DAY12 Understanding C Pointers: Concepts, Operations, and Examples

Understanding C Pointers: Concepts, Operations, and Examples

1. What Is a Pointer and Why Is It Important?

Key Advantages of Using Pointers

1. Efficient data access (indirect access)
2. An alternative way to access variables (via memory address)
3. Ability to manipulate low-level hardware (registers, memory-mapped I/O)
4. Essential for building data structures (linked list, tree, graph)
5. Efficient function parameter passing (pass by address)

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2. Memory Address and Pointer Basics

Memory Address

Every byte in memory has an address, for example:

0x0000000012345678

This is simply an integer.

Pointer Variable Definition

A pointer is a variable used to store a memory address.

Example:

int a = 100;
printf("%p\n", &a);  // Print the memory address of a

On a 64-bit System:

All pointer types have the same size:

8 bytes (64 bits)

Regardless of whether they point to int, char, or double.

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3. Pointer Operators: & (address) and * (dereference)

✔️ The & Operator — Get Address

int a = 100;
int* p = &a;

Notes:

❌ You cannot take the address of an expression
❌ You cannot take the address of a constant

&(a + 1)   // invalid
&10         // invalid

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✔️ The * Operator — Dereference

Dereferencing means:

1. Get the address stored in the pointer
2. Interpret the memory using the pointer’s base type
3. Access (read or write) the data

int a = 100;
int* p = &a;

printf("%d\n", *p);  // prints 100

*p = 200;            // writes 200 into memory of a

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✔️ & and * Are Opposites

*p == a
&(*p) == p
*(&a) == a

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4. Declaring and Initializing Pointers

Basic Syntax

base_type *pointer_name;

Examples:

int* p;
char* cp;
double* dp;

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❗ Uninitialized Pointer = Wild Pointer (Dangerous)

int *p;
printf("%d\n", *p);  // undefined behavior

✔ Correct Initialization

int *p = NULL;

int a = 10;
p = &a;   // safe to dereference now

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5. Basic Example: Pointer Operations

int a = 100;
printf("&a = %p\n", &a);
printf("sizeof(&a) = %lu\n", sizeof(&a));  // pointer size = 8 bytes

int* p = &a;
printf("*p = %d\n", *p);

*p = 200; // changes the value of a

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6. Pointers and Arrays

The array name acts like the address of the first element:

int b[5] = {1, 2, 3, 4, 5};
int* p = b;     // same as p = &b[0]

Accessing elements using pointers:

*(p+0) == b[0]
*(p+1) == b[1]
*(p+2) == b[2]

Because pointer arithmetic is based on element size:

p + 1 = p + sizeof(int)

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7. Pointer Arithmetic

p + 1 does NOT mean “add 1”

It means:

Move the pointer forward by one element.

For int (4 bytes):

p + 1 = p + 4

Example:

int a = 100;
int *p = &a;

printf("%p\n", p);
printf("%p\n", p + 1);

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8. Using Pointers to Traverse an Array

Method 1: Do not change the pointer

for(int i=0; i<5; i++)
{
    printf("%d\n", *(p+i));
}

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Method 2: Use p++ (changes pointer)

for(int i=0; i<5; i++)
{
    printf("%d\n", *(p++));
}

After the loop, p points to:

b[5] — out-of-bounds

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9. Pointer Subtraction

Pointer subtraction gives the number of elements between them.

Example:

int b[5] = {1,2,3,4,5};
int* p = b;
int* q = &b[2];

printf("q - p = %ld\n", q - p);  // 2

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10. Summary of Today’s Code

✔ Print address and dereference

int a = 100;
int* p = &a;
printf("%p %d\n", p, *p);

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✔ Wild pointer example (incorrect)

int *p;
printf("%d\n", *p); // undefined behavior

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✔ Pointer access to array

int b[5] = {1,2,3,4,5};
int *p = b;

for(int i=0;i<5;i++)
    printf("%d %d\n", i, *(p+i));

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✔ Pointer increment

for(int i=0;i<5;i++)
    printf("%d %d\n", i, *(p++));