Programming fundamentals 2:pointers in c++ clearly explained

Programming fundamentals 2
Chapter 3:Pointer
Miss:Hanan Hardam
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Computer Skills2 for Scientific
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Pointers in C++
Topics to cover:
• Overview of Pointers
• Pointer Declaration
• Pointer Assignment
• Pointer Arithmetic
• Relations Between Pointers and Arrays
• Pointers and Strings

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Overview of Pointers
• A Pointer in C++ is variable whose value is a memory
address.
• With pointers many memory locations can be
referenced.
• Some data structures use pointers (e.g. linked list,
tree).
• The * and & operators
- & operator is the address operator
- * operator is the dereferencing operator. It is used in
pointers declaration

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Pointer Declaration
• Pointers are declared as follows:
<type> * variable_name ;
e.g.
int * xPtr; // xPtr is a pointer to data of type integer
char * cPtr; //cPtr is a pointer to data of type character
void * yPtr; // yPtr is a generic pointer,
// represents any type

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Pointer Assignment
• Assignment can be applied on pointers of the same type
• If not the same type, a cast operator must be used
• Exception: pointer to void does not need casting to convert a
pointer to void type
• void pointers cannot be dereferenced
• Example
int *xPtr, *yPtr; int x = 5;
…
xPtr = & x; // xPtr now points to address of x
yPtr = xPtr; // now yPtr and xPtr point to x

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Pointer Arithmetic
• Increment / decrement pointers ( ++ or -- )
• Add / subtract an integer to/from a pointer
( + or += , - or -= )
• Pointers may be subtracted from each other
• Pointer arithmetic is meaningless unless
performed on an array

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Pointer Arithmetic (Cont.)
• Example
Consider an integer array of 5 elements on a machine using 4
bytes for integers.
1000 1004 1008 1012 1016
Pointer variable vPtr
- vPtr pointes to first element V[0] (location 1000)
i.e. vPtr = 1000
- vPtr +=2; sets vPtr to 1008
i.e. vPtr points to V[2]
V[0] V[1] V[2] V[3] V[4]

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Pointer Arithmetic (Cont.)
• Subtracting pointers
- Returns the number of elements between two
addresses
e.g. if v is an array and
vPtr1 = v[0];
vPtr2 = v[2];
then vPtr2 – vPtr1 = 2 (i.e. 2 addresses)

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Relations Between Pointers and Arrays
• Arrays and pointers are closely related.
- Array name is like constant pointer
- Pointers can do array subscribing operations
- If we declare an array A[4] and a pointer aPtr
 aPtr is equal to A
aPtr == A
 aPtr is equal to the address of the first
element of A
aPtr == & A[0]

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Relations Between Pointers and Arrays (Cont.)
• Accessing array elements with pointers:
- Element A[i] can be accessed by *(aPtr+i)
 This is called pointer/offset notation
- Array itself can use pointer arithmetic
 A[3] is same as *(A+3)
- Pointers can be subscripted
(i.e. pointer/subscript notation)
 aPtr [3] is same as A[3]

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Examples on Pointers
//File: swap.cpp
//A program to call a function to swap two numbers using reference parameters
#include <iostream.h>
void swap(int *, int *); // This is swap's prototype
void main()
{ int x = 5, y = 7;
swap(&x , &y); // calling swap with reference parameters
cout << "n x is now "<< x << " and y is now " << y << 'n';
}
// swap function is defined here using dereferencing operator ‘*’
void swap(int *a, int *b)
{ int temp;
temp = *a;
*a = *b;
*b = temp;
}

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Examples on Pointers (Cont.)
//File: pointers.cpp
//A program to test pointers and references
void main ( )
{ int intVar = 10;
int *intPtr; // intPtr is a pointer
intPtr = & intVar;
cout << "nLocation of intVar: " << & intVar;
cout << "nContents of intVar: " << intVar;
cout << "nLocation of intPtr: " << & intPtr;
cout << "nContents of intPtr: " << intPtr;
cout << "nThe value that intPtr points to: " << * intPtr;
}

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Pointers and Strings
• Pointers can be used to declare strings and with string
functions (see next lecture slides)

When calling, the pointer formal parameter will points to the
actual parameter.
void Increment(int*);
void main() {
int A = 10;
Increment(&A);
cout<<A<<endl;
}
void Increment(int *X) { ++*X; }
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Programming fundamentals 2:pointers in c++ clearly explained

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