Programming Essentials in C - Lectures 9

What is a Pointer

A pointer in C is a derived data type used to store the address of another variable.
It allows access and modification of data located in memory. Pointers are essential for:

• Efficient data passing between functions
• Creating dynamic data structures (linked lists, trees, graphs)

Pointer Declaration

To declare a pointer, use the dereferencing operator (*) with a valid C data type:

Syntax:
type *varname

• type is the base type of the pointer
• varname is the name of the pointer

Note: The * in the declaration is not multiplication—it signifies that the variable is a pointer.

Example:

int *intPtr;     // integer pointer
float *floatPtr; // float pointer
char *charPtr;   // char pointer

Pointer Initialization

A pointer is initialized using the address-of operator (&) to assign it the address of a variable.

Example:

int var = 5;
// stores the address of `var` in memory
int *ptr = &var;

// dereference the pointer to access its value
printf("%d => %d\n", var, *ptr); // 5 => 5

Referencing and Dereferencing Pointers

• & (Address-of Operator): Used to get the address of a variable.
• * (Dereference Operator): Used to access the value at the memory address held by a pointer.

Access and Manipulate Values Using Pointers

You can modify the value of a variable via its pointer.

int var = 5;
int *ptr = &var;

printf("%d => %d\n", var, *ptr); // 5 => 5

*ptr = 10; // modifies both `ptr` and `var`

printf("%d => %d\n", var, *ptr); // 10 => 10

Printing the address of a variable:

int var = 5;
int *ptr = &var;

printf("value of &var: %p\n", &var); // using %p for pointers
printf("value of ptr: %p\n", ptr);   // prints the same address

NOTE

The type of a variable and its pointer must match.

Output:

value of &var: 0x7ffd5cbbef94
value of ptr:  0x7ffd5cbbef94

Pointer to a Pointer

A pointer can also store the address of another pointer.

int var = 5; // integer variable
int* ptr = &var; // integer pointer to a variable
int** ptrToptr = &ptr; // integer pointer to a pointer

Pointer Arithmetic in C

Increment and Decrement of a Pointer

Incrementing a pointer moves it to the next memory location based on the data type size.

int x = 10;
int *y = &x;

printf("Value of y before increment: %d\n", y);
y++;
printf("Value of y after increment: %d\n", y);

Output:
Pointer y increases by sizeof(int) (usually 4 bytes).
Notice that the address has changed (0x7ff...44 -> 0x7ff...48).

Value of y before increment: 0x7ffc32820844
Value of y after increment:  0x7ffc32820848

NOTE

Memory addresses change with each run.

Decrementing a pointer in an array

int arr[] = { 10, 20, 30 };
int* ptr = &arr[2];

for (int i = 0; i < 3; i++) {
  printf("%d ", *ptr); // print the current value
  ptr--; // move the pointer to the previous element
}

Addition and Subtraction of Integer to Pointer

Adding or subtracting an integer to a pointer moves it forward or backward in memory.

int arr[] = { 10, 20, 30, 40, 50 };
int* ptr = arr; // points to arr[0]

for (int i = 0; i < 5; i++) {
  // move the pointer by `i` positions in memory then print its value
  printf("%d ", *(ptr + i));
}

Summary

Expression	Description
ptr + n	Move forward to the $n^{th}$ next element
ptr - n	Move backward to the $n^{th}$ previous element
*(ptr + n)	Value at the $n^{th}$ next element
*(ptr - n)	Value at the $n^{th}$ previous element

Reverse Printing a String Using Pointer

#include <stdio.h>
#include <string.h> // for strlen

int main() {
  char str[] = "Pointer";
  char* ptr = &str[strlen(str) - 1];

  printf("Reversed string: ");
  while (ptr >= str) {
    printf("%c", *ptr);
    ptr--;
  }

  printf("\n");
  return 0;
}

Output:

Reversed string: retnioP

Pointer to an Array

A pointer to an array points to the whole array, not just its first element.

int arr[5] = { 10, 20, 30, 40, 50 };

int (*ptr)[5] = &arr; // Pointer to the entire array

for (int i = 0; i < 5; i++) {
  printf("%d ", (*ptr)[i]);
}