POINTER

1. POINTER

2. Pointer ? • One of the features of the C language that gives C language its sophistication, power, and complexity • Every byte in computer memory has an address and the address of the first allocated byte of the variable is called a pointer to the variable • a variable that holds an address is called a pointer variable.

3. 65 is the ASCII code for letter A ch address is 1060 1060 65 1 byte allocated to ch 1061 num address is 1062 1062 10 1063 2 byte allocated to num 1064 Conceptual View of a variable in Memory int num =10; char ch = ‘A’; Memory

4. Address operator, & • To obtain the address of a specifies variable • Examples int num = 10; char ch = ‘A’; &num - address of the variable num called pointer to num &ch - address of the variable ch called pointer to ch NOT VALID - & 60, & ‘A’, & (K+2), &(Index/6)

5. More about variables • Every variable is associated with 5 attributes • variable type • variable name • variable contents • variable address • variable size

6. type(int) name(num) size is 2 bytes sizeof(int) contents(value) Address is 1062 (&num) Attributes of a Variable int num = 10; 1060 1061 1062 1063 1064

7. Examples #include <stdio.h> int main( ) { int num; num = 10; printf(“The address of num is %p\n”, &num); return 0; }

8. Input Function scanf( ) • Used to read input data from the keyboard and store it in the specified storage. • One of the function in stdio.h library • Syntax scanf(“control string”, arg1, arg2,….) • Examples int num; /* declare variables */ scanf(“%d”, &num); /* read from keyboard */

9. declaration statement conversion specification address of (pointer to) num assumed num memory location scanf ( ) function int num; scanf(“%d”, &num); num 1062

10. Examples /* Definition: This program asks for a name and 2 integer numbers. It displays the name and sum, product of the numbers. */ #include <stdio.h> /* include the stdio.h header file */ int main ( ) { int num1, num2 ; /* declare the variables */ printf ( " \nEnter two numbers and press the Enter key: " ) ; /* read 2 integer numbers from the keyboard */ scanf ( "%d %d", &num1, &num2 ) ; printf ( " \n %d + %d = %d \n ", num1, num2, num1+ num2 ) ; printf ( " \n %d - %d = %d \n ", num1, num2, num1 - num2 ) ; printf ( " \n %d * %d = %d \n ", num1, num2, num1 * num2 ) ; return 0 ; }

11. Program Output Enter two numbers and press the Enter key : 10 20 10 + 20 = 30 10 - 20 = -10 10 * 20 = 200

12. Conversion specification conversion description Example %c interpret input as a character scanf(“%c”, &ch); %d or %I interpret input as a signed integer scanf(“%d”, &inum); %f interpret input as floating-point scanf(“%f”,&fnum); number %e interpret input as floating-point scanf(“%e”,&fnum); number, e notation %s interpret input as a character string scanf(“%s”,name);

13. Character Strings • C doesn’t provides data type for strings • declare an array of char type to store character strings • Examples char Last_Name [40]; scanf(“%s”, Last_Name);

14. x x \0 String x character ‘x’ the character x “x” the string x null character, terminating the string

15. Program Example /* Definition: This program asks for a name and displays the name with a smile. */ #include <stdio.h> int main ( ) { char name [ 40 ] ; /* declare the variables */ printf ( " \nEnter your name and press the Enter key: " ); scanf ( "%10s", name ) ; /* read name from the keyboard */ printf ( " \n :-) Hi %s! \n ", name ) ; /* display the last name */ return 0; } Program Output Enter your name and press the Enter key : Unimas :-) Hi Unimas !

16. Formatting Input Data char name [40]; scanf(“%10s”,name); in this case scanf() stops after reading 10 characters or encountering a whitespace character, whichever comes first

17. Conversion Specification Char ch, name[40]; int inum; float fnnum; Statement Input data Read by scanf( ) scanf(“%s”, name); Unimas Sarawak Unimas scanf(“%9s”, name); Unimas Sarawak Unimas scanf(“%4s”, name); Unimas Sarawak Unim scanf(“%3d”,&inum); 123456 123 scanf(‘%3.3f”,&fnum); 123.3 123.300

18. Pointer Declarations Syntax data type *identifier • data type is any valid C data type such as int, or float • *(asterisk) the pointer indicator • identifier is any valid variable name Examples int *pint; /* declare a pointer variable of int type */

19. More example short *sp; /* the pointer variable sp points to short integer type */ int *ip; /* the pointer variable ip points to an integer type */ char *cp; /* the pointer variable cp points to a character type */ float *fp; /* the pointer variable fp points to a floating point type */ double *dp; /* the pointer variable dp points to a double precision type */

20. Pointer Initializations VALID int num; int *pnum=&num; NOT VALID int *pnum=&num; int num;

21. pnum num num address 2 bytes are accessed Storing Addresses int *pnum, num; /*declaring a pointer and an int type variables */ pnum = &num; /*assigning the address of the variable num to the pointer variable pint */ declaration statements storing the address of num in pnum int *pnum; int num; pnum=&num;

22. Indirection Operator * • Used for manipulating data by referring to variables by their addresses rather than their names. • Called indirection or dereferencing operator, is placed before a pointer variable name to obtain the value of the object that the pointer is pointing to • indirection operator * is a unary operator and can only be applied to pointer variables • used both to declare a pointer variable and to access the variable whose address is stored in a pointer

23. Examples int *pnum; /* declare a pointer variable */ int num=100, value; /*declare int type variables */ pnum = &num; /*assign the address of the variable num to the pointer variable pnum */ value = *pnum; /*use the indirection operator to obtain the contents of the variable that is pointed to by pnum */

24. Address 2000 2002 100 2004 2006 2002 2008 2120 Conceptual View Declaration and initialization statements int num = 100; int *pnum; storing the address of num in pnum pnum=&num; ppnum=&pnum num &num pnum *pnum &pnum ________________________________________ 100 2002 2002 100 2008

25. Direct and Indirect Methods of Accessing a Variable /* direct method */ /* pointer method */ int num 5; int num = 5; int value; int value; int *pnum = &num; value = num; value = *pnum;

26. Common Errors • Attempting to obtain the address of a constant value and constant value doesn’t have an address ptr = &45; /* invalid assignment */ ptr = &(Index + 10); /* invalid assignment */ • initializing pointer incorrectly int *ptr = 5; /* invalid initialization */

27. Multiple Indirection • Pointer point to another pointer - multiple indirection • have to declare a pointer to pointer variable before we can use it int **newpointer Example int num, *pnum, **ppnum; num=100; /* store 10 in num */ pnum=&num; /* store num address in pnum */ ppnum=&pnum; /* store pnum address in ppnum */

28. Address 2000 100 num 2002 2004 pnum 2006 2000 2008 2006 2010 ppnum Conceptual View Declaration and initialization statements int num = 100; int *pnum, **pnum; storing the address of num in pnum, and address of pnum ppnum pnum = &num; ppnum = &pnum; num &num pnum *pnum **ppnum *ppnum &pnum &ppnum 100 2000 2000 100 100 2006 2006 2010