CS1001 Structured Programming Approach Pointers in C programming

CS-1001 ( Structured Programming Approach) Pointers in C programming

Introduction to pointer Pointer declaration and initialization. Pointer arithmetic. Topic to be cover Void pointer Pointer with Array Pointer with String Pointer with function

1. Introduction to pointer • A pointer is a special variable stores the address of another variable of same type. • Value of the pointer variable is the address of another variable, i. e. , direct address of the memory location. • Like any variable or constant, you must declare a pointer before using it to store any variable address. • Some C programming tasks are performed more easily with pointers. • Dynamic memory allocation, cannot be performed without using pointers. • So it becomes necessary to learn pointers to become a perfect C programmer.

Introduction to pointer (conti…. ) • Consider the declaration, int i = 3 ; This declaration tells the C compiler to: ü Reserve space in memory to hold the integer value. ü Associate the name i with this memory location. ü Store the value 3 at this location.

Introduction to pointer (conti…. ) We can print this address number through the following program: main( ) { int i = 3 ; printf ( "n. Address of i = %u", &i ) ; printf ( "n. Value of i = %d", i ) ; } Output Address of i = 65524 Value of i = 3

Introduction to pointer (conti…. ) • ‘address of’ Operator (&) : ü In first printf( ) statement carefully. ‘&’ used in this statement is C’s ‘address of’ operator. ü The expression &i returns the address of the variable i, which in this case happens to be 65524. ü Since 65524 represents an address, there is no question of a sign being associated with it. • Hence it is printed out using %u, which is a format specifier for printing an unsigned integer. • value at address (*): ü The other pointer operator available in C is ‘*’, called ‘value at address’ operator. ü It gives the value stored at a particular address. ü The ‘value at address’ operator is also called ‘indirection’ operator.

Introduction to pointer (conti…. ) main( ) { int i = 3 ; int *p=&i; printf ( "n. Address of i = %u", &i ) ; printf ( "n. Value of i = %d", *( &i ) ) ; printf ( "n. Value of i = %d", *p ) ; } Output Address of i = 65524 Value of i = 3

2. Pointer Declaration and initialization. • Like any variable or constant, you must declare a pointer before using it to store any variable address. • The general form of a pointer variable declaration is − type *var-name; • Here, type is the pointer's base type; it must be a valid C data type and var-name is the name of the pointer variable. • The asterisk * used to declare a pointer is the same asterisk used for multiplication. • The asterisk is being used to designate a variable as a pointer.

Pointer Declaration and initialization. (Conti. ) • Take a look at some of the valid pointer declarations üint *ip; üdouble *dp; üfloat *fp; üchar *ch /* pointer to an integer */ /* pointer to a double */ /* pointer to a float */ /* pointer to a character */ • The actual data type of the value of all pointers is the same, which is a long hexadecimal number that represents a memory address. • The only difference between pointers of different data types is the data type of the variable or constant that the pointer points to.

Pointer Declaration and initialization (Conti…) • Once the pointer declared you can assign the address of variable of same type in the pointer using & operator using following syntax. Pointer_Name = & name of the variable main( ) { int i = 3 ; int *j ; j = &i ; // assignment printf ( "n. Address of printf ( "n. Value of j printf ( "n. Value of i } i i j = = = %u", &i ) ; = %u", j ) ; = %u", &j ) ; %u", j ) ; %d", i ) ; %d", *( &i ) ) ; %d", *j ) ; Output: Address of i = 65524 Address of j = 65522 Value of j = 65524 Value of i = 3

3. Pointer arithmetic. • A pointer in C is an address, which is a numeric value. • Therefore, you can perform arithmetic operations on a pointer just as you can on a numeric value. • There are four arithmetic operators that can be used on pointers: ++, -, +, and ü The content of a pointer can be copied to another pointer variable provided both pointers are of same type. (ptr=ptr 1) üA pointer variable can be assigned a null(zero) value. (e. g ptr=NULL, where NULL is a symbolic constant that represents the value 0). üAn integer quantity can be added to or subtracted from a pointer variable. (e. g ptr+1, ++ptr, ptr-3, ptr- - etc).

Pointer arithmetic. (Conti…) • Let us consider that ptr is an integer pointer which points to the address 1000. Assuming 32 -bit integers ( 4 Bytes), let us perform the following arithmetic operation on the pointer − int *ptr; ptr++ • The ptr will point to the location 1004 because each time ptr is incremented, it will point to the next integer location which is 4 bytes next to the current location. • This operation will move the pointer to the next memory location without impacting the actual value at the memory location. • If ptr points to a character whose address is 1000, then the above operation will point to the location 1001 because the next character will be available at 1001

4. Void pointer • A void pointer is called as generic pointer which can point to any type of memory block. But to access the complete memory one has to specify casting explicitly. #include<stdio. h> main() { int a =1; char c=’a’; void *ptr; ptr = &a; printf(“%d”, *(int *) ptr); ptr = &c; printf(“%c”, *(char *) ptr); } • Here the void pointer ptr is used as integer and character pointers suing explicit casting method.

5. Pointer with Array • In C, there is a strong relationship between pointers and arrays because Any operation that can be achieved by array subscripting can also be done with pointers. • The pointer version will in general be faster but, at least to the uninitiated, somewhat harder to understand. • Consider the following declaration int a[10]; • Above statement defines an array of size 10, that is, a block of 10 consecutive objects named a[0], a[1], . . . , a[9] as shown in the figure on the screen. • The notation a[i] refers to the i-th element of the array.

Pointer with Array (Conti…) Now if we create a pointer to integer int *pa; // If pa is a pointer to an integer pa=&a[0] // assigning the address of first element of the array to pointer to integer. Then • pa+1 points to the next location and *(pa+1) will refer to the contents of a[1], • pa+i is the address of a[i], and *(pa+i) is the contents of a[i].

Pointer with Array (Conti…) Program #include <stdio. h> int main() { int x[5] = {1, 2, 3, 4, 5}; int* ptr; // ptr is assigned the address of the third element ptr = &x[2]; printf("*ptr = %d n", *ptr); // 3 printf("*(ptr+1) = %d n", *(ptr+1)); // 4 printf("*(ptr-1) = %d", *(ptr-1)); // 2 return 0; }

6. Pointer with String #include <stdio. h> int main(void) { char str[6] = "Hello"; char *ptr = str; // print the string while(*ptr != '') { printf("%c", *ptr); ptr++; } return 0; } // string variable // pointer variable // move the ptr pointer to the next memory location //it will increment the pointer location by 4 bytes

7. Pointer with function • The pointer can be used to pass the argument in the function. So whenever the pointer to the variable passed as argument, changed made with that variable will be reflected in the calling environment too. • To understand the use of pointer to pass argument in the function.

Pointer with function (Conti…) #include <stdio. h> void swap(int*, int*); // function declaration int main() { int x, y; printf("Enter the value of x and yn"); scanf("%d%d", &x, &y); //let user enter 10 and 20 printf("Before Swappingnx = %dny = %dn", x, y); swap(&x, &y); printf("After Swappingnx = %dny = %dn", x, y); return 0; } void swap(int *a, int *b) { int temp; temp = *b; *b = *a; *a = temp; }

Quires