Lecture 9 - Pointers 1

Transcription

1 Lecture 9 - Pointers 1

2 Outline Introduction Pointer Variable Definitions and Initialization Pointer Operators Calling Functions by Reference Pointer Expressions and Pointer Arithmetic Relationship between Pointers and Arrays Arrays of Pointers 2

3 Introduction Pointer is the address (i.e. a specific memory location) of an object. It can refer to different objects at different times. Pointers are used in C programs for a variety of purposes: To return more than one value from a function (using pass by reference) To create and process strings To manipulate the contents of arrays and structures To construct data structures whose size can grow or shrink dynamically 3

4 Pointer Variable Definitions and Initialization Pointer variables Contain memory addresses as their values Normal variables contain a specific value (direct reference) num 7 Pointer contains an address of a variable that has a specific value (indirect reference) Indirection referencing a pointer value numptr num 7 4

5 Pointer Variable Definitions and Initialization Pointer definitions * is used with pointer variables int *numptr; Defines a pointer to an int (pointer of type int *) Multiple pointers require using a * before each variable definition int *numptr1, *numptr2; Can define pointers to any data type Initialize pointers to 0, NULL, or an address 0 or NULL points to nothing (NULL preferred) int *numptr = NULL; or int *numptr = 0; 5

6 Pointer Operators Symbol & is called address operator Returns address of operand int num = 7; int *numptr; numptr = # /* numptr gets address of num */ numptr numptr points to num num numptr num Address of num is value of numptr 6

7 Pointer Operators Symbol * is called indirection/dereferencing operator Returns a synonym/alias of what its operand points to *numptr returns num (because numptr points to num) * can also be used for assignment Returns alias to an object *numptr = 10; /* changes num to 10 */ show pictures!! Dereferenced pointer (operand of *) must be an lvalue (no constants) * and & are inverses They cancel each other out 7

8 #include <stdio.h> int main() { int num; int *numptr; int num1=5; Sample program num = 7; printf("number = %d\n", num); numptr = &num; number = 7 numptr points to num whereby the value is = 7 Address of numptr : Contents of numptr : Address of num : Dereferencing pointer, *numptr = 15 num = 20 *numptr = 20 *numptr + num1 = 25 printf("numptr points to num whereby the value is = %d\n",*numptr); printf("address of numptr : %d Contents of numptr : %d\n", &numptr, numptr); printf("address of num : %d\n\n", &num); } *numptr = 15; printf("dereferencing pointer, *numptr = %d\n", *numptr); num = num + num1; printf( num = %d\n, num); printf("*numptr = %d\n", *numptr); printf("*numptr + num1 = %d\n", *numptr + num1); return 0; 8

9 Calling Functions by Reference Call by reference with pointer arguments Passes address of argument using & operator Allows you to change actual location in memory Arrays are not passed with & because the array name is already a pointer * operator Used as alias or nickname for variable inside of function void fun1 (int *number) { *number = 2 * (*number); } *number used as nickname for the variable passed 9

10 #include <stdio.h> #include <string.h> Remember..last time char read(); void find_count_vc(char, int*, int*); void print(int,int); int main() { char ch, choice; int count_v=0,count_c=0; do { ch = read(); find_count_vc(ch, &count_v, &count_c); printf("do you want to continue?"); scanf("%c", &choice); getchar(); }while((choice == 'y') (choice =='Y')); print(count_v,count_c); return 0; } char read() { char ch1; printf("enter character : "); scanf("%c", &ch1); getchar(); return(ch1); } void find_count_vc(char ch1, int *vowel, int *consonant) { switch(ch1) } { case 'A': case 'a': case 'E': case 'e': case 'I': case 'i': case 'O': case 'o': case 'U': } Enter character : f Do you want to continue?y Enter character : I Do you want to continue?y Enter character : k Do you want to continue?n Number of vowel : 1 Number of consonant : 2 Functions that return more than one value i.e. arguments are passed by ref case 'u': *vowel = *vowel +1;break; default: *consonant = *consonant + 1; void print(int vowel, int consonant) { printf("number of vowel : %d\n", vowel); printf("number of consonant : %d\n", consonant); } 10

11 Pointer Expressions and Pointer Arithmetic Arithmetic operations can be performed on pointers Increment/decrement pointer (++ or --) Add an integer to a pointer (+ or +=, - or -=) Pointers may be subtracted from each other Operations meaningless unless performed on an array 11

12 Pointer Expressions and Pointer Arithmetic 5 element int array on machine with 4 byte ints vptr points to first element v[ 0 ] at location 3000 (vptr = 3000) vptr += 2; sets vptr to 3008 vptr points to v[ 2 ] (incremented by 2), but the machine has 4 byte ints, so it points to address 3008 location v[0] v[1] v[2] v[3] v[4] pointer variable vptr 12

13 Pointer Expressions and Pointer Arithmetic Subtracting pointers Returns number of elements from one to the other. If vptr2 = &v[ 2 ]; vptr = &v[ 0 ]; vptr2 - vptr would produce 2 Pointer comparison ( <, ==, > ) See which pointer points to the higher numbered array element Also, see if a pointer points to 0 13

14 Example of Pointer Operations #include <stdio.h> int main() {int *vptr; int *vptr2; int v[5] = {10,20,30,40,50}; int temp; int *p, *q; vptr= v; printf("address of vptr : %d Contents of vptr : %d\n", &vptr, vptr); printf("address of v[0] : %d\n", &v); vptr +=2; printf("address of vptr + 2: %d\n", vptr); vptr +=2; printf("address of vptr + 4: %d\n", vptr); Address of vptr : Contents of vptr : Address of v[0] : Address of vptr + 2: Address of vptr + 4: Contents of temp : 2 Contents of p : q: vptr2=&v[2]; vptr=&v[0]; temp=vptr2-vptr; printf("contents of temp : %d\n", temp); p=q; printf("contents of p : %d q: %d\n", p,q); return 0;} 14

15 The Relationship between Pointers and Arrays Arrays and pointers are closely related Array name like a constant pointer Pointers can do array subscripting operations Define an array b[5] and a pointer bptr To set them equal to one another use: bptr = b; The array name (b) is actually the address of first element of the array b[5] bptr = &b[0]; Explicitly assigns bptr to the address of first element of b 15

16 The Relationship between Pointers and Arrays Element b[3] Can be accessed by *(bptr + 3) where * is the offset. Called pointer/offset notation Can be accessed by bptr[3] Called pointer/subscript notation bptr[3] same as b[3] Can be accessed by performing pointer arithmetic on the array itself *(b + 3) 16

17 Address of bptr : Contents of bptr : Address of b : Contents of b[0]: bptr points to b[0] = 10 Example I am accessing element b[3]!! Let see how many ways I can do it b[3] = 40 *(bptr + 3) = 40 *(b + 3) = 40 bptr[3] = 40 #include <stdio.h> int main() { int *bptr ;int i; int b[10]={10,20,30,40,50}; bptr = b; b[0] = 10 b[1] = 20 b[2] = 30 b[3] = 40 b[4] = 50 b[5] = 0 b[6] = 0 b[7] = 0 b[8] = 0 b[9] = 0 printf("address of bptr : %d Contents of bptr : %d\n", &bptr, bptr); printf("address of b : %d Contents of b[0]:%d %d %d\n", &b, b[0], *bptr, *b); printf("bptr points to b[0] = %d\n", *bptr); printf("\ni am accessing element b[3]!!\nlet see how many ways I can do it\n"); printf("b[3] = %d\n", b[3]); printf("*(bptr + 3) = %d\n", *(bptr + 3)); printf("*(b + 3) = %d\n", *(b + 3)); printf("bptr[3] = %d\n\n", bptr[3]); for(i=0;i<10;i++) printf("b[%d] = %d\n", i, *(bptr+i)); return 0; } 17

18 Arrays of Pointers Arrays can contain pointers For example: an array of strings char *suit[4] = { Hearts, Diamonds, Clubs, Spades }; Strings are pointers to the first character char * each element of suit is a pointer to a char The strings are not actually stored in the array suit, only pointers to the strings are stored 18

19 Arrays of Pointers suit[0] H e a r t s \0 suit[1] suit[2] suit[3] D i a m o n d s \0 C l u b s \0 S p a d e s \0 suit array has a fixed size, but strings can be of any size 19

20 Example #include <stdio.h> #define N 5 int main() { char *studentname[n]; int i; for(i=0;i<5;i++) { printf("enter student[%d] name : ", i); scanf("%s", studentname + i); printf("you just entered :\n%s\n", studentname + i); } return 0; } Enter student[0] name : ali You just entered : ali Enter student[1] name : abu You just entered : abu Enter student[2] name : cheah You just entered : cheah Enter student[3] name : dali You just entered : dali Enter student[4] name : gheeta You just entered : gheeta 20