Understanding Pointers in C: Definitions, Operations, and Examples

1. Pointer • To hold the location of another variable • Declaration: • a type and a name with an asterisk • E.g. int *ptr; • Assigning a value • ptr = &k;

2. More on Pointer int k, j, *ptr; k = 25; j = k; ptr = &k; • Recall: Variable • Name • Value • Memory location: address • Pointer • Record the address of another variable k 2 1335 1336 1337 j ptr 2 1336

3. Dereferencing • Refer to the value of that which it points to by using the indirection operator * • Called dereferencing operator sometimes • *ptr = 2; • Same as: • k = 2; • Different from • ptr = 2; • Common mistake: forget * when dereferencing

4. Pointer operation • Commonly used operation • +, - • Note: make sure the pointer is pointing to a valid address • Example • ptr++ • The pointer moves to the next integer value in memory (since ptr is an integer pointer defined before) ptr

5. Pointer and Array (I) • Consider the following array: • int a[ ] = {1, 2, 3, 4, 5, 6} • Alternatively access them via a pointer as follows: • int *ptr; • ptr = &a[0]; ptr a[0] a[1]

6. Pointer and Array (II) int a[ ] = {1, 2, 3, 4, 5, 6} int *ptr; ptr = &a[0]; ptr ++; *ptr = 10; ptr a[0] a[1] 10

7. Example int a[6] = {1, 2, 3, 4, 5, 6}; int *ptr; int sum = 0; for (ptr = &a[0]; ptr <= &a[5]; ptr++) sum += *ptr; for (ptr = a; ptr <= a + 5; ptr++) sum += *ptr;

8. Example (II) int a[6] = {1, 2, 3, 4, 5, 6}; int *p, *q; int b[6], i; p = a; q = b; for (i = 0; i < 6; i++) *q++ = *p++;

9. Null pointer • A regular pointer of any pointer type • with a special value NULL (0 in most OS) • Signify that the pointer intentionally does not have a target . • not pointing to any valid reference or memory address. • E.g., ptr = NULL; • Good programming habit: • Set any pointer as NULL initially

10. string.h • string: an array of characters • Functions in string.h • #include <string.h> char * strcpy (char *s1, const char *s2); • char str[5]; strcpy(str, “abcd”); char * strcat(char *s1, const char *s2); • Concatenate s2 to s1 and return s1. char str1[10] = “abcd”; strcat(str1, “efg”); • Make sure s1 gets enough space

11. String.h – cont. int strcmp(const char * s1, const char * s2); • How ? • returning negative value if s1<s2, zero if s1 and s2 are identical, positive value if s1>s2 size_t strlen(const char * s) • Returns length of s

12. Multidimensional Array and Pointers (I) • Pointers can point to multidimensional arrays too • Row major • E.g. char A[3][4] A[0][0] A[0][1] A[0][2] A[0][3] A[1][0] A[1][1] A[1][2] A[1][3] A[2][0] A[2][1] A[2][2] A[2][3]

13. p Multidimensional Array and Pointers (II) char * p; p = &A[0][0] • How do we get to A[0][1]? p += 1; • How do we get to A[1][0]? p += 3; • How do we get to A[2][3]; p += 7; or p = &A[2][3]; A[0][0] A[0][1] A[0][2] A[0][3] A[1][0] A[1][1] A[1][2] A[1][3] A[2][0] A[2][1] A[2][2] A[2][3]

14. Multidimensional Array and Pointers (III) • For one dimensional array • int a[10], *p; • p = a; • For multidimensional array • int b[10][10], *p; • p = &b[0][0]; • If p = b; • “warning: assignment from incompatible pointer type” • *b is a integer pointer, not b; • Remember: when passing a 2D array to a function, specify the number of columns

15. Array of Pointers char aname[][15] = { ``no month'', ``jan'', ``feb” }; char *name[] = { ``no month'', ``jan'', ``feb'', ... };

16. Pointers and Structures • Declaration struct date *datePtr; • Initialization datePtr = &today; • Access members • (*datePtr).year • datePtr->year • struct date{ • int day; • char month[10]; • int year; • }; • struct date today;

17. Structures contains pointers • Pointers can be members in a structure struct date{ int day; char* month; int year; }; • Special case: struct date{ int day; char* month; int year; struct date *next; };

18. Linked List • What is a list? • Node • Link • An alternative to Array • Pros/Cons • + More flexible • + Can grow and shrink as necessary • + Insert and delete node with particular index • - Cannot be randomly accessed • + sorted (ordered list) • - non-uniformed find time

19. Linked List • Operations • Start a list • Create a node • Insert • Search • Query • Print

20. Dynamically allocate memory • Why • Allocate memory at run-time • Recall variable definition • Pre-claim memory during compiling • Has to prepare for the worst case • Maybe waste of memory space • Dynamically memory allocation apply for memory when needed • Most frequently associated with array, string and struct

21. Memory Allocation • malloc is by far most frequently used • Defined in <stdlib.h> void *malloc(size_t size); • Allocate memory space with size bytes • Why does it return a void pointer? • Because it doesn't matter to malloc to what type this memory will be used for • If no memory of the size available, will return null • Be sure to check whether it is null;

22. Example int *ip; ip = malloc(12); if (ip == NULL) { printf("ERROR: Malloc failed"); exit(-1); }

23. Function sizeof • Helpful when dynamically allocating memory • sizeof(data type) returns a size_t of the data type in byte(s) • For a typical 32-bit machine • sizeof(int) returns 4

24. Example: malloc array • Difficulty in estimating the proper size of array ahead of time. int *a; a = malloc(n * sizeof(int)); • Can we just say a = malloc(n* 4)? • If success, you got all the space needed • Then you are able to use a[i] or *(a+i) to access the elements

25. Free Allocated Space • Very important • System won’t automatically take back memory space allocated through malloc • If not free, a memory leak • How? • Use function • void free(void *ptr); • It release the memory space referenced by ptr • Note that free can take in NULL, as specified by ANSI

26. Build a Linked List • Build a structure representing a node struct studentRecord{ int idNum; struct studentRecord *next; }; • Initialize the node struct studentRecord *first = NULL;

27. Create a node • Follow these steps: • Allocate memory for the node • Set data into the node struct studentRecord *new_student; new_student = malloc(sizeof(struct studentRecord )); (* new_student).idNum = 1; new_student next = NULL;

28. Insert a node: insert in the front • Follow these steps: • Create a node • Set the node pointing to the front of the list • Set it as the starting node of this list new_student = malloc(sizeof(struct studentRecord )); new_student  idNum = 2; new_student next = first; first = new_student ;

29. Insert a node: insert in middle • To insert a new node after node called pt, follow these steps: • Create a node • Set the node pointing to the next node after pt in the list • Set it as the next node of this list after pt new_student = malloc(sizeof(struct studentRecord )); new_student  idNum = 2; new_student next = pt  next; pt  next = new_student ;

30. Traversing along the List for (p = first; p != NULL; p = pnext) { …. } int Length(struct studentRecord* first) { int count = 0; struct studentRecord* current = first; while (current != NULL) { count++; current=current->next; } return(count); }

31. Get nth Element in List int GetNth(struct studentRecord * first, int index) { struct studentRecord * current = first; int count = 0; // the index of the node while (current != NULL) { if (count == index) return(current idNum ); count++; current = current  next; } return(-1); // if we get to this line, the caller was asking // for a non-existent element }

32. Delete • Delete (almost reverse of insertion) • locating the node to be deleted (see search a linked list) • altering the previous node to bypass the deleted node • calling free to reclaim the space occupied by the deleted node

33. Example for deleting • To delete a node p from a linked list, you should also know the node which is right ahead of p in the list if (p != NULL) { q  next = p  next; free(p); } • If deleting the first node in the list if( p == first ) { first = p->next; free(p); }