POINTERS IN C Pointer Basics, Pointer Arithmetic, Pointer to arrays and Pointer in functions

Introduction to Pointer  Variables are stored in the memory  Every variable has unique memory location(Address)  Address can be accessed using ampersand (&) operator

Program to display the address of the variable #include void main () { int var1; printf("Address of var1 variable: %x\n", &var1 ); } Address of var1 variable: bff5a400

Pointers  A pointer is a variable whose value is the address of another variable  You must declare a pointer before you can use it to store any variable address  Declaring Pointer  Syntax type *var-name;  Eg : int *p1;

Program to access the memory location using pointer #include void main () { int var = 20; /* actual variable declaration */ int *ip; /* pointer variable declaration */ ip = &var; /* store address of var in pointer variable*/ printf("Address of var variable: %x\n", &var ); printf("Address stored in ip variable: %x\n", ip ); printf("Value of *ip variable: %d\n", *ip ); } Address of var variable: Address stored in ip variable: Value of *ip variable: 20 Address of var variable: Address stored in ip variable: Value of *ip variable: 20

Program Explanation Memory 20 var int var = 20; 20 var int *ip; ip ip = &var; ip 65558

Other pointer types int *ip; /* pointer to an integer */ double *dp; /* pointer to a double */ float *fp; /* pointer to a float */ char *ch /* pointer to a character */

Null Pointer in C  A pointer that is assigned NULL is called a null pointer.  It is always a good practice to assign a NULL value to a pointer variable in case you do not have exact address to be assigned.  The NULL pointer is a constant with a value of zero

Null Pointer – Example Program #include void main () { int *ptr = NULL; printf("The value of ptr is : %x\n", ptr ); } The value of ptr is 0

Guess the output of the following program # include void main() { int *p, x=20; p=&x; x= *p+15; printf(“%d”, x); printf(“%d”, *p); }

Pointer arithmetic  int *a;  Increment Operator  a++ : address will be incremented by 2 (if it is int)  (*a)++ : content will be incremented by 1  Example  int *a, b=30; /* assume address of b is 2000 */  a=&b;  b++ => 2002  *b++ => 31

Pointer arithmetic with Array – Example1 #include void main () { int var[ ] = {10, 100, 200}; int i, *ptr; ptr = &var[0]; printf(“the value is %d\n”, *ptr); ptr++; printf(“the value is %d\n”, *ptr); printf(“the address is %x\n”, ptr); } Assume starting address of var[] is 2500 the value is 10 the value is 100 the address is 2502 the value is 10 the value is 100 the address is 2502

Pointer arithmetic with array – Example2 #include void main () { int var[ ] = {10, 100, 200}; int i, *ptr; ptr = &var[0]; printf(“the value is %d\n”, *ptr); (*ptr)++; printf(“the value is %d\n”, *ptr); printf(“the address is %x\n”, ptr); } Assume starting address of var[] is 2500 the value is 10 the value is 11 the address is 2500 the value is 10 the value is 11 the address is 2500

Sum of array using pointer – Example 3 #include void main() { int a[5]={1,2,3,4,5}; int i,sum=0; int *b; b=a; for(i=0;i<5;i++) { sum=sum + *b; b++; } printf("\n The Sum is %d",sum); } The Sum is 15

Pointer and Functions  Call by value  Call by Reference  Returning pointers

Call by value # include int add(int, int); void main() { int no1,no2, sum; printf(“Enter two numbers\t”); scanf(“%d%d”,&no1,&no2); sum=add(no1,no2); printf(“the sum is %d \t”, sum); } int add(int x, int y) { int s; s=x+y; return(s); } Call by Value

Call by Reference – Example(swapping of two numbers) # include void swap(int*, int*); void main() { int no1=5, no2=8; printf(“values before swapping %d\t %d\n”, no1,no2); swap(&no1,&no2); printf(“values after swapping %d\t %d”, no1,no2); } void swap(int *x, int *y) { int t; t= *x; *x=*y; *y=t; } Call by Reference values before swapping 58 values after swapping 85 values before swapping 58 values after swapping 85

Dynamic memory allocation  Static Memory Allocation  Allocating memory to variable during compile time of your program  Eg. : int a[20];  Disadvantage : Wastage of Memory Space  Dynamic Memory allocation  Allocating memory to variables during run time of your program  Advantage : No wastage of Memory Space

Allocating memory during run time # include void main() { int n, *a,k=1; printf(“Enter no. of values “); scanf(“%d”, &n); a= (int *)malloc(n*sizeof(int)); /* malloc(n*2) */ for(i=0;i<n;i++) { *a=k; a++; k++; }