pgt 106 c programming pointers. pgt 106 c programming outline introduction pointer variable...

PGT 106 C Programming

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


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


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

iNum

7

iNum7

piNum


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 definitionint *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;


Pointer Operators Symbol & is called address operator

Returns address of operandint iNum = 7;int *piNum; piNum = &iNum; /* piNum gets address of iNum */piNum “points to” iNum

piNum

iNum7

piNum

500000 600000

iNum

600000 7

Address of iNum is value of piNum


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


#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; }

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

Dereferencing pointer, *piNum = 15iNum = 20*piNum = 20*piNum + iNum1 = 25


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);

} *piNumber used as nickname for the variable

passed

10

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 : fDo you want to continue?yEnter character : IDo you want to continue?yEnter character : kDo you want to continue?nNumber of vowel : 1Number of consonant : 2

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


Remember…last time#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;

}

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

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

aiX[iCounter] = 0;}


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


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


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

pointer variable piVPtr

aiV[0] aiV[1] aiV[2] aiV[4]aiV[3]

3004 3008 3012 3016location3000


Pointer Expressions and Pointer Arithmetic Subtracting pointers

Returns number of elements from one to the other. IfpiVPtr2 = &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


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

UniMAP SemII-09/10 PGT 106 C 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] : 1245020Address of piVPtr + 2: 1245028Address of piVPtr + 4: 1245036Contents of temp : 2Contents of piP : 2147323904 piQ : 2147323904


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


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)


Example

Address of piBPtr : 1245064 Contents of piBPtr : 1245016Address of aiB : 1245016 Contents of aiB[0]:10 10 10piBPtr points to aiB[0] = 10

I am accessing element aiB[3]!!Let see how many ways I can do itaiB[3] = 40*(piBPtr + 3) = 40*(aiB + 3) = 40piBPtr[3] = 40

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

#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;}


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


Arrays of Pointers

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

acSuit[3]

acSuit[2]

acSuit[1]

acSuit[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’


Example#include <stdio.h>#define N 5int 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 : aliYou just entered :aliEnter student[1] name : abuYou just entered :abuEnter student[2] name :

cheahYou just entered :cheahEnter student[3] name : daliYou just entered :daliEnter student[4] name :

gheetaYou just entered :gheeta

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