Structures, ADT

1. Structures, ADT Lecture 25 14/3/2002 Sudeshna Sarkar, CSE, IIT Kharagpur

2. Announcements • Lab Test 2 on the week of 18th – 22nd March • Syllabus for lab test : arrays, structures, data types, ….. Sudeshna Sarkar, CSE, IIT Kharagpur

3. The List ADT • A list : <A1, A2, ... , AN> of size N. • Special list of size 0 : an empty list • Operations: • makenull () : returns an empty list • makelist (elem) : makes a list containing a single element • printlist (list) • search(elem, list) : searches whether a key is in the list • insert (elem, list) • delete (elem, list) • findKth (list) Sudeshna Sarkar, CSE, IIT Kharagpur

4. Array Implementation of List typedef int ETYPE; typedef struct { ETYPE elements[MAXS]; int size; } LIST; LIST makenull () ; LIST makeList (ETYPE) ; void printList (LIST) ; int IsEmpty (LIST) ; int search (ETYPE, LIST) ; void delete (ETYPE, LIST * ); void insert (ETYPE, LIST * ) Sudeshna Sarkar, CSE, IIT Kharagpur

5. Complex Number ADT typedef struct { float real; float imag; } COMPLEX; COMPLEX makecomplex (float, float) ; COMPLEX addc (COMPLEX, COMPLEX); COMPLEX subc (COMPLEX, COMPLEX); COMPLEX multc (COMPLEX, COMPLEX); COMPLEX divc (COMPLEX, COMPLEX); Sudeshna Sarkar, CSE, IIT Kharagpur

6. SET ADT • Interface functions (1): SET makenullset () ; int member (ETYPE, SET) ; SET adjoin (ETYPE, SET); SET union (SET, SET) ; SET intersection (SET, SET); Void printset (SET) ; Interface functions (2): SET makenullset () ; int member (ETYPE, SET) ; void adjoin(ETYPE, SET *); void union (SET, SET, SET*); void intersection (SET, SET, SET*); Void printset (SET) ; Sudeshna Sarkar, CSE, IIT Kharagpur

7. Concrete implementation of SET ADT typedef struct { ETYPE elem[MAX]; int size; } SET; Implementation 1 : sorted array adjoin : Sorted insert member : Binary search delete : ? union : merge 2 sorted arrays intersection : ? Sudeshna Sarkar, CSE, IIT Kharagpur

8. Concrete implementation of SET ADT typedef struct { ETYPE elem[MAX]; int size; } SET; Implementation 2 : unsorted array keep the elements in the array unsorted. adjoin : Insert at the end member : Search till found or till the end delete : Go through the array sequentially until element is found, or reach the end. Then left shift the array. union , intersection ? Sudeshna Sarkar, CSE, IIT Kharagpur

9. Arrays of Structures • A struct represents a single record. • Typically structs are used to deal with collections of such records • Examples : student records, employee records, book records, ... • In each case we will hav multiple instances of the struct type. Arrays of structs are the natural way to do this. Sudeshna Sarkar, CSE, IIT Kharagpur

10. Arrays of structs : declaration & use Each declaration below declares an array, where each array element is a structure: point corner_points[10] ; StudentRecord btech01[MAXS] ; We access a field of a struct in an array by specifying the array element and then the field : btech01[i].name corner_points[4].x Sudeshna Sarkar, CSE, IIT Kharagpur

11. Naming in struct Arrays point pentagon[5]; pentagon : an array of points x y pentagon[1] : a point structure x y x y pentagon[4].x : a double x y x y Sudeshna Sarkar, CSE, IIT Kharagpur

12. Using Arrays of structs StudentRecord class[MAXS]; ... for (i=0; i<nstudents; i++) { scanf (“%d%d”, &class[i].midterm, &class[i].final); class[i].grade = (double)(class[i].midterm+class[i].final)/50.0; } Sudeshna Sarkar, CSE, IIT Kharagpur

13. struct Array elements as parameters void draw_line (point p1, point p2) { ... } ... point pentagon[5]; ... for (i=0;i<4;i++) draw_line (pentagon[i], pentagon[i+1]); draw_line (pentagon[4], pentagon[0]); Sudeshna Sarkar, CSE, IIT Kharagpur

14. structs as Parameters • A single struct is passed by value. • all of its components are copied from the argument (actual parameter) to initialize the (formal) parameter. point set_midpt (point a, point b) { ... } int main (void) { point p1, p2, m; ... m = set_midpt(p1, p2); } Sudeshna Sarkar, CSE, IIT Kharagpur

15. Passing Arrays of structs • An array of structs is an array. • When any array is an argument (actual parameter), it is passed by reference, not copied [As for any array] • The parameter is an alias of the actual array argument. int avg (StudentRec class[MAX]) { ... } int main (void) { StudentRec bt01[MAX]; int average; ... average = avg_midpt(bt01) ; } Sudeshna Sarkar, CSE, IIT Kharagpur

16. Dynamic Memory Allocation,Structure pointers Lecture 26 14.3.2002. Sudeshna Sarkar, CSE, IIT Kharagpur

17. Basic Idea • Many a time we face situations where data is dynamic in nature. • Amount of data cannot be predicted beforehand. • Number of data item keeps changing during program execution. • Such situations can be handled more easily and effectively using dynamic memory management techniques. Sudeshna Sarkar, CSE, IIT Kharagpur

18. C language requires the number of elements in an array to be specified at compile time. • Often leads to wastage or memory space or program failure. • Dynamic Memory Allocation • Memory space required can be specified at the time of execution. • C supports allocating and freeing memory dynamically using library routines. Sudeshna Sarkar, CSE, IIT Kharagpur

19. Memory Allocation Process in C Local variables Stack Free memory Heap Global variables Permanent storage area Instructions Sudeshna Sarkar, CSE, IIT Kharagpur

20. The program instructions and the global variables are stored in a region known as permanent storage area. • The local variables are stored in another area called stack. • The memory space between these two areas is available for dynamic allocation during execution of the program. • This free region is called the heap. • The size of the heap keeps changing Sudeshna Sarkar, CSE, IIT Kharagpur

21. Memory Allocation Functions • malloc: Allocates requested number of bytes and returns a pointer to the first byte of the allocated space. • calloc: Allocates space for an array of elements, initializes them to zero and then returns a pointer to the memory. • free : Frees previously allocated space. • realloc: Modifies the size of previously allocated space. Sudeshna Sarkar, CSE, IIT Kharagpur

22. Dynamic Memory Allocation • used to dynamically create space for arrays, structures, etc. int main () { int *a ; int n; .... a = (int *) calloc (n, sizeof(int)); .... } a = malloc (n*sizeof(int)); Sudeshna Sarkar, CSE, IIT Kharagpur

23. Space that has been dynamically allocated with either calloc() or malloc() does not get returned to the function upon function exit. • The programmer must use free() explicitly to return the space. • ptr = malloc (...) ; • free (ptr) ; Sudeshna Sarkar, CSE, IIT Kharagpur

24. void read_array (int *a, int n) ; int sum_array (int *a, int n) ; void wrt_array (int *a, int n) ; int main () { int *a, n; printf (“Input n: “) ; scanf (“%d”, &n) ; a = calloc (n, sizeof (int)) ; read_array (a, n) ; wrt_array (a, n) ; printf (“Sum = %d\n”, sum_array(a, n); } Sudeshna Sarkar, CSE, IIT Kharagpur

25. void read_array (int *a, int n) { int i; for (i=0; i<n; i++) scanf (“%d”, &a[i]) ; } void sum_array (int *a, int n) { int i, sum=0; for (i=0; i<n; i++) sum += a[i] ; return sum; } void wrt_array (int *a, int n) { int i; ........ } Sudeshna Sarkar, CSE, IIT Kharagpur

26. Arrays of Pointers • Array elements can be of any type • array of structures • array of pointers Sudeshna Sarkar, CSE, IIT Kharagpur

27. int main (void) { char word[MAXWORD]; char * w[N]; /* an array of pointers */ int i, n; /* n: no of words to sort */ for (i=0; scanf(“%s”, word) == 1); ++i) { w[i] = calloc (strlen(word)+1, sizeof(char)); if (w[i] == NULL) exit(0); strcpy (w[i], word) ; } n = i; sortwords (w, n) ; wrt_words (w, n); return 0; } Sudeshna Sarkar, CSE, IIT Kharagpur

28. Input : A is for apple or alphabet pie which all get a slice of come taste it and try w 0 A \0 1 i s \0 2 f o r \0 3 a p p l e \0 17 t r y \0 Sudeshna Sarkar, CSE, IIT Kharagpur

29. void sort_words (char *w[], int n) { int i, j; for (i=0; i<n; ++i) for (j=i+1; j<n; ++j) if (strcmp(w[i], w[j]) > 0) swap (&w[i], &w[j]) ; } void swap (char **p, char **q) { char *tmp ; tmp = *p; *p = *q; *q = tmp; } Sudeshna Sarkar, CSE, IIT Kharagpur

30. Before swapping w w[i] f o r \0 a p p l e \0 w[j] Sudeshna Sarkar, CSE, IIT Kharagpur

31. After swapping w w[i] f o r \0 a p p l e \0 w[j] Sudeshna Sarkar, CSE, IIT Kharagpur

32. Pointers to Structure Sudeshna Sarkar, CSE, IIT Kharagpur

33. Pointers and Structures • You may recall that the name of an array stands for the address of its zero-th element. • Also true for the names of arrays of structure variables. • Consider the declaration: struct stud { int roll; char dept_code[25]; float cgpa; }class[100], *ptr ; Sudeshna Sarkar, CSE, IIT Kharagpur

34. The name class represents the address of the zero-th element of the structure array. • ptr is a pointer to data objects of the type struct stud. • The assignment ptr = class ; • will assign the address of class[0] to ptr. • When the pointer ptr is incremented by one (ptr++) : • The value of ptr is actually increased by sizeof(stud). • It is made to point to the next record. Sudeshna Sarkar, CSE, IIT Kharagpur

35. Once ptr points to a structure variable, the members can be accessed as: ptr –> roll ; ptr –> dept_code ; ptr –> cgpa ; • The symbol “–>” is called the arrow operator. Sudeshna Sarkar, CSE, IIT Kharagpur

36. Warning • When using structure pointers, we should take care of operator precedence. • Member operator “.” has higher precedence than “*”. • ptr –> roll and (*ptr).roll mean the same thing. • *ptr.roll will lead to error. • The operator “–>” enjoys the highest priority among operators. • ++ptr –> roll will increment roll, not ptr. • (++ptr) –> roll will do the intended thing. Sudeshna Sarkar, CSE, IIT Kharagpur

37. Program to add two complex numbers using pointers typedef struct { float re; float im; } complex; main() { complex a, b, c; scanf (“%f %f”, &a.re, &a.im); scanf (“%f %f”, &b.re, &b.im); add (&a, &b, &c) ; printf (“\n %f %f”, c,re, c.im); } Sudeshna Sarkar, CSE, IIT Kharagpur

38. void add (complex * x, complex * y, complex * t){ • t->re = x->re + y->re ; • t->im = x->im + y->im ; • } Sudeshna Sarkar, CSE, IIT Kharagpur