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Lecture 9 - Pointers

Lecture 9 - Pointers. 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 . Introduction.

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Lecture 9 - Pointers

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  1. Lecture 9 - Pointers

  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

  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

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

  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; orint *numPtr = 0;

  6. numPtr num num 7 numPtr Address of num is value of numPtr 500000 600000 600000 7 Pointer Operators • Symbol & is called address operator • Returns address of operand int num = 7; int *numPtr; numPtr = # /* numPtr gets address of num */ numPtr “points to” num

  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 tonum) • * 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

  8. Sample program #include <stdio.h> int main() { int num; int *numPtr; int num1=5; num = 7; printf("number = %d\n", num); numPtr = &num; 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; } number = 7 numPtr points to num whereby the value is = 7 Address of numPtr : 1245060 Contents of numPtr : 1245064 Address of num : 1245064 Dereferencing pointer, *numPtr = 15 num = 20 *numPtr = 20 *numPtr + num1 = 25

  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); } • *numberused as nickname for the variable passed

  10. #include <stdio.h> #include <string.h> 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': 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); } 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 Remember..last time Functions that “return” more than one value i.e. arguments are passed by ref

  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

  12. v[0] v[1] v[2] v[4] v[3] 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 3000 3004 3008 3012 3016 pointer variablevPtr

  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

  14. #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); 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;} Address of vPtr : 1245064 Contents of vPtr : 1245020 Address of v[0] : 1245020 Address of vPtr + 2: 1245028 Address of vPtr + 4: 1245036 Contents of temp : 2 Contents of p : 2147323904 q: 2147323904 Example of Pointer Operations

  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

  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)

  17. Address of bPtr : 1245064 Contents of bPtr : 1245016 Address of b : 1245016 Contents of b[0]:10 10 10 bPtr points to b[0] = 10 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 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 Example #include <stdio.h> int main() { int *bPtr ;int i; int b[10]={10,20,30,40,50}; bPtr = b; 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; }

  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

  19. ’\0’ ’\0’ ’\0’ ’\0’ ’n’ ’d’ ’o’ ’u’ ’a’ ’s’ ’d’ ’b’ ’m’ ’H’ ’s’ ’a’ ’D’ ’i’ ’a’ ’s’ ’l’ ’C’ ’r’ ’s’ ’S’ ’p’ ’e’ ’t’ ’e’ suit[0] suit[1] suit[2] suit[3] Arrays of Pointers • suit array has a fixed size, but strings can be of any size

  20. #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 Example

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