UNIT 1: POINTERS & DYNAMIC MEMORY ALLOCATION. Programs demonstrated in class. Tojo Mathew Asst. Professor CSE Dept., NIE Mysuru.

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1 UNIT 1: POINTERS & DYNAMIC MEMORY ALLOCATION Programs demonstrated in class. Tojo Mathew Asst. Professor CSE Dept., NIE Mysuru.

2 Table of Contents 1. Simple pointer program demo Chaining of pointers D Arrays & array of pointers Pointers as function parameters: pointer to an integer Pointers as function parameters: pointer to char array Pointers as function parameters : pointer to char array Pointers as function return type Dynamic memory allocation: memory for single integer Dynamic memory allocation: memory for an array of elements Dynamic memory allocation: memory for structure instance Dynamic memory allocation: memory for array of structure instances Dynamic memory allocation: memory for an array using calloc Dynamic memory allocation: 2D arrays & Dynamic memory allocation Dynamic memory allocation: realloc & reducing size of allocated block Pointer to structure and member access Pointer to structure and member access. Structure having pointer type member Address of functions Function pointers Void pointer (void *) and automatic type conversion btn void * & int * Demo program to illustrate use of void *...25

3 1. Simple pointer program demo. int x =10; int *p1 = &x; int **p2; p2 = &p1; printf("\np2 address(&p2)= %X\np2 destination(p2)\ = %X\np2 1stlevel dest value(*p2) =%X\np2 2nd\ level dest value(**p2):%x\n",&p2, p2, *p2, **p2); printf("\np1 address(&p1)= %X\np1 destination(p1)\ = %X\np1 dest value(*p1) =%X\nvalue(x)=\ %X\n",&p1, p1, *p1, x);

4 2. Chaining of pointers int x = 10; int *p1 = &x; printf("x = %d\n",*p1); int **p2 = &p1; printf("x = %u\n",**p2); // Display what???

5 3. 2D Arrays & array of pointers int x[2][3] = 1,2,3,4,5,6; int i,j; // 2-D Array printf("\starting address of 2-D array: %u \n",x); for(i=0;i<2;i++) for(j=0;j<3;j++) printf("%d ",x[i][j]); printf("\n"); //1. pointer to the array. int *p = x; printf("\accessing as a linear array using pointer:linear\n"); for(i=0;i<2*3;i++) printf("%d ",*(p+i)); // 3= no.cols printf("\accessing as a 2D array using pointer\n"); for(i=0;i<2;i++) for(j=0;j<3;j++) printf("%d ",*(p+i*3+j)); //2. Pointer to array of 3 elements(here each row has 3 elems) int (*p1)[3] = x ; printf("\n21. Addres Val p1 = %u (p1+1) = %u\n",p1, (p1+1)); printf("\n22. Addres Val *p1 = %u *(p1+1) = %u\n",*p1, *(p1+1)); for(i=0;i<2;i++) for(j=0;j<3;j++) printf("%d ",*(*(p1+i)+j)); //3. Array of pointers to point to each row separately int *p2[2]; p2[0] = &x[0]; p2[1] = &x[1]; printf("\n31.address Val &p2 = %u p2 = %u *p2 = %u\n",&p2, p2, *p2); printf("\n32.address Val p2+1 = %u *(p2+1) = %u\n",p2+1, *(p2+1)); for(i=0;i<2;i++)

6 for(j=0;j<3;j++) printf("%d ",*(*(p2+i)+j));

7 5. Pointers as function parameters: pointer to an integer. void display_int(int *x); int num =10; display_int(&num); printf("x = %d",num); void display_int(int *p) printf("int val = %d\n",*p); *p =20; printf("int val = %d\n",*p);

8 6. Pointers as function parameters: pointer to char array. void display_string(char *); char s[] = "hello world!\n"; display_string(s); void display_string(char *str) while(*str!= '\0') putc(*str,stdout); str++;

9 7. Pointers as function parameters : pointer to char array. void display_numbers(int *, int); int nums[] = 1,2,3,4,5; display_numbers(nums,5); printf("\nagain display.\n"); display_numbers(nums,5); void display_numbers(int *n, int count) int i; for(i=0;i<count ; i++,n++) printf(" %d ",*n); (*n)++; // Modifiying the array elems

10 8. Pointers as function return type. int* larger(int *x,int *y); int p =30, q =20; int *z = larger(&p, &q); // pass-by-ref printf("larger number is %d\n",*z); *z = 100; //?? printf("number is %d\n",p); // What is printed int* larger(int *x, int *y) if(*x>*y) return x; else return y;

11 9. Dynamic memory allocation: memory for single integer #include <stdlib.h> int *x; x = (int*)malloc(sizeof(int)); *x = 100; printf("starting address = %u Value = %u\n",x, *x); free(x);

12 10. Dynamic memory allocation: memory for an array of elements. #include <stdlib.h> float *x; int numel = 10; // sie of array x = (float*)malloc(numel*sizeof(float)); // initializing the array int i=0; for(;i<numel;i++) *(x+i) = 100+i; // Displaying array printf("the dynamically allocated array contents are:\n"); for(i = 0;i<numel;i++) printf("%.0f",*(x+i - 1)); free(x); // freeing the memory

13 11. Dynamic memory allocation: memory for structure instance #include <stdlib.h> #include <string.h> struct student char name[20]; int usn; ; struct student * s; s = (struct student*)malloc(sizeof(struct student)); printf("size of struct student is %d\n",sizeof(struct student)); // Initializing strcpy(s->name, "Sachin Tendulkar"); s->usn = 200; printf("name is %s USN is %d\n",s->name,s->usn); free(s); // freeing the memory

14 12. Dynamic memory allocation: memory for array of structure instances. #include <stdlib.h> #include <string.h> struct student char name[20]; int usn; ; struct student * s; int n = 10; s = (struct student*)malloc(n*sizeof(struct student)); int i=0; // Initializing struct student * temp = s; for(;i<n;i++) strcpy(temp->name, "Sachin Tendulkar"); temp->usn = 100+i; temp++; // Displaying temp = s; for(i = 0;i<n;i++) printf("name is %s USN is %d\n",temp->name,temp->usn); temp++; free(s); // freeing the memory

15 13. Dynamic memory allocation: memory for an array using calloc #include <stdlib.h> float *x; int numel = 10; // size of array x = (float*)calloc(numel, sizeof(float)); // initializing the array int i=0; for(;i<numel;i++) *(x+i) = 100+i; // Displaying array printf("the dynamically allocated array contents are:\n"); for(i = 0;i<numel;i++) printf("%.2f ",*(x+i)); free(x); // freeing the memory

16 14. Dynamic memory allocation: 2D arrays & Dynamic memory allocation // Dynamic memory allocation: 2D arrays & Dynamic memory allocation #include <stdlib.h> int **x; int rows = 3,cols =4; printf("enter num. of rows & cols:\n"); scanf("%d%d",&rows,&cols); // Creating array of pointers for printing to each row x = (int**)malloc(rows*sizeof(int*)); // Allocating memory for each row int i=0; for(;i<rows;i++) *(x+i) = (int*)malloc(cols*sizeof(int)); int j =0; printf("enter %d elements of the martix:\n",rows*cols); // Initializing each elemet for(i=0;i<rows;i++)

17 for(j=0;j<cols;j++) //*(*(x+i)+j) = j; scanf("%d",*(x+i)+j); // Displaying array // The matrix is : for(i=0;i<rows;i++) for(j=0;j<cols;j++) printf("%d ",*(*(x+i)+j)); printf("\n"); // Freeing the memory: Free the rows for(i = 0;i<rows;i++) free(*(x+i)); // Free the array of pointers to rows free(x); // freeing the memory

18 15. Dynamic memory allocation: realloc & reducing size of allocated block. #include <stdlib.h> float *x; int i=0; int numel = 10; // sie of array x = (float*)malloc(numel*sizeof(float)); // initializing the array printf("address located: %u\n",x); for(;i<numel;i++) *(x+i) = 100+i; // or scanf( %d,x+i); // Displaying array printf("the dynamically allocated array contents are:\n"); for(i = 0;i<numel;i++) printf("%.2f ",*(x+i)); x = (float*)realloc(x,5*sizeof(float)); printf("new Address located: %u\n",x); // address remains same for(i = 0;i<numel;i++) printf("%.2f ",*(x+i)); free(x); // freeing the memory

19 16. Pointer to structure and member access. #include <stdlib.h> #include <string.h> struct student char name[20]; // Array in struct int usn; // Normal variable in struct ; struct student * s; s = (struct student*)malloc(sizeof(struct student)); printf("size of struct student is %d\n",sizeof(struct student)); printf("enter firstname & usn:"); scanf("%s%d",s->name, &s->usn); printf("name is %s USN is %d.\n",s->name,s->usn); free(s); // freeing the memory

20 17. Pointer to structure and member access. Structure having pointer type member. #include <stdlib.h> #include <string.h> struct student char name[20]; // Array in struct int usn; // Normal variable in struct char *section; // Pointer type member in structure ; struct student * s; s = (struct student*)malloc(sizeof(struct student)); printf("size of struct student is %d\n\n",sizeof(struct student)); // Initializing strcpy(s->name, "Sachin Tendulkar"); // or scanf("%s%d",s->name, &s->usn); s->usn = 200; // Allocating memory for pointer type member of structure s->section = (char*)malloc(sizeof(char)); *s->section = 'A'; // Initializing section printf("name:%s \nusn:%d \nsection:%c\n",s->name,s->usn, *s->section); free(s->section); // Freeing memory allocated for char variable free(s); // freeing the memory

21 18. Address of functions.. void fun(int x) printf("function fun() called\n"); printf("address of function main():%u\n", main); printf("address of function fun():%u\n", fun);

22 19. Function pointers void fun(int x) printf("function fun() called %d\n",x); void fun2(int x) printf("function fun2() called %d\n",x); void (*fptr)(int); // Function pointer defined. fptr = fun; //initializing function pointer with address of fun() fun(5); fptr(10); // calling fun() using function pointer. fptr = fun2; // calling fun() using function pointer. fptr(100);

23 20. Void pointer (void *) and automatic type conversion btn void * & int * int x = 99; int *p = &x; void *vp = p; int *ip = vp; printf("%d",*ip);

24 21. Demo program to illustrate use of void *. #include <string.h> struct student char name[20]; ; struct faculty char name[20];int empid; ; struct principal char name[20]; char hdegree; ; void display(void * ptr, int type) if(type == 1) // Student struct student *s = ptr; printf("student name %s\n",s->name); else if(type == 2) // faculty struct faculty *f = ptr; printf("faculty name %s empid:%d\n",f->name,f->empid); else if(type == 3) // principal // no harm with explicite casting. Improves code readability. struct principal *p = (struct principal *)ptr; printf("principal name %s degree: %c\n",p->name, p->hdegree); struct student s1; strcpy(s1.name, "Akhil"); struct faculty f1; strcpy(f1.name, "Narayana"); f1.empid = 401; struct principal p1; strcpy(p1.name, "Dr. Bhaskar"); p1.hdegree = 'P'; display(&s1,1); display(&f1,2); display(&p1,3);

Computer Programming Unit 3

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