 'a'; i++; ctext[i] = '\0'; int main(void) char ptext[smax], char ctext[smax]; int key; printf("enter plaintext: "); scanf("%s", ptext); printf("enter key: "); scanf("%d", &key); EncShift(ptext, ctext, key); printf("ciphertext: %s\n", ctext); return 0;

16 Structures A structure is a collection of one or more variables, possibly of different types struct person char name[10]; int age; char sex; ; Struct person introduces a structure which contains three members, i.e., name, age, and sex Person is called a structure tag Once the structure tag is defined, we can declare structure variables struct person per1, per2; struct person per1 = Lee, 20, f ;

17 Operations on Structures A member of a structure variable is referred to by the following form: structure-variable.member For example, we can print the members of per1 as follows: printf("%s, %d, %c", per1.name, per1.age, per1.sex); A structure can be copied as a unit For example, per2 = per1; copies per1.name to per2.name, per1.age to per2.age, and per1.sex to per2.sex

18 Operations on Structures (contd.) We can take the address of a structure with & pp is declared as a pointer to struct person, and the address of per1 is assigned to pp (i.e., pp points to per1) *pp means per1, and (*pp).name, (*pp).age, (*pp).sex refer to per1 s members struct person *pp; pp = &per1;

19 Operations on Structures (contd.) If pp is a pointer to a structure, an alternative notation to refer to a member is pp->structure-member pp->name, pp->age, and pp->sex refer to the members of per1

20 Array of Structures When we need to maintain a list of persons, we can declare an array of structures: struct person per[10];

21 Array of Structures (contd.) #include <stdio.h> struct person char name[10]; int age; char sex; ; void PrintPerson(struct person *pp) printf("name: %s, age: %d, sex: %c\n", pp->name, pp->age, pp->sex); int main(void) int i; struct person per[3]= Lee, 20, 'f', Kim, 25, 'm', Park, 22, f ; for (i=0; i<3; i++) PrintPerson(&per[i]); return 0;

22 Structure Example The following program declares a structure for complex numbers and a function for complex number multiplication #include <stdio.h> struct complex double x; double y; ; struct complex cmult(struct complex a, struct complex b) struct complex c; c.x = a.x * b.x - a.y * b.y; c.y = a.x * b.y + a.y * b.x; return c;

23 Structure Example (contd.) int main(void) struct complex a, b, c; printf("enter a: "); scanf("%lf %lf", &a.x, &a.y); printf("enter b: "); scanf("%lf %lf", &b.x, &b.y); c = cmult(a, b); printf("mult: %f + %f i\n", c.x, c.y); return 0;

24 Abstract Data Types An abstract data type is a collection of objects together with a collection of operations Lists, sets, and stacks are examples of abstract data types

25 Abstract Data Types (contd.) A list is simply a list of elements a1, a2,, and ak Two operations on the list Linsert(list, x) inserts element x into list Lsearch(list, x) searches list for element x We may define more operations such as Ldelete A set a1, a2,, ak along with some set operations such as union and intersection can be an abstract data type A stack is an important abstract data type which has many applications in programming

26 Linked Lists A linked list is a list of nodes linked by pointers A node of a linked list requires a structure It has two members element and next which is a pointer to struct node struct node int element; struct node *next; ;

27 Linked Lists (contd.) Linsert(list, x) gets list, which is a pointer to a linked list, and element x as its input It inserts x at the front of the list Lsearch(list, x)gets list, which is a pointer to a linked list, and x as its input If x is in the list, it returns the pointer of the node that contains x; NULL otherwise

28 Linked Lists (contd.) #include <stdio.h> struct node int element; struct node *next; ; struct node *Linsert(struct node *list, int x) struct node *pnew; pnew = malloc(sizeof(struct node)); if (pnew == NULL) printf( malloc error\n ); return NULL; pnew->element = x; pnew->next = list; return pnew;

29 Linked Lists (contd.) struct node *Lsearch(struct node *list, int x) struct node *pn; for (pn = list; pn!= NULL; pn = pn->next) if (pn->element == x) return pn; return NULL; void PrintList(struct node *list) if (list == NULL) printf( \n ); return; printf( %d, list->element); PrintList(list->next);

30 Linked lists (contd.) int main(void) struct node *list = NULL; int x; x); while (1) printf( enter x (0 to terminate): ); scanf( %d, &x); if (x == 0) break; list = Linsert(list, x); PrintList(list); printf( enter x: ); scanf( %d, &x); if (Lsearch(list, x)!= NULL) printf( %d is in the list\n, else printf( %d is not in the list\n, x); return 0;

31 malloc() In the standard library (stdlib.h) To dynamically create storage data structures void* malloc( size_t s ) Allocates contiguous space in memory with a size of s bytes without initialization Typically typedef unsigned int size_t; in stdlib.h x = malloc( n * sizeof( int )); If successful, returns a pointer to the base of the space, otherwise, returns NULL sizeof( type ) Returns the number of bytes needed to store a data item in type

32 Stacks A stack is a list of elements with the restriction that elements are inserted and deleted only at one place called the top Fundamental stack operations are push and pop push(st, x) gets st, which is a pointer to a stack, and element x as its input It pushes x into the stack. pop(st) gets st, which is a pointer to a stack, and it pops and returns the element at the top

33 Stacks (contd.) #include <stdio.h> #define MAX 100 struct stack int starray[max]; int top; ; struct stack *create(void) struct stack *st; st = malloc(sizeof(struct stack)); if (st == NULL) printf( malloc error\n ); return NULL; st->top = 0; return st;

34 Stacks (contd.) int is_empty(struct stack *st) return st->top == 0; void push(struct stack *st, int x) if (st->top == MAX) printf( stack is full\n ); return; st->starray[st->top++] = x; int pop(struct stack *st) if (is_empty(st)) printf( stack is empty\n ); return; return st->starray[--st->top];

35 Stacks (contd.) int main(void) struct stack *st; int x; st = create(); while (1) printf( enter x (0 to terminate): ); scanf( %d, &x); if (x == 0) break; push(st, x); while (!is_empty(st)) printf( %d, pop(st)); printf( \n ); return 0;

36 Programming style Programming style is a set of rules or guidelines for writing computer programs A good programming style is a subject matter, and thus it is difficult to define However, there are several elements common to many programming styles

37 Programming style (contd.) Indentation helps identify control flows Blank lines can divide a program into logical units Also spaces should be used properly to enhance readability of programs Variable names and function names: names should carry appropriate meanings

38 Programming style (contd.) Write clearly what you are doing in comments Modular design: use functions for independent tasks Write first in an easy-to-understand language, and then translate into a programming language