Ekt120 Lecture09 Pointers

8/12/2019 Ekt120 Lecture09 Pointers

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UniMAP SemII-09/10 EKT 120:Computer Programming

POINTERS


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Outline Introduction

Pointer Variable Definitions and Initialization

Pointer Operators

Calling Functions by Reference

Using the const Qualifier with Pointers

Pointer Expressions and Pointer Arithmetic

Relationship between Pointers and Arrays

Arrays of Pointers


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Introduction Pointer is the address (i.e. a specific memory

location) of an object.

It can refer to different objects at different times.

Pointers are used in C programs for a variety ofpurposes: To return more than one value from a function(using pass

by reference)

To create and process strings

To manipulate the contents of arrays and structures

To construct data structures whose size can grow or shrinkdynamically


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Pointer Variable Definitions and

Initialization Pointer variables

Contain memory addresses as their values

Normal variables contain a specific value(directreference)

Pointercontains an addressof a variable that hasa specific value (indirect reference)

Indirectionreferencing a pointer value

num

7

num

7

numPtr


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Pointer Variable Definitions and

Initialization Pointer definitions

*used with pointer variables

int *numPtr; Defines a pointer to an int(pointer of type int *)

Multiple pointers require using a *before each variabledefinition

int *numPtr1, *numPtr2;

Can define pointers to any data type

Initialize pointers to 0, NULL, or an address 0or NULLpoints to nothing (NULLpreferred)

int *numPtr = NULL; orint *numPtr = 0;


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Pointer Operators Symbol &is called address operator

Returns address of operandint num = 7;

int *numPtr;numPtr = &num; /* numPtr gets address of num */

numPtr points to num

numPtr

num

7

numPtr

500000 600000

num

600000 7

Address of

numis value

of numPtr


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Pointer Operators Symbol *is called indirection/dereferencing

operator Returns a synonym/alias of what its operand points to

*numPtrreturns num(because numPtrpoints tonum)

*can also be used for assignment

Returns alias to an object*numPtr = 10; /* changes num to 10 */ show pictures!!

Dereferenced pointer (operand of *) must be an lvalue (no

constants)

*and are inverses They cancel each other out


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#include int main(){ int num;

int *numPtr;int num1=5;

num = 7;printf("number = %d\n", num);numPtr = &num;printf("numPtr points to num whereby the value is = %d\n",*numPtr);printf("Address of numPtr : %d Contents of numPtr : %d\n", &numPtr, numPtr);printf("Address of num : %d\n\n", &num);

*numPtr = 15;

printf("Dereferencing pointer, *numPtr = %d\n", *numPtr);num = num + num1;printf(num = %d\n, num);printf("*numPtr = %d\n", *numPtr);printf("*numPtr + num1 = %d\n", *numPtr + num1);return 0;

}

Sample programnumber = 7numPtr points to num whereby the value is = 7Address of numPtr : 1245060 Contents of numPtr : 1245064Address of num : 1245064

Dereferencing pointer, *numPtr = 15num = 20

*numPtr = 20*numPtr + num1 = 25


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Calling Functions by Reference Call by reference with pointer arguments

Passes address of argument using &operator Allows you to change actual location in memory

Arrays are not passed with because the array name isalready a pointer

*operator Used as alias or nickname for variable inside of function

void fun1 (int *number)

{*number = 2 *(*number);}

*numberused as nickname for the variable passed


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Remember..last time#include #include char read();void find_count_vc(char, int*,int*);void print(int,int);int main()

{ char ch, choice; int count_v=0,count_c=0;do{ ch = read();

find_count_vc(ch, &count_v,&count_c);printf("Do you want to continue?");scanf("%c", &choice);getchar();

}while((choice == 'y') ||(choice =='Y'));print(count_v,count_c);

return 0;}char read(){ char ch1;

printf("Enter character : ");scanf("%c", &ch1);getchar();return(ch1);

}

void find_count_vc(char ch1, int *vowel,int *consonant){

switch(ch1){ case 'A':

case 'a':case 'E':

case 'e':case 'I':case 'i':case 'O':case 'o':case 'U':case 'u':*vowel = *vowel +1;break;default:*consonant = *consonant + 1;

}

}void print(int vowel, int consonant){

printf("Number of vowel : %d\n", vowel);printf("Number of consonant : %d\n", consonant);

}

Enter character : f

Do you want to continue?y

Enter character : I

Do you want to continue?y

Enter character : k

Do you want to continue?n

Number of vowel : 1

Number of consonant : 2

Functions that returnmore than one value i.e.

arguments are passed byref


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Rememberlast time#include const int arraySize = 10;

void initializeArray (int x [], int sizeX);void fillArray (int x [], int sizeX);void printArray (const int x [], int sizeX);

int sumArray (const int x [], int sizeX);int indexLargestElement (const int x [], int

sizeX);void copyArray (const int x [], int y [], int

length);

int main(){

int listA [arraySize] = {0};int listB [arraySize];

printArray (listA, arraySize);

initializeArray (listB, arraySize);printArray (listB, arraySize);fillArray (listA, arraySize);printArray (listA, arraySize);sumArray (listA, arraySize);copyArray (listA, listB, arraySize);printArray (listB, arraySize);return 0;

}

void initializeArray (int x [ ], int sizeX){

int counter;

for (counter = 0; counter < sizeX; counter ++)x [counter] = 0;

}


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Using the constQualifier

with Pointers constqualifier

Variable cannot be changed Use constif function does not need to change a variable Attempting to change a constvariable produces an error

constpointers Point to a constant memory location Must be initialized when defined int *const myPtr = &x;

Type int *constconstant pointer to an int const int *myPtr = &x;

Regular pointer to a const int const int *const Ptr = &x;

constpointer to a const int xcan be changed, but not *Ptr


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Pointer Expressions and

Pointer Arithmetic Arithmetic operations can be performed on

pointers

Increment/decrement pointer (++or --) Add an integer to a pointer( +or +=, -or -=)

Pointers may be subtracted from each other

Operations meaningless unless performed on an

array


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Pointer Arithmetic 5 element intarray on machine with 4 byte ints

vPtrpoints to first element v[ 0 ]

at location 3000 (vPtr = 3000)

vPtr += 2;sets vPtrto 3008

vPtrpoints to v[ 2 ](incremented by 2), but themachine has 4 byte ints, so it points to address 3008

pointer variable

vPtr

v[0] v[1] v[2] v[4]v[3]

3004 3008 3012 3016

location

3000


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Pointer Arithmetic Subtracting pointers

Returns number of elements from one to theother. If

vPtr2 = &v[ 2 ];vPtr = &v[ 0 ];

vPtr2 - vPtrwould produce 2

Pointer comparison ( ) See which pointer points to the higher numbered

array element Also, see if a pointer points to 0


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Pointer Expressions andPointer Arithmetic

Pointers of the same type can be assigned toeach other

If not the same type, a cast operator must beused

Exception: pointer to void(type void *)

Generic pointer, represents any type

No casting needed to convert a pointer to voidpointer voidpointers cannot be dereferenced


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Example of Pointer Operation#include int main(){int *vPtr; int *vPtr2;int v[5] = {10,20,30,40,50}; int temp;int *p, *q;

vPtr= v;

printf("Address of vPtr : %d Contents of vPtr: %d\n", &vPtr, vPtr);

printf("Address of v[0] : %d\n", &v);vPtr +=2;printf("Address of vPtr + 2: %d\n", vPtr);vPtr +=2;printf("Address of vPtr + 4: %d\n", vPtr);

vPtr2=&v[2];vPtr=&v[0];temp=vPtr2-vPtr;printf("Contents of temp : %d\n", temp);

p=q;printf("Contents of p : %d q: %d\n", p,q);

return 0;}

Address of vPtr : 1245064 Contents of vPtr : 1245020Address of v[0] : 1245020Address of vPtr + 2: 1245028Address of vPtr + 4: 1245036Contents of temp : 2Contents of p : 2147323904 q: 2147323904


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The Relationship betweenPointers and Arrays

Arrays and pointers are closely related Array name like a constant pointer Pointers can do array subscripting operations

Define an array b[5]and a pointer bPtr To set them equal to one another use:

bPtr = b;

The array name (b) is actually the address of firstelement of the array b[ 5 ]

bPtr = b[0];

Explicitly assignsbPtrto the address of first element ofb


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The Relationship betweenPointers and Arrays

Element b[3]

Can be accessed by *(bPtr + 3) Where *is the offset. Called pointer/offset notation

Can be accessed by bPtr[3] Called pointer/subscript notation

bPtr[3]same as b[3]

Can be accessed by performing pointer

arithmetic on the array itself*(b + 3)


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Example

Address of bPtr : 1245064 Contents of bPtr : 1245016Address of b : 1245016 Contents of b[0]:10 10 10bPtr points to b[0] = 10

I am accessing element b[3]!!Let see how many ways I can do itb[3] = 40*(bPtr + 3) = 40*(b + 3) = 40

bPtr[3] = 40

b[0] = 10b[1] = 20b[2] = 30b[3] = 40b[4] = 50b[5] = 0b[6] = 0b[7] = 0b[8] = 0b[9] = 0

#include int main()

{ int *bPtr ;int i;int b[10]={10,20,30,40,50};bPtr = b;printf("Address of bPtr : %d Contents of bPtr : %d\n", &bPtr, bPtr);printf("Address of b : %d Contents of b[0]:%d %d %d\n", &b, b[0], *bPtr, *b);printf("bPtr points to b[0] = %d\n", *bPtr);

printf("\nI am accessing element b[3]!!\nLet see how many ways I can do it\n");

printf("b[3] = %d\n", b[3]);printf("*(bPtr + 3) = %d\n", *(bPtr + 3));printf("*(b + 3) = %d\n", *(b + 3));printf("bPtr[3] = %d\n\n", bPtr[3]);

for(i=0;i


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Arrays of Pointers

Arrays can contain pointers

For example: an array of strings

char suit[4] = {Hearts,Diamonds,Clubs,Spades};

Strings are pointers to the first character

char *each element of suitis a pointer to a char

The strings are not actually stored in the array suit, only

pointers to the strings are stored


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Arrays of Pointers

suitarray has a fixed size, but strings can

be of any size

suit[3]

suit[2]

suit[1]

suit[0] H e a r t s \0

D i a m o n d s \0

C l u b s \0

S p a d e s \0


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Example

#include #define N 5int main(){

char *studentName[N]; int i;

for(i=0;i

Ekt120 Lecture09 Pointers

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