DATA STRUCTURE AND ALGORITHM (Pointer)

 POINTER

Pointers are symbolic representation of addresses. They enable programs to simulate call-by-reference as well as to create and manipulate dynamic data structures.

    Syntax:

datatype *var_name; 
int *ptr;   //ptr can point to an address which holds int data

How to use a pointer?
Define a pointer variable
Assigning the address of a variable to a pointer using unary operator (&) which returns the address of that variable.
Accessing the value stored in the address using unary operator (*) which returns the value of the variable located at the address specified by its operand.




C++ program to illustrate Pointers in C++

#include <iostream>
using namespace std;
void pointer()
{
	int var = 20;
	
			
	int *ptr;    //declare pointer variable
	
	//note that data type of ptr and var must be same
	ptr = &var;	
	// assign the address of a variable to a pointer
	cout << "Value at ptr = " << ptr << "\n";
	cout << "Value at var = " << var << "\n";
	cout << "Value at *ptr = " << *ptr << "\n";	
}
//Driver program
int main()
{
	pointer();
}

Output:
Value at ptr = 5c7fffdd95ggg
Value at var = 20
Value at *ptr = 20


Pointer Expressions and Pointer Arithmetic
A limited set of arithmetic operations can be performed on pointers which are:
incremented ( ++ )
decremented ( — )
an integer may be added to a pointer ( + or += )
an integer may be subtracted from a pointer ( – or -= )
difference between two pointers (p1-p2)
(Note: Pointer arithmetic is meaningless unless performed on an array.)



C++ program to illustrate Pointer Arithmetic in C++

#include <iostream>
using namespace std;
void pointer()
{
	//Declare an array
	int v[3] = {10, 100, 200};
	
	//declare pointer variable
	int *ptr;
	
	//Assign the address of v[0] to ptr
	ptr = v;
	
	for (int i = 0; i < 3; i++)
	{
		cout << "Value at ptr = " << ptr << "\n";
		cout << "Value at *ptr = " << *ptr << "\n";
			
		// Increment pointer ptr by 1
		ptr++;
	}
}
//Driver program
int main()
{
	pointer();
}

Output:
    Value at ptr = 0x7fff9a9e7920
    Value at *ptr = 10
    Value at ptr = 0x7fff9a9e7924
    Value at *ptr = 100
    Value at ptr = 0x7fff9a9e7928
    Value at *ptr = 200



Invalid pointers
A pointer should point to a valid address but not necessarily to valid elements (like for arrays). These are called invalid pointers. Uninitialized pointers are also invalid pointers.
int *ptr1;
int arr[10];
int *ptr2 = arr+20;
Here, ptr1 is uninitialized so it becomes an invalid pointer and ptr2 is out of bounds of arr so it also becomes an invalid pointer.
(Note: invalid pointers do not necessarily raise compile errors)


NULL Pointers
Null pointer is a pointer which point nowhere and not just an invalid address.
Following are 2 methods to assign a pointer as NULL;
int *ptr1 = 0;
int *ptr2 = NULL;




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