NBC 103 • Unit 314 min read★ High Exam Frequency

Pointers, Call by Reference, 2D Arrays & Strings

Q: What is a NULL pointer vs a Dangling pointer?

A NULL pointer explicitly points to address 0 (safe/nothing). A Dangling Pointer points to a memory location that has been freed/deallocated, causing undefined behavior if dereferenced. (Examiner Tip: Always set ptr = NULL after free(ptr).)

Unit 3: Arrays, Pointers & String Manipulation • Problem Solving Using C

👨‍🏫 Professor's Mental Model: The House Address Analogy

Variable tumhara ghar hai jisme data (jaise 42) rehta hai. Pointer ek paper slip hai jisme us ghar ka address (jaise 0x1000) likha hai. Address-of operator (&) address pata karta hai, aur Dereference operator (*) us address par jakar andar ka saman nikalta hai!

Pointer Operators (& and *) & Memory Addressing

Pointer variable stores the hexadecimal memory address of another variable. Syntax: int *ptr = &x; - & (Address-of): Returns address of variable in RAM. - * (Dereferencing): Accesses value stored at the target address. Pointer Arithmetic: Incrementing (ptr++) advances the address by sizeof(*ptr) bytes (e.g. +4 bytes for int).

Call by Reference (Simulation via Pointers)

Passing memory addresses (&a, &b) allows a function to modify variables in the caller's stack frame directly, overcoming C's single return value limitation.

Strings in C: Null-Terminated Character Arrays

A string is a 1D character array ending with '\0'. Important <string.h> functions: strlen(), strcpy(), strcat(), strcmp(), strrev().

Pointer Memory Architecture Diagram

Pointer: ptr (int *)

Stores Address0x1000

At RAM: 0x2000

Dereference *ptr

Variable: num (int)

Holds Value42

At RAM: 0x1000

💡 Professor Key Point: Memory address 0x1000 is passed to functions in Call by Reference.

Interactive Tested Code Example

#include <stdio.h>

void swap(int *x, int *y) {
    int temp = *x;
    *x = *y;
    *y = temp;
}

int main(void) {
    int a = 10, b = 25;
    printf("Before Swapping: a = %d, b = %d\n", a, b);
    swap(&a, &b); // Passing memory addresses
    printf("After Swapping:  a = %d, b = %d\n", a, b);
    return 0;
}

💡 Note:Demonstrates Call by Reference: swap() dereferences pointers *x and *y to exchange the original variables stored in main().

🎯 University Exam Scoring Blueprint

Draw memory blocks showing addresses (e.g. 0x1000) whenever explaining pointers.
Explain that array name (arr) acts as a constant pointer to its first element (&arr[0]).
State that uninitialized pointers point to random memory and are called Wild Pointers.