Pointers in C and C++ | Set 1 (Introduction, Arithmetic and Array)

2.4

Pointers store address of variables or a memory location.

// General syntax
datatype *var_name; 

// An example pointer "ptr" that holds
// address of an integer variable or holds
// address of a memory whose value(s) can
// be accessed as integer values through "ptr"
int *ptr;  

Using a Pointer:

To use pointers in C, we must understand below two operators.

  • To access address of a variable to a pointer, we use the unary operator & (ampersand) that returns the address of that variable. For example &x gives us address of variable x.
    // The output of this program can be different
    // in different runs. Note that the program
    // prints address of a variable and a variable
    // can be assigned different address in different
    // runs.
    #include <stdio.h>
    
    int main()
    {
        int x;
    
        // Prints address of x
        printf("%p", &x);
    
        return 0;
    }
    
  • One more operator is unary * (Astrik) which is used for two things :
    • To declare a pointer variable: When a pointer variable is declared in C/C++, there must a * before its name.

      // C program to demonstrate declaration of
      // pointer variables.
      #include <stdio.h>
      int main()
      {
          int x = 10;
      
          // 1) Since there is * in declaration, ptr
          // becomes a pointer varaible (a variable
          // that stores address of another variable)
          // 2) Since there is int before *, ptr is
          // pointer to an integer type variable
          int *ptr;
      
          // & operator before x is used to get address
          // of x. The address of x is assigned to ptr.
          ptr = &x;
      
          return 0;
      }
    • To access the value stored in the address we use the unary operator (*) that returns the value of the variable located at the address specified by its operand.
      // C program to demonstrate use of * for pointers in C
      #include <stdio.h>
      
      int main()
      {
          // A normal integer variable
          int Var = 10;
      
          // A pointer variable that holds address of var.
          int *ptr = &Var;
      
          // This line prints value at address stored in ptr.
          // Value stored is value of variable "var"
          printf("Value of Var = %d\n", *ptr);
      
          // The output of this line may be different in different
          // runs even on same machine.
          printf("Address of Var = %p\n", ptr);
      
          // We can also use ptr as lvalue (Left hand
          // side of assignment)
          *ptr = 20; // Value at address is now 20
      
          // This prints 20
          printf("After doing *ptr = 20, *ptr is %d\n", *ptr);
      
          return 0;
      }
      

      Output :

      Value of Var = 10
      Address of Var = 0x7fffa057dd4
      After doing *ptr = 20, *ptr is 20
      

      Below is pictorial representation of above program:
      pointers in c

 

Pointer Expressions and Pointer Arithmetic

A limited set of arithmetic operations can be performed on pointers. A pointer may be:

  • incremented ( ++ )
  • decremented ( — )
  • an integer may be added to a pointer ( + or += )
  • an integer may be subtracted from a pointer ( – or -= )

(Note: Pointer arithmetic is meaningless unless performed on an array.)

// C++ program to illustrate Pointer Arithmetic
// in C/C++
#include <bits/stdc++.h>

// Driver program
int main()
{
    // 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++)
    {
        printf("Value of *ptr = %d\n", *ptr);
        printf("Value of ptr = %p\n\n", ptr);

        // Increment pointer ptr by 1
        ptr++;
    }
}
Output:Value of *ptr = 10
Value of ptr = 0x7ffcae30c710

Value of *ptr = 100
Value of ptr = 0x7ffcae30c714

Value of *ptr = 200
Value of ptr = 0x7ffcae30c718

Untitled presentation (3)

 

Array Name as Pointers

An array name acts like a pointer constant. The value of this pointer constant is the address of the first element.
For example, if we have an array named val then val and &val[0] can be used interchangeably.

// C++ program to illustrate Array Name as Pointers in C++
#include <bits/stdc++.h>
using namespace std;

void geeks()
{
    // Declare an array
    int val[3] = { 5, 10, 20 };

    // Declare pointer variable
    int *ptr;

    // Assign address of val[0] to ptr.
    // We can use ptr=&val[0];(both are same)
    ptr = val ;
    cout << "Elements of the array are: ";
    cout << ptr[0] << " " << ptr[1] << " " << ptr[2];

    return;
}

// Driver program
int main()
{
    geeks();
    return 0;
}
Output:
Elements of the array are: 5 10 20

Untitled presentation (2)
Now if this ptr is sent to a function as an argument then the array val can be accessed in a similar fashion.

 

Pointers and Multidimensional Arrays

Consider pointer notation for the two-dimensional numeric arrays. consider the following declaration

int nums[2][3]  =  { {16, 18, 20}, {25, 26, 27} };

In general, nums[i][j] is equivalent to *(*(nums+i)+j)

Screenshot (22)

Related Articles:

 

QuizzesQuiz on Pointer Basics , Quiz on Advanced Pointer

Reference:
https://www.ntu.edu.sg/home/ehchua/programming/cpp/cp4_PointerReference.html

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