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PointersPointers

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VariableVariable

y A variable is a named memory location.

y Variables provide direct access to its memory

location.y A variable has a name, an address, a type,and a

value:

y

"the name identifies the variable to theprogrammer

y "the address specifies where in main memorythe variable is located

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What is a variable?What is a variable?

y "the type specifies how to interpret the

data stored in main memory and howlong the variable is

y "the value is the actual data stored in the

variable after if has been interpreted

according to a given type

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Pointer variablePointer variabley A pointer is a variable that contains the memory

location ofanother variable.

y Syntax:-

y type * variable namey You start by specifying the type of data stored in the

location identified by the pointer.

y The asterisk tells the compiler that youare creating a

pointer variable.

y Finally you give the name of the variable.

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Declaring a Pointer VariableDeclaring a Pointer Variable

To declare ptr as an integer pointer:

int *ptr;

To declare ptr as a character pointer:

char *ptr;

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Address operator:Address operator:

y Once we declare a pointer variable we mustpoint it to something we can do this byassigning to the pointer the address of the

variable you want to point as in the following

example:

ptr=#

y This places the address where num is stores

into the variable ptr. If num is stored inmemory 21260 address then the variable ptrhas the value 21260.

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Address and PointersAddress and Pointers

y Memory can beconceptualized as alinear set of datalocations.

y Variables reference thecontents ofalocations

y Pointers have a valueof the address ofagiven location

Lect 14 P. 7

Contents1

Contents11

Contents16

ADDR1ADDR2ADDR3ADDR4

ADDR5ADDR6***

ADDR11

**

ADDR16

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Pointer VariablePointer VariableAssume ptr is a pointer variable and x is an integer variable

x

ptr

x = 10

10

ptr = &x

&x

Now ptrcanaccessthe value ofx.

HOW!!!!

Write: *variable . Forexample:

printf(%d\n, *ptr);

Can youtellme why weput%d????

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Variables, Addresses and PointersVariables, Addresses and Pointers

y main()

{

int a = 5;

int b = 6;

int* c;

// c points to a

c = &a;}

y MemoryValue

y a (1001) 5

y b (1003) 6

y c (1005) 1001

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include< stdio.h >

{int num, *intptr;

float x, *floptr;char ch, *cptr;num=123;

x=12.34;

ch=a;intptr=&num;cptr=&ch;floptr=&x;

printf(Num %d stored at address %u\n,*intptr,intptr);

printf(Value %f stored at address %u\n,*floptr,floptr);printf(Character %c stored at address %u\n,*cptr,cptr);}

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Run this codeRun this code

int main()

{

int x;

int *ptr;

x = 10;

ptr = &x;

*ptr = *ptr + 1;

printf(x = %d\n, x);

}

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Manipulating Pointer VariableManipulating Pointer Variable

y Once a variable is declared, we can get its address bypreceding its name with the unary & operator, asin &k.

y We can "dereference" a pointer, i.e. refer to the value

of that which it points to, by using the unary '*'operator as in *ptr

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ReferenceReference

y Reference &

y Retrieve the memory address of a variable

y int a = 6;

y int* c = &a; // &a is the memory location of variable a

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DereferenceDereference

y Dereference * Accessing the variable (content)the pointer points to

y (Indirection)

y int a = 6;

y int* c = &a;

y *c = 7; /* Changes content of variable a by

using its address stored in pointer c */y equivalent to

y a = 7;

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Constant PointersConstant Pointers

y A constant pointer,ptr, is a pointer that

is initialized with an address, and cannotpoint to anything else.

yWe can useptr to change the contentsof value

y Example

int value = 22;int * constptr = &value;

CIS 15 Pointer Arithmetic 15

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Constant PointerConstant Pointer Constant pointer means the pointer is constant.Constant pointer is NOT pointer to constant. For eg:

int * const ptr2 indicates that ptr2 is a pointerwhich is constant. This means that ptr2 cannot bemade to point to another integer.

However the integer pointed by ptr2 can bechanged.

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//const pointer void

main()

{

int i = 100,k;

int* const pi =&i;

*pi = 200;

pi=&k; //won't compile

}

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Constant Pointers to ConstantsConstant Pointers to Constants

A constant pointer to a constant is:y a pointer that points to a constant

y a pointer that cannot point to anything

except what it is pointing to

Example:int value = 22;

const int * const ptr =&value;

CIS 15 Pointer Arithmetic 18

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y //const pointer to a const void

y f3()

y {

y int i = 100;

y const int* const pi =&i;

y //*pi = 200;

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Pointer to constantPointer to constant

Pointer to constant is

const int * ptr1 indicates that ptr1 is a

pointer that points to a constant integer.The integer is constant and cannot be

changed.However, the pointer ptr1 can

be made to point to some other integer.

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y //pointer to a const

void f1()

{

int i = 100;

const int* pi =&i;

//*pi = 200;

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Pointer arithmeticPointer arithmetic

Valid operations on pointers include:

- the sum of a pointer and an integer

- the difference of a pointer and an integer

- pointer comparison

-the difference of two pointers.

-Increment/decrement in pointers

-assignment operator used in pointers

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ExampleExample

void main(){

int a=25,b=78,sum;

int *x,*y;x=&a;

y=&b;

sum= *x + *y;

printf(Sum is : %d,sum);

}

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Assignment in pointersAssignment in pointers

y Pointer variables can be "assigned":int *p1, *p2;p2 = p1;

Assigns one pointer to another "Make p2 point to where p1 points"

y Do not confuse with:*p1 = *p2;

Assigns "value pointed to" by p1, to "valuepointed to" by p2

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Diagrammatic representationDiagrammatic representation

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Comparison in pointersComparison in pointers

y Two pointers of the same type, p and q, may becompared as long

y as both of them point to objects within a single memory

blocky Pointers may be compared using the , =,

== , !=

y When youare comparing two pointers, youare

comparing the

y values of those pointers rather than the contents ofmemorylocations pointed to by these pointers

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Increment and decrementIncrement and decrement

Data Type Initialaddress

Operation Addressafter

operation s

Requiredbytes

Int i=2 4046 ++ -- 4048 4044 2bytes

Char c=x 4053 ++ -- 4054 4042 1bytes

Float f=2.2 4050 ++ -- 4054 4046 4bytes

Long l=2 4060 ++ -- 4064 4056 4bytes

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y You can add an integer to a pointer but you cannot add a pointer to apointer.

y

If the pointer p points to the first element in an array, the followingexpression causes the pointer to point to the third element in the samearray:

y p = p + 2; If you have two pointers that point to the same array, you cansubtract one pointer from the other. This operation yields the number ofelements in the array that separate the two addresses that the pointers

refer to.y You can compare two pointers with the following

operators: ==, !=,< ,>,=.

y Pointer comparisons are defined only when the pointers point toelements of the same array. Pointer comparisons usingthe == and != operators can be performed even when the pointers point

to elements of different arrays.y You can assign to a pointer the address ofa data object, the value of

another compatible pointer or the NULL pointer.

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Pointer ArithmeticPointer Arithmetic

y Operations on pointer variables:

CIS 15 Pointer Arithmetic 30

Operation Exampleint vals[]={4,7,11};

int *valptr = vals;++, -- valptr++; // points at 7

valptr--; // now points at 4

+, - (pointerand int) cout

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Generic pointersGeneric pointers

yWhen a variable is declared as being a pointerto type void it is known as a generic pointer.

y Since you cannot have a variable of type void,

the pointer will not point to any dataandtherefore cannot be dereferenced.

y It is stilla pointer though, to use it youjust

have to typecast it to another kind of pointerfirst.Hence the termGeneric pointer.

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y This is very useful when you want a pointer to

point to data of different types at differenttimes.

y Syntax:

void * variable name;

Print value stored in variable

*(data_type*)name of variable;

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void main()

{ int i;

char c;

void *data;

i = 6;

c = 'a';

data=&i;

printf("the_data points to the integer value %d\n", *(int*)data);

data=&c;

printf("the_data now points to the character %c\n", *(char*)

data);

}

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Null PointerNull Pointer

y NULL value can be assigned to any pointer, nomatter what its type.

y

void *p = NULL;y int i = 2;

y int *ip =&i;

y

p = ip;y printf("%d", *p);

y printf("%d", *((int*)p ) );

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Pointers and ArraysPointers and Arrays

The concept ofarray is very much bound to

the one of pointer. In fact, the identifier of

an array is equivalent to the address of its

first element, as a pointer is equivalent to

the address of the first element that it

points to, so in fact they are the same

concept.

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Pointers and ArraysPointers and Arrays

y Pointers have close relationship with array

.

y An array name by itself is an address or

pointer.

y Pointers are linked with both types of

array

A)one dimensionalarray pointer B)two dimensionalarray pointer

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For example,

int numbers [20]; int * p;

The following assignment operationwould be valid:

p = numbers;

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*p = 30;p = numbers + 3;

*p = 40; p = numbers;

*(p+4) = 50;for(int n=0; n

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In arrays we used brackets ([]) several times

in order to specify the index ofan element

of the array to which we wanted to refer.

Well, these bracket sign operators [] are

also adereference operator known as

offset operator. They dereference the variable

they follow just as * does, but they also add

the number between brackets to the

address being dereferenced

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For example:

a[5] = 0; // a [offset of 5] = 0

*(a+5) = 0; // pointedby (a+5) = 0

These two expressions are equivalent andvalid both ifa is a pointer or ifa is an

array.

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Example ofExample of1D array1D arrayV

oid main(){

int a[5]={1,2,3,4,5};

int *p,i;

p=&a;

printf(displaying data using pointer );for(i=0;i

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Example ofExample of2D array2D array

Void main()

{

int a[5][2]={ {1,2},{3,4},{5,6},{7,8},{9,10} };

int *p,j,i;

p=&a;

printf(displaying data using pointer );

for(i=0;i

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Pointers and Character StringPointers and Character String

void main(){

char name[] = sanjima;

char *ptr;

int i=0;count=0;ptr=&name;

while(*(ptr+i)!=\0)

{

count++;i++;

}

printf(length of string is %d ,count);

getch();

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ExampleExample

void main(){

char str1[]=hello;char str2[10];

char *s= Good Morning;char *q;

str2=str1; /error/

q= s; /works/return;}

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Array of pointersArray of pointers

y It is nothing but a collection of address.

Void main()

{

int a[5]={1,2,3,4,5},i;

int *p[5];

for(i=0;i

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Pointers to pointersPointers to pointersy

A pointer variable containing address ofanother pointer variable is called pointer

to pointer

void main()

{

int a=2, *p, **q;

p=&a;

q=&p;

printf(%d value is stored at %uand pointer

is stored at %u,a,p,q);}

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