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POINTERS. 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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pointers

POINTERS

EKT 120:Computer Programming

outline
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

EKT 120:Computer Programming

introduction
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 of purposes:
    • 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 shrink dynamically

EKT 120:Computer Programming

pointer variable definitions and initialization

iNum

piNum

iNum

7

7

Pointer Variable Definitions and Initialization
  • Pointer variables
    • Contain memory addresses as their values
    • Normal variables contain a specific value (direct reference)
    • Pointer contains an address of a variable that has a specific value (indirect reference)
    • Indirection – referencing a pointer value

EKT 120:Computer Programming

pointer variable definitions and initialization1
Pointer Variable Definitions and Initialization
  • Pointer definitions
    • * used with pointer variables

int *piNum;

    • Defines a pointer to an int (pointer of type int *)
    • Multiple pointers require using a * before each variable definition

int *piNum1, *piNum2;

    • Can define pointers to any data type
    • Initialize pointers to 0, NULL, or an address
      • 0 or NULL– points to nothing (NULL preferred)
      • int *piNum = NULL; or int *piNum = 0;

EKT 120:Computer Programming

pointer operators

piNum

iNum

iNum

7

piNum

Address of iNum is value of piNum

500000

600000

600000

7

Pointer Operators
  • Symbol & is called address operator
    • Returns address of operand

int iNum = 7;

int *piNum;

piNum = &iNum; /* piNum gets address of iNum */

piNum “points to” iNum

EKT 120:Computer Programming

pointer operators1
Pointer Operators
  • Symbol * is called indirection/dereferencing operator
    • Returns a synonym/alias of what its operand points to
      • *piNum returns iNum (because piNum points to iNum)
    • * can also be used for assignment
      • Returns alias to an object

*piNum = 10; /* changes iNum to 10 */ show pictures!!

    • Dereferenced pointer (operand of *) must be an lvalue (no constants)
  • * and & are inverses
    • They cancel each other out

EKT 120:Computer Programming

sample program
Sample program

#include <stdio.h>

int main()

{ int iNum;

int *piNum;

int iNum1=5;

iNum = 7;

printf("number = %d\n", iNum);

piNum = &iNum;

printf(“piNum points to iNum whereby the value is = %d\n",*piNum);

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

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

*piNum = 15;

printf("Dereferencing pointer, *piNum = %d\n", *piNum);

iNum = iNum + iNum1;

printf(“iNum = %d\n”, iNum);

printf("*piNum = %d\n", *piNum);

printf("*piNum + iNum1 = %d\n", *piNum + iNum1);

return 0;

}

number = 7

piNum points to iNum whereby the value is = 7

Address of piNum : 1245060 Contents of piNum : 1245064

Address of iNum : 1245064

Dereferencing pointer, *piNum = 15

iNum = 20

*piNum = 20

*piNum + iNum1 = 25

EKT 120:Computer Programming

calling functions by reference
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 is already a pointer
  • * operator
    • Used as alias or nickname for variable inside of function

void fnFun1 (int *piNumber)

{

*piNumber = 2 * (*piNumber);

}

    • *piNumberused as nickname for the variable passed

EKT 120:Computer Programming

remember last time
#include <stdio.h>

#include <string.h>

char fnRead();

void fnFindCountVC(char, int*, int*);

void fnPrint(int,int);

int main()

{ char cCh, cChoice; int iCountV=0, iCountC=0;

do

{ cCh = fnRead();

fnFindCountVC(cCh, &iCountV, &iCountC);

printf("Do you want to continue? ");

scanf("%c", &cChoice);

getchar();

}while((cChoice == \'y\') ||(cChoice ==\'Y\'));

fnPrint(iCountV,iCountC);

return 0;

}

char fnRead()

{ char cCh1;

printf("Enter character : ");

scanf("%c", &cCh1);

getchar();

return(cCh1);

}

void fnFindCountVC(char cCh1, int *piVowel, int *piConsonant)

{

switch(cCh1)

{ case \'A\':

case \'a\':

case \'E\':

case \'e\':

case \'I\':

case \'i\':

case \'O\':

case \'o\':

case \'U\':

case \'u\': *piVowel = *piVowel +1;break;

default: *piConsonant = *piConsonant + 1;

}

}

void fnPrint(int iVowel, int iConsonant)

{

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

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

}

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

Remember..last time

Functions that “return” more than one value i.e. arguments are passed by reference

remember last time1
#include <stdio.h>

const int iArraySize = 10;

void fnInitializeArray (int aiX[], int iSizeX);

void fnFillArray (int aiX[], int iSizeX);

void fnPrintArray (const int aiX[], int iSizeX);

int fnSumArray (const int aiX[], int iSizeX);

int fnIndexLargestElement (const int aiX[], int iSizeX);

void fnCopyArray (const int aiX[], int aiY[], int iLength);

int main()

{

int aiListA [iArraySize] = {0};

int aiListB [iArraySize];

fnPrintArray (aiListA, iArraySize);

fnInitializeArray (aiListB, iArraySize);

fnPrintArray (aiListB, iArraySize);

fnFillArray (aiListA, iArraySize);

fnPrintArray (aiListA, iArraySize);

fnSumArray (aiListA, iArraySize);

fnCopyArray (aiListA, aiListB, iArraySize);

fnPrintArray (aiListB, iArraySize);

return 0;

}

Remember…last time

void fnInitializeArray (int aiX[ ], int iSizeX)

{

int iCounter;

for (iCounter = 0; iCounter < iSizeX; iCounter++)

aiX[iCounter] = 0;

}

EKT 120:Computer Programming

using the const qualifier with pointers
Using the const Qualifier with Pointers
  • const qualifier
    • Variable cannot be changed
    • Use const if function does not need to change a variable
    • Attempting to change a const variable produces an error
  • const pointers
    • Point to a constant memory location
    • Must be initialized when defined
    • int *const piMyPtr = &iX;
      • Type int *const– constant pointer to an int
    • const int *piMyPtr = &iX;
      • Regular pointer to a const int
    • const int *const piPtr = &iX;
      • const pointer to a const int
      • iX can be changed, but not *piPtr

EKT 120:Computer Programming

pointer expressions and pointer arithmetic
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

EKT 120:Computer Programming

pointer expressions and pointer arithmetic1

aiV[0]

aiV[1]

aiV[2]

aiV[4]

aiV[3]

Pointer Expressions and Pointer Arithmetic
  • 5 element int array on machine with 4 byte ints
    • piVPtr points to first element aiV[ 0 ]
      • at location 3000 (piVPtr = 3000)
    • piVPtr += 2; sets piVPtr to 3008
      • piVPtr points to aiV[ 2 ] (incremented by 2), but the machine has 4 byte ints, so it points to address 3008

location

3000

3004

3008

3012

3016

EKT 120:Computer Programming

pointer variable piVPtr

pointer expressions and pointer arithmetic2
Pointer Expressions and Pointer Arithmetic
  • Subtracting pointers
    • Returns number of elements from one to the other. If

piVPtr2 = &aiV[ 2 ];

piVPtr = &aiV[ 0 ];

    • piVPtr2 - piVPtr would produce 2
  • Pointer comparison ( <, == , > )
    • See which pointer points to the higher numbered array element
    • Also, see if a pointer points to 0

EKT 120:Computer Programming

pointer expressions and pointer arithmetic3
Pointer Expressions and Pointer Arithmetic
  • Pointers of the same type can be assigned to each other
    • If not the same type, a cast operator must be used
    • Exception: pointer to void (type void *)
      • Generic pointer, represents any type
      • No casting needed to convert a pointer to void pointer
      • void pointers cannot be dereferenced

EKT 120:Computer Programming

example of pointer operation
#include <stdio.h>

int main()

{int *piVPtr; int *piVPtr2;

int aiV[5] = {10,20,30,40,50}; int iTemp;

int *piP, *piQ;

piVPtr= aiV;

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

printf("Address of aiV[0] : %d\n", &aiV);

piVPtr +=2;

printf("Address of piVPtr + 2: %d\n", piVPtr);

piVPtr +=2;

printf("Address of piVPtr + 4: %d\n", piVPtr);

piVPtr2=&aiV[2];

piVPtr=&aiV[0];

iTemp=piVPtr2-piVPtr;

printf("Contents of iTemp : %d\n", iTemp);

piP=piQ;

printf("Contents of piP : %d piQ : %d\n", piP, piQ);

return 0;}

Address of piVPtr : 1245064 Contents of piVPtr : 1245020

Address of aiV[0] : 1245020

Address of piVPtr + 2: 1245028

Address of piVPtr + 4: 1245036

Contents of temp : 2

Contents of piP : 2147323904 piQ : 2147323904

Example of Pointer Operation

EKT 120:Computer Programming

the relationship between pointers and arrays
The Relationship between Pointers and Arrays
  • Arrays and pointers are closely related
    • Array name like a constant pointer
    • Pointers can do array subscripting operations
  • Define an array aiB[5]and a pointer piBPtr
    • To set them equal to one another use:

piBPtr = aiB;

      • The array name (aiB) is actually the address of first element of the array aiB[ 5 ]

piBPtr = &aiB[0];

      • Explicitly assigns piBPtr to the address of first element of aiB

EKT 120:Computer Programming

the relationship between pointers and arrays1
The Relationship between Pointers and Arrays
  • Element aiB[3]
    • Can be accessed by *(piBPtr + 3)
      • Where * is the offset. Called pointer/offset notation
    • Can be accessed by piBPtr[3]
      • Called pointer/subscript notation
      • piBPtr[3] same as aiB[3]
    • Can be accessed by performing pointer arithmetic on the array itself

*(aiB + 3)

EKT 120:Computer Programming

example

Address of piBPtr : 1245064 Contents of piBPtr : 1245016

Address of aiB : 1245016 Contents of aiB[0]:10 10 10

piBPtr points to aiB[0] = 10

I am accessing element aiB[3]!!

Let see how many ways I can do it

aiB[3] = 40

*(piBPtr + 3) = 40

*(aiB + 3) = 40

piBPtr[3] = 40

aiB[0] = 10

aiB[1] = 20

aiB[2] = 30

aiB[3] = 40

aiB[4] = 50

aiB[5] = 0

aiB[6] = 0

aiB[7] = 0

aiB[8] = 0

aiB[9] = 0

Example

#include <stdio.h>

int main()

{ int *piBPtr ;int iIndex;

int aiB[10]={10,20,30,40,50};

piBPtr = aiB;

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

printf("Address of aiB : %d Contents of aiB[0]:%d %d %d\n", &aiB, aiB[0], *piBPtr, *aiB);

printf(“piBPtr points to aiB[0] = %d\n", *piBPtr);

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

printf(“aiB[3] = %d\n", aiB[3]);

printf("*(piBPtr + 3) = %d\n", *(piBPtr + 3));

printf("*(aiB + 3) = %d\n", *(aiB + 3));

printf(“piBPtr[3] = %d\n\n", piBPtr[3]);

for(iIndex=0;iIndex<10;iIndex++)

printf(“aiB[%d] = %d\n", iIndex, *(piBPtr+iIndex));

return 0;

}

EKT 120:Computer Programming

arrays of pointers
Arrays of Pointers
  • Arrays can contain pointers
  • For example: an array of strings

char *acSuit[4] = {“Hearts”,“Diamonds”,“Clubs”,“Spades”};

    • Strings are pointers to the first character
    • char *– each element of acSuit is a pointer to a char
    • The strings are not actually stored in the array acSuit, only pointers to the strings are stored

EKT 120:Computer Programming

arrays of pointers1

’\0’

’\0’

’\0’

’\0’

’o’

’n’

’a’

’s’

’d’

’u’

’d’

’b’

’a’

’t’

’i’

’D’

’s’

’H’

’r’

’e’

’s’

’a’

’m’

’e’

’p’

’C’

’l’

’S’

’s’

acSuit[0]

acSuit[1]

acSuit[2]

acSuit[3]

Arrays of Pointers
  • acSuit array has a fixed size, but strings can be of any size

EKT 120:Computer Programming

example1
#include <stdio.h>

#define N 5

int main()

{

char *acStudentName[N]; int iIndex;

for(iIndex=0;iIndex<5;iIndex++)

{ printf("Enter student[%d] name : ", iIndex);

scanf("%s", acStudentName + iIndex);

printf("You just entered :\n%s\n", acStudentName + iIndex);

}

return 0;

}

Enter student[0] name : ali

You just entered :

ali

Enter student[1] name : abu

You just entered :

abu

Enter student[2] name : cheah

You just entered :

cheah

Enter student[3] name : dali

You just entered :

dali

Enter student[4] name : gheeta

You just entered :

gheeta

Example

EKT 120:Computer Programming

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