pointer in c++ dr. ahmed telba. topics covered introduction to pointers pointers and arrays...

Pointer in C++

Dr. Ahmed Telba

Topics Covered

• Introduction to Pointers• Pointers and arrays• Character Pointers, • Arrays and Strings• Examples

Introduction to Pointers• When we declare a variable some memory is allocated for it. The memory location

can be referenced through the identifier “i”. Thus, we have two properties for any variable : its address and its data value. The address of the variable can be accessed through the referencing operator “&”. “&i” gives the memory location where the data value for “i” is stored.

• A pointer variable is one that stores an address. We can declare pointers as follows int* p; .This means that p stores the address of a variable of type int.

Introduction to Pointers• Q: Why is it important to declare the type of the variable that a pointer points to?

Aren’t all addresses of the same length?• A: It’s true that all addresses are of the same length, however when we perform an

operation of the type “p++” where “p” is a pointer variable, for this operation to make sense the compiler needs to know the data type of the variable “p” points to. If “p” is a character pointer then “p++” will increment “p” by one byte (typically), if “p” were an integer pointer its value on “p++” would be incremented by 2 bytes (typically).

Introduction to Pointers• Summary of what was learnt so far:

– Pointer is a data type that stores addresses, it is declared as follows: int* a; char* p; etc.

– The value stored in a pointer p can be accessed through the dereferencing operator “*”.– The address of a memory location of a variable can be accessed through the reference

operator “&”.– Pointer arithmetic: only addition and subtraction are allowed.

Pointers and Arrays• The concept of array is very similar to the concept of pointer. The identifier of an

array actually a pointer that holds the address of the first element of the array.• Therefore if you have two declarations as follows:

– “int a[10];” “int* p;” then the assignment “p = a;” is perfectly valid– Also “*(a+4)” and “a[4]” are equivalent as are “*(p+4)” and “p[4]” .– The only difference between the two is that we can change the value of “p” to any

integer variable address whereas “a” will always point to the integer array of length 10 defined.

Character Pointers, Arrays and Strings

• What is a String?– A string is a character array that is ‘\0’ terminated.– E.g. “Hello”

• What is a Character array?– It is an array of characters, not necessarily ‘\0’ terminated– E.g. char test[4] = {‘a’, ‘b’, ‘c’, ‘d’}; <this char array is not zero terminated>

• What is a character pointer?– It is a pointer to the address of a character variable.– E.g. char* a; <this pointer is not initialized>

Character Pointers, Arrays and Strings

• How do we initialize a Character pointer?– Initialize it to NULL. char* a = NULL;– Let it point to a character array.

• char* a; char b[100]; a = b;– Initialize to a character string.

• char* a = “Hello”; a pointer to the memory location where ‘H’ is stored. Here “a” can be viewed as a character array of size 6, the only difference being that a can be reassigned another memory location.

Examples• char* a = “Hello”;

– a -> gives address of ‘H’– *a -> gives ‘H’– a[0] -> gives ‘H’– a++ -> gives address of ‘e’– *a++ -> gives ‘e’– a = &b; where b is another char variable is perfectly LEGAL. However “char a[100];” “a

=&b;” where b is another char variable is ILLEGAL.

first program in pointer

• // my first pointer • #include <iostream>• using namespace std;• int main ()• {• int a = 5, b = 15;• int *ptr;• ptr= &a;• *ptr = 10;• ptr = &b;• *ptr = 20;• cout<<"a="<<a<<" , b="<<b;• return 0;• }

• #include <iostream>• using namespace std;• int main ()• {• int a = 5, b = 15;• int *p1, *p2;• p1 = &a;• p2 = &b;• *p1 = 10;• *p2 = *p1;• p1 = p2;• *p1 = 20;• cout << "a=" << a << " , b=" << b;• return 0;}

Array in pointer • #include <iostream>• using namespace std;• int main ()• {• int numbers[5];• int * p;• p = numbers; *p = 10;• p++; *p = 20;• p = &numbers[2]; *p = 30;• p = numbers + 3; *p = 40;• p = numbers; *(p+4) = 50;• for (int n=0; n<5; n++)• cout << numbers[n] << ", ";• return 0;• }

Value and address of pointer• Pointer type: int ,float double, char,long,short• Pointer name: *p,*AH• #include <iostream.h>• main()• {• int *ptr,i;• i=10;• ptr=&i;

• cout << "ptr = " <<*ptr<<endl;• //cout << "ptr = " <<*ptr<<"\t"<<&ptr<<endl;• system("pause");• }

• #include <iostream.h>• main()• {• int* grades = NULL;• int numberOfGrades;

• cout << "Enter the number of grades: ";• cin >> numberOfGrades;• grades = new int[numberOfGrades];

• for (int i = 0; i < numberOfGrades; i++)• cin >> grades[i];

• for (int j = 0; j < numberOfGrades; j++)• cout << grades[j] << " ";

• delete [] grades;• grades = NULL;• system("pause");• }

Pointer in array • #include <iostream.h>• main()• {• int arr[2] ; • int *ptr;• arr[0]=10;• arr[1]=20;• ptr=&arr[0];

• cout << "ptr = " <<*ptr<<"\t"<<*++ptr<<endl;• system("pause");• }

Pointer in string

• #include <iostream.h>• main()• {• int arr[3] ; • char *ptr="GE 211";• cout << "ptr = " <<ptr<<endl;• system("pause");• }

String

• #include <iostream.h>• main()• {• • char arr[300];

• cout << "enter string in = " ;• cin>>arr;• cout <<arr<<endl;• system("pause");• }

Pointer in array

• #include <iostream.h>• main()• {• • int arr[3]={100,200,300};• int *ptr,i;• ptr=arr;• for(i=0;i<3;i++)• cout <<i<<"\t"<<ptr[i]<<endl;• system("pause");• }

Pointer to another pointer • #include <iostream.h>

• main()• {• int p;• int *pt;• int **ptr;• p=10;• • cout << p << endl;• pt =&p;• ptr =&pt;• **ptr =5;• cout << p << endl;• system("pause");• }

Pointers in C++int number;int * p = &number;Its equvelant to int number;int * p;p = &number;

Return by value • #include<iostream.h>• int cubeByValue(int); // prototype• int main( )• {• int number = 5;• cout <<" The original value of number is " <<number<<endl;• number = cubeByValue(number);• cout << " The new value of number is " << number<< endl;• system("pause");• return 0;• }• int cubeByValue(int n)• {• return n*n*n; // cube local variable n• }

call-by-reference with a pointer argument

• #include<iostream.h>• void cubeByReference (int *); // prototype• main( )• {• int number = 5;• cout<< " The original value of number is " << number • <<endl;• cubeByReference(&number);• cout<< " The new value of number is " << number <<endl;• system("pause");• return 0;• }• void cubeByReference (int *nPtr)• {• *nPtr = *nPtr * *nPtr * *nPtr; // cube number in main• }

Pointer in array

• int array1[3]={1,2,3};• for (int j=0;j<3;j++)• cout<<endl<<array1[j];• ****• int array1[3]={1,2,3};• for (int j=0;j<3;j++)• cout<<endl<< *(array1+j);

Pointer in string

• char str1[ ] =”with array";• char str2[ ] =”with pointer";• cout <<endl<<str1;• cout <<endl<<str2;• str2++;• cout <<endl<<str2;

• #include<iostream.h>• main ( )• {• int a ; //a is an integer• int *aptr; // aptr is apointer to an integer• a = 7;• aptr = &a; // aptr set to address of a• cout <<" The address of a is " << &a <<endl• << "The value of aptr is " << aptr<< endl<< endl;

• cout << "The value of a is " << a<< endl• << "The value of *aptr is " << *aptr<< endl<<endl;• cout<<" Proving that * and & are complement of "• << "each other." <<endl<< " & *ptr = "<< & *aptr• << endl<< " *&aptr = " << *&aptr <<endl;• system("pause");• return 0;• }

• call-by-value• call-by-reference • call by reference with pointer arguments.

Pointers and Arrays• The concept of array is very similar to the concept of pointer. The identifier of an

array actually a pointer that holds the address of the first element of the array.• Therefore if you have two declarations as follows:

– “int a[10];” “int* p;” then the assignment “p = a;” is perfectly valid– Also “*(a+4)” and “a[4]” are equivalent as are “*(p+4)” and “p[4]” .– The only difference between the two is that we can change the value of “p” to any

integer variable address whereas “a” will always point to the integer array of length 10 defined.

Character Pointers, Arrays and Strings

• What is a String?– A string is a character array that is ‘\0’ terminated.– E.g. “Hello”

• What is a Character array?– It is an array of characters, not necessarily ‘\0’ terminated– E.g. char test[4] = {‘a’, ‘b’, ‘c’, ‘d’}; <this char array is not zero terminated>

• What is a character pointer?– It is a pointer to the address of a character variable.– E.g. char* a; <this pointer is not initialized>

Adress in memory

• char * p = "hello";

• char* cars[4]• char[0]="Mercedes";• char[1]="Nissan";• char[2]="Ferrari";• char[3]=“Toyota";

• *p++; • important note that ++is priority than *• *p++ = *q++;• *p = *q;• p++;• q++;

Pointer to pointer

• char a;• char * b;• char ** c;• a = 'z';• b = &a;• c = &b;

Character Pointers, Arrays and Strings

• How do we initialize a Character pointer?– Initialize it to NULL. char* a = NULL;– Let it point to a character array.

• char* a; char b[100]; a = b;– Initialize to a character string.

• char* a = “Hello”; a pointer to the memory location where ‘H’ is stored. Here “a” can be viewed as a character array of size 6, the only difference being that a can be reassigned another memory location.

Examples• char* a = “Hello”;

– a -> gives address of ‘H’– *a -> gives ‘H’– a[0] -> gives ‘H’– a++ -> gives address of ‘e’– *a++ -> gives ‘e’– a = &b; where b is another char variable is perfectly LEGAL. However “char a[100];” “a

=&b;” where b is another char variable is ILLEGAL.

• #include <iostream>• using namespace std;• int main ()• {• int a = 5, b = 15;• int *ptr;• ptr= &a;• *ptr = 10;• ptr = &b;• *ptr = 20;• cout<<"a="<<a<<" , b="<<b<<endl;• system("pause");• return 0;}

• //DISPLAY 7.1 Program Using an Array• //Reads in 5 scores and shows how much each• //score differs from the highest score.• #include <iostream>

• int main( )• {• using namespace std;• int i, score[5], max;

• cout << "Enter 5 scores:\n";• cin >> score[0];• max = score[0];• for (i = 1; i < 5; i++)• {• cin >> score[i];• if (score[i] > max)• max = score[i];• //max is the largest of the values score[0],..., score[i].• }

• cout << "The highest score is " << max << endl• << "The scores and their\n"• << "differences from the highest are:\n";• for (i = 0; i < 5; i++)• cout << score[i] << " off by "• << (max - score[i]) << endl;• system("pause");• return 0;• }

• #include <iostream>• using namespace std;• int main ()• {• int a = 5, b = 15;• int *p1, *p2;• p1 = &a;• p2 = &b;• *p1 = 10;• *p2 = *p1;• p1 = p2;• *p1 = 20;• cout << "a=" << a << " , b=" << b<< endl;• system("pause");• return 0;}

• #include <iostream>• using namespace std;• int main ()• {• int numbers[5];• int * p;• p = numbers; *p = 10;• p++; *p = 20;• p = &numbers[2]; *p = 30;• p = numbers + 3; *p = 40;• p = numbers; *(p+4) = 50;• for (int n=0; n<5; n++)• cout << numbers[n] << ", ";• system("pause");• return 0;• }

• In pointer & array • a[5] = 0;• *(p+5) = 0;

Dynamic Allocation

• int *pNumber;• pNumber = new int;• double *pDouble;• pDouble = new double;• #include <iostream.h>• int *pPointer;• void SomeFunction()• {• Pointer = new int;• *pPointer = 25;• }• main()• {• SomeFunction(); // pPointer • cout<<"Value of *pPointer: “<< *pPointer;• }

• //DISPLAY 9.2 Arrays and Pointer Variables

• #include <iostream>• using namespace std;

• int main( )• {• int *p1, *p2;

• p1 = new int;• *p1 = 42;• p2 = p1;• cout << "*p1 == " << *p1 << endl;• cout << "*p2 == " << *p2 << endl;

• *p2 = 53;• cout << "*p1 == " << *p1 << endl;• cout << "*p2 == " << *p2 << endl;

• p1 = new int;• *p1 = 88;• cout << "*p1 == " << *p1 << endl;• cout << "*p2 == " << *p2 << endl;• cout << "Hope you got the point of this example!\n";• system("pause");• return 0;• }

//DISPLAY 9.4 Arrays and Pointer Variables//Program to demonstrate that an array variable is a kind of pointer variable.#include <iostream>using namespace std;

typedef int* IntPtr;

int main( ){ IntPtr p; int a[10]; int index;

for (index = 0; index < 10; index++) a[index] = index;

p = a;

for (index = 0; index < 10; index++) cout << p[index] << " "; cout << endl;

for (index = 0; index < 10; index++) p[index] = p[index] + 1;

for (index = 0; index < 10; index++) cout << a[index] << " "; cout << endl; system("pause"); return 0;}

//DISPLAY 9.6 A Two-Dimensional Dynamic Array #include <iostream>using namespace std;

typedef int* IntArrayPtr;

int main( ){ int d1, d2; cout << "Enter the row and column dimensions of the array:\n"; cin >> d1 >> d2;

IntArrayPtr *m = new IntArrayPtr[d1]; int i, j; for (i = 0; i < d1; i++) m[i] = new int[d2]; //m is now a d1 by d2 array.

cout << "Enter " << d1 << " rows of " << d2 << " integers each:\n"; for (i = 0; i < d1; i++) for (j = 0; j < d2; j++) cin >> m[i][j];

cout << "Echoing the two-dimensional array:\n"; for (i = 0; i < d1; i++) { for (j = 0; j < d2; j++) cout << m[i][j] << " "; cout << endl; } for (i = 0; i < d1; i++) delete[] m[i]; delete[] m;system("pause"); return 0;}

Pointer in array • #include <iostream.h>• main()• {• int Array[3];• Array[0] = 10;• Array[1] = 20;• Array[2] = 30;• int *pArray;• pArray = &Array[0];• cout<<"pArray points to the value "<<endl << *pArray<<endl;• system("pause");• }

#include <iostream.h> main(){ int Array[3];Array[0] = 10;Array[1] = 20;Array[2] = 30;int *pArray;pArray = &Array[0];cout<<"pArray points to the value "<<endl << *pArray<<endl;pArray++;cout<<"pArray points to the value "<<endl << *pArray<<endl;pArray++;cout<<"pArray points to the value "<<endl << *pArray<<endl;system("pause");}

• #include <iostream>// (int arg[]) =printarray• using namespace std;• void printarray(int arg[], int length) {• for (int n=0; n<length; n++)• cout << arg[n] << " ";• cout << "\n";• }• int main ()• {• int firstarray[] = {5, 10, 15};• int secondarray[] = {2, 4, 6, 8, 10};• printarray (firstarray,3);• printarray (secondarray,5);• system("pause");• return 0;• }

Adding content of array • #include <iostream>• using namespace std;• int main()• {• int A[]={5,6,1,9,12,4,7};• int sum=0;• for(int i=0;i<7;i++)• {• cout<<A[i]<<" ";• sum+=A[i]; //sum =sum+A[i];• }• cout<<"\nsum= "<<sum<<endl;• system("pause");• return 0;}

• #include <iostream>// find char in array• using namespace std;• int main()• {• int arr[10],V,i;• bool found=false;• cout<<"enter 10 numbers please!\n";• for(i=0;i<10;i++)• {• cout<<"number"<<i+1<<"= ";• cin>>arr[i];• }• cout<<"enter V: ";• cin>>V;• for(i=0;i<10;i++)• if(arr[i]==V)• {• found=true;• break;• }• if(found) //same as if(found==true)• cout<<"V is in the array\n";• else• cout<<"V is not in the array\n";• system("pause");• return 0;}

String in array• #include <iostream>• using namespace std;• #include <string>• int main ()• {• char MyName[20];• strcpy (MyName,"AHMED TELBA");• cout << MyName<<endl;• system("pause");• return 0;• }

• //DISPLAY 7.1 Program Using an Array• //Reads in 5 scores and shows how much each• //score differs from the highest score.• #include <iostream>

• int main( )• {• using namespace std;• int i, score[5], max;

• cout << "Enter 5 scores:\n";• cin >> score[0];• max = score[0];• for (i = 1; i < 5; i++)• {• cin >> score[i];• if (score[i] > max)• max = score[i];• //max is the largest of the values score[0],..., score[i].• }

• cout << "The highest score is " << max << endl• << "The scores and their\n"• << "differences from the highest are:\n";• for (i = 0; i < 5; i++)• cout << score[i] << " off by "• << (max - score[i]) << endl;• system("pause");• return 0;• }

//DISPLAY 7.3 Indexed Variable as an Argument//Illustrates the use of an indexed variable as an argument.//Adds 5 to each employee's allowed number of vacation days.#include <iostream>const int NUMBER_OF_EMPLOYEES = 3;int adjust_days(int old_days);//Returns old_days plus 5.int main( ){ using namespace std; int vacation[NUMBER_OF_EMPLOYEES], number; cout << "Enter allowed vacation days for employees 1" << " through " << NUMBER_OF_EMPLOYEES << ":\n"; for (number = 1; number <= NUMBER_OF_EMPLOYEES; number++) cin >> vacation[number-1]; for (number = 0; number < NUMBER_OF_EMPLOYEES; number++) vacation[number] = adjust_days(vacation[number]); cout << "The revised number of vacation days are:\n"; for (number = 1; number <= NUMBER_OF_EMPLOYEES; number++) cout << "Employee number " << number << " vacation days = " << vacation[number-1] << endl; system("pause"); return 0;}

int adjust_days(int old_days){ return (old_days + 5);}

Dynamic Allocation• int *pNumber;• pNumber = new int;• double *pDouble;• pDouble = new double;• #include <iostream.h>• int *pPointer;• void SomeFunction()• {• Pointer = new int;• *pPointer = 25;• }• void main()• {• SomeFunction(); // pPointer • printf("Value of *pPointer: %d\n", *pPointer);• }

• //DISPLAY 9.2 Arrays and Pointer Variables

• #include <iostream>• using namespace std;

• int main( )• {• int *p1, *p2;

• p1 = new int;• *p1 = 42;• p2 = p1;• cout << "*p1 == " << *p1 << endl;• cout << "*p2 == " << *p2 << endl;

• *p2 = 53;• cout << "*p1 == " << *p1 << endl;• cout << "*p2 == " << *p2 << endl;

• p1 = new int;• *p1 = 88;• cout << "*p1 == " << *p1 << endl;• cout << "*p2 == " << *p2 << endl;• cout << "Hope you got the point of this example!\n";• system("pause");• return 0;• }

//DISPLAY 9.4 Arrays and Pointer Variables//Program to demonstrate that an array variable is a kind of pointer variable.#include <iostream>using namespace std;

typedef int* IntPtr;

int main( ){ IntPtr p; int a[10]; int index;

for (index = 0; index < 10; index++) a[index] = index;

p = a;

for (index = 0; index < 10; index++) cout << p[index] << " "; cout << endl;

for (index = 0; index < 10; index++) p[index] = p[index] + 1;

for (index = 0; index < 10; index++) cout << a[index] << " "; cout << endl; system("pause"); return 0;}

• #include <cstdlib>• #include <cstddef>• typedef int* IntArrayPtr;• void fill_array(int a[], int size);• //Precondition: size is the size of the array a.• //Postcondition: a[0] through a[size-1] have been• //filled with values read from the keyboard.• void sort(int a[], int size);• //Precondition: size is the size of the array a.• //The array elements a[0] through a[size-1] have values.• //Postcondition: The values of a[0] through a[size-1] have been rearranged• //so that a[0] <= a[1] <= ... <= a[size-1].• int main( )• { using namespace std;• cout << "This program sorts numbers from lowest to highest.\n";• int array_size;• cout << "How many numbers will be sorted? ";• cin >> array_size;• IntArrayPtr a;• a = new int[array_size];• fill_array(a, array_size);• sort(a, array_size);• cout << "In sorted order the numbers are:\n";• for (int index = 0; index < array_size; index++)• cout << a[index] << " ";• cout << endl;• delete [] a;• system("pause");• return 0;}• //Uses the library iostream:• void fill_array(int a[], int size)• { using namespace std;• cout << "Enter " << size << " integers.\n";• for (int index = 0; index < size; index++)• cin >> a[index];}• sort(int a[], int size)• //Any implementation of sort may be used. This may or may not require some • //additional function definitions. The implementation need not even know that • //sort will be called with a dynamic array. For example, you can use the • //implementation in Display 7.12 (with suitable adjustments to parameter names).

//DISPLAY 9.6 A Two-Dimensional Dynamic Array #include <iostream>using namespace std;

typedef int* IntArrayPtr;

int main( ){ int d1, d2; cout << "Enter the row and column dimensions of the array:\n"; cin >> d1 >> d2;

IntArrayPtr *m = new IntArrayPtr[d1]; int i, j; for (i = 0; i < d1; i++) m[i] = new int[d2]; //m is now a d1 by d2 array.

cout << "Enter " << d1 << " rows of " << d2 << " integers each:\n"; for (i = 0; i < d1; i++) for (j = 0; j < d2; j++) cin >> m[i][j];

cout << "Echoing the two-dimensional array:\n"; for (i = 0; i < d1; i++) { for (j = 0; j < d2; j++) cout << m[i][j] << " "; cout << endl; } for (i = 0; i < d1; i++) delete[] m[i]; delete[] m;system("pause"); return 0;}

• #include <iostream.h>• int mult( int a, int b)• {• return (a*b);• }

• main( )• {• int x, y, z;• cin >> x >> y >> z;• cout << "x = " << x << " y = " << y << " z = " << z << endl;• cout << "product1 ="<< mult (x ,y) << endl;• cout << "product2 " << mult (x +2, y) << endl;• system("pause");• return 0;• }