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Pointers

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Pointers Main memory is a sequence of memory locations

that are addressed 0, 1, 2, Pointers are variables that contain memory addresses

as their values. A pointer containing an address of some location is

said to be a pointer to that location. A pointer variable can have NULL (or 0) value

meaning that the pointer does not point to any

memory location. Zero is the only integer value that can be assigned to

a pointer. Size of pointer is often 4 bytes

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Concept of Address and Pointers Memory can be

conceptualized as alinear set of data

locations.

Variables reference thecontents of a locations

Pointers have a value of

the address of a givenlocation

Contents1

Contents11

Contents16

ADDR1

ADDR2

ADDR3

ADDR4

ADDR5ADDR6

***

ADDR11

**

ADDR16

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Pointers Pointers are variables that contain memory

addresses as their values.

A variable name directly references a value.

A pointer indirectly references a value.Referencing a value through a pointer is

called indirection. A pointer variable must be declared before it

can be used.

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Accessing the address of a variable

We can obtain the value of address of thevariable stored using address operator (&).

The value thus obtained is known as pointervalue. The variable that contains pointer value is

called as pointer variable Form:

p = &q;

& (address of operator) can be used with avariable and array element

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Accessing the address of a variable

Example:

#include int j, k;int *ptr;void main(void)

{j = 1;k = 2;ptr = &k;clrscr();printf("\n");printf("j has the value %d and is stored at %u\n", j, &j);

printf("k has the value %d and is stored at %u\n", k, &k);printf("ptr has the value %p and is stored at %u\n", ptr, &ptr);getch();

}/*Output:j has the value 1 and is stored at 978k has the value 2 and is stored at 976

ptr has the value 03D0 and is stored at 974*/

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Declaring Pointer variable Pointer variables contain addresses that belong to a

separate data type, they must be declared aspointers before we use them.

Form:

data_type *pt_name

This tells the compiler about pt_name:1. The asterisk (*) tells that variable pt_name is a pointer

variable2. pt_name needs a memory location

3. pt_name points to a variable of type data_type.

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Pointer declarations Examples: int *a; // integer pointer

float*b; // float pointerchar *c; //character pointer

The asterisk, when used as above in the declaration, tells thecompiler that the variable is to be a pointer, and the type ofdata that the pointer points to, butNOT the type of the value ofthe pointer.

This declarations cause the memory to allocate memorylocations for pointer variable (a, b, c). Since memory locationshave not been assigned by any values, these locations maycontain some unknown values in them and therefore they pointto the unknown location.

?a ?

Containsgarbage

Points to

unknownlocation

int *a;

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Initialization of pointer variable The process of assigning the address of variable to a pointer variable is

known as initialization. Once the pointer variable is declared we can use the assignment

operator to initialize the variable. Example:

int q;Int *p; //Declaration

P=&q; //Initialization

OR

int x,*p=&q; //three in one. Valid

int *p=&q,x; Invalid

We can define pointer variable with initial value of NULL or 0 (zero) No other constant value can be assigned to pointer variable.Eg: int *p=NULL; Valid

int *p=0; Validint *p=5360; Invalid

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Accessing a variable through its pointer

This is done by asterisk operator

It is also called as indirection operator

dereferencing operator * : Value at address

Example: p=&q;

n= *p;

n=*&q;

n=q;Note: You can not access the value stored at the address 5368 by just *5368

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Example// program to demonstrate the use of pointers# includevoid main(){

int number=0;int *ptr;clrscr();printf("\n Enter number:");scanf("%d", &number);ptr=&number;printf("\n %d is stored at %u",number,ptr);

getch();}/*OutputEnter number:2323 is stored at 65524*/

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Chain of pointers It is possible to make pointer to point another pointer.

Example:main(){Int x, *p1, **p2;

X=100;P1=&x;P2=&p1;Printf(%d,**p2);}

address2 address1 address1

p2 p1 Variable

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Pointer Expression Like variables, pointer variables can be used in expression.// Pointer arithmeticvoid main(){

int a=0, b=0, *p1, *p2;clrscr();printf("\n Enter first number:");scanf("%d", &a);printf("\n Enter second number:");scanf("%d", &b);p1=&a; p2=&b;

printf("\n %d is at %u and %d is at %u", a,p1,b,p2);printf("\n p1-p2= %d " , p1-p2);printf("\n *p1 + *p2=%d ", *p1 + *p2);printf("\n *p1*5= %d", *p1*5);printf("\n*p1/*p2= %d", *p1/ *p2);printf("\n *p1/5= %d", *p1/5);printf("\n p1++ =%u",++p1);

getch();

/*Output:Enter first number:25Enter second number:2025 is at 65524 and 20 is at 6552p1-p2= 1

*p1 + *p2=45*p1*5= 125*p1/*p2= 1*p1/5= 5p1++ =65526 */

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Arithmetic and Logical Operations on Pointers

A pointer may be incremented or decremented

An integer may be added to or subtracted from apointer.

Pointer variables may be subtracted from oneanother.

Pointer variables can be used in comparisons, butusually only in a comparison to NULL.

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Arithmetic Operations on Pointers

When an integer is added to or subtracted from apointer, the new pointer value is changed by theinteger times the number of bytes in the datavariable the pointer is pointing to.

For example, if the pointer valptr contains theaddress of a double precision variable and that

address is 870, then the statement:valptr = valptr + 2;

would change valptr to 878

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sizeof Thiskeyword can be used to determine the number

of bytesin a data type, a variable, or an array

Example:double array [10];

sizeof (double); /* Returns the value 8 */

sizeof (array); /* Returns the value 80 */

sizeof(array)/sizeof(double); /* Returns 10 */

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Pointers and arrays When array is declared, the compiler allocates a

base address and sufficient amount of memorystorage to contain all the members of array in

contiguous memory locations. Base address is a location of first element in an

array (index 0) The compiler also defines the array name as

constant pointer to first element.

Eg: int x[5]={1,2,3,4,5}; Assume base address of x is 1000. Each integer

requires 2 bytes. The five elements can be storedas:

X[0] X[1] X[2] X[3] X[4]

1 2 3 4 5

1000 1002

1004 1006 1007

Elements

value

addressBase address

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Pointers and arrays If p is declared as constant pointer pointing to first element

x[0] and therefore value of p is 1000p=&x[0]=1000

Pointer p is declared as pointer to an array: int p = &x[0]; Now we can access each value of x using p++ shown as:

P=&x[0] (=1000)P+1=&x[1] (=1002)P+2=&x[2] (=1004)P+3=&x[3] (=1006)P+4=&x[4] (=1008)

Adress of an element can be calculated:address of x[3] = base address + (3*scale factor of int)

1000+(3*2)=1006

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Example#include

void main(){

char *c , ch[10];int *i , j[10];float *f , g[10];int x;clrscr();c = ch;i = j;

f = g;

for ( x=0 ; x

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Example program to accept array elements and print them

with there address locations using pointers.

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// program to accept array elements and

print them using pointers.void main(){int arr[5],i=0;int *ptr;clrscr();for(i=0;i

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Pointers and character strings

void main() {char *ptr;char arrayChars[8] = {'F','r','i','e','n','d','s','\0'};clrscr();ptr = arrayChars;printf("The array reads %s.\n", arrayChars);*ptr = 'f'; /* ptr points to the first element */printf("now it reads %s.\n", arrayChars);printf("The 3rd character of the array is %c.\n",*(ptr+=2));

getch();}/*Output:The array reads Friends.now it reads friends.The 3rd character of the array is i.*

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Program to copy one string into anotherstring; pointer version

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/*strcpy: copy t to s; pointer version */

#includevoid main(){char arr[] = {'A','B','C','D'};char arr1[10];

char *s, *t;clrscr();s = arr;t = arr1;while ((*t = *s) != '\0'){

s++;t++;

}printf("%s",arr1);getch();}//ABCD

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Array of pointers One important use while handling the

table of stringsEg: char name [3] [25];

Name contains three names with maximum length of25 caharacters.

Storage required is 75 bytes

We know that individual string will be of equal lengths.

Therefore, making each row a fixed number of characters, we can meke ita pointer to a string of varying length.

Eg: char * name [3] ;

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Array of pointers# includevoid main(){char *str[5]={"Keyboard",

"Monitor","CPU" ,"Hard disk","SMPS"};

int i=0;clrscr();

for(i=0;i

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Pointers as function arguments// Addition of two numbers using call by reference.

int add(int *, int*);# includemain()

{int a=0,b=0;clrscr();printf("\n Enter first number:");scanf("%d", &a);printf("\n Enter second number:");scanf("%d",&b);printf("\nAddition = %d",add(&a,&b));getch();}int add(int *x, int *y){return *x+*y;}

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Pointer to function#include

int add(int x, int y); /* declare function */

void main() {

int x=6, y=9;int (*ptr)(int, int); /* declare pointer to function*/clrscr();ptr = add; /* set pointer to point to "add" function */

printf("%d plus %d equals %d.\n", x, y, (*ptr)(x,y));/* call function using pointer */getch();

}

int add(int x, int y) { /* function definition */return x+y;}//6 plus 9 equals 15

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Passing array to function using pointer#includevoid zero_cnt(int *);void main(){

int arr[5], *p;int i=0;clrscr();for(i=0;i

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Pointer and structurevoid main(){struct movie{char movie_nm[20];

char actor[15];int rel_year;};struct movie *ptr;clrscr();printf("\n Enter movie_nm:");scanf("%s", ptr->movie_nm);

printf("\n Enter movie actor:");scanf("%s", ptr->actor);printf("\n Enter release year:");scanf("%d",&ptr->rel_year);printf("\n Movie name - %s", ptr->movie_nm);printf("\n Actor-%s",ptr->actor);printf("\n Release year-%d",ptr->rel_year);

getch();

/*Output:Enter movie_nm: chamatkarEnter movie actor:sridevi

Enter release year:1986

Movie name - chamatkarActor-srideviRelease year-1986 */

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Reference variable in C++ References allow you to create a second

name for the a variable that you can use toread or modify the original data stored in thatvariable.

A reference variable must be initialized at thetime of declaration.

A reference variable provides an alias for

previously defined variable Form:data_type & reference name = variable_name

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//Demo of Reference variable#include#includevoid main(){

clrscr();float total=100;float &sum=total;

cout

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Difference between Pointer

and Reference1. A pointer can be re-assigned any

number of times while a reference can

not be reassigned after initialization.

2. A pointer can point to NULL whilereference can never point to NULL

3. There's no "reference arithmetics

http://www.student.cs.uwaterloo.ca/~cs343/documents/refvsptr.html

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