C Programming Notes — BCA Sem 1

C Programming — Introduction

C ek compiled, procedural, general-purpose programming language hai jo 1972 mein Dennis Ritchie ne Bell Labs mein banai.

Kyon C seekhein?

• Operating systems, kernels C mein likhe hain (Linux, Windows kernel)
• Hardware-level programming possible hai
• Fast execution — no garbage collection
• Sabhi modern languages C se influenced hain

Program Structure:

#include <stdio.h>   // Header file — printf, scanf
#include <stdlib.h>  // malloc, free, exit

int main() {          // Entry point — every C program starts here
    printf("Hello, BCA World!\n");
    return 0;          // 0 = success, non-zero = error
}

Compilation process:

Source (.c) → [Preprocessor] → [Compiler] → [Assembler] → [Linker] → Executable

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1. Data Types & Variables

// Basic data types
int     age = 20;          // 4 bytes, -2B to +2B
float   gpa = 8.5f;        // 4 bytes, ~7 decimal digits
double  pi = 3.14159265;   // 8 bytes, ~15 decimal digits
char    grade = 'A';       // 1 byte, ASCII character
long    population = 7800000000L;  // 8 bytes

// Type modifiers
short int   x = 100;      // 2 bytes
unsigned int u = 4294967295;  // no negative, 0 to ~4B
long double  ld = 3.14L;  // 10-16 bytes

// Constants
#define PI 3.14159   // Preprocessor constant (no memory)
const int MAX = 100; // Const variable (has memory, type-safe)

Format Specifiers (printf/scanf): | Type | Specifier | |------|-----------| | int | %d | | float | %f | | double | %lf | | char | %c | | string | %s | | pointer | %p | | hex | %x |

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2. Operators

// Arithmetic
int a = 10, b = 3;
printf("%d\n", a + b);   // 13
printf("%d\n", a - b);   // 7
printf("%d\n", a * b);   // 30
printf("%d\n", a / b);   // 3 (integer division)
printf("%d\n", a % b);   // 1 (remainder/modulus)

// Relational — return 0 or 1
printf("%d\n", a == b);  // 0 (false)
printf("%d\n", a != b);  // 1 (true)
printf("%d\n", a > b);   // 1 (true)

// Logical
printf("%d\n", (a > 5) && (b < 5));  // 1 (AND)
printf("%d\n", (a < 5) || (b < 5));  // 1 (OR)
printf("%d\n", !(a > 5));            // 0 (NOT)

// Bitwise
printf("%d\n", 5 & 3);  // 1  (101 & 011 = 001)
printf("%d\n", 5 | 3);  // 7  (101 | 011 = 111)
printf("%d\n", 5 ^ 3);  // 6  (101 ^ 011 = 110)
printf("%d\n", ~5);     // -6 (bitwise NOT)
printf("%d\n", 5 << 1); // 10 (left shift = multiply by 2)
printf("%d\n", 8 >> 2); // 2  (right shift = divide by 4)

// Increment/Decrement
int x = 5;
printf("%d\n", x++); // 5 (print then increment)
printf("%d\n", ++x); // 7 (increment then print)

---

3. Control Flow

// if-else if-else
int marks = 75;
if (marks >= 90)      printf("A+\n");
else if (marks >= 75) printf("A\n");    // This runs
else if (marks >= 60) printf("B\n");
else                  printf("Fail\n");

// switch-case (for discrete values)
char grade = 'B';
switch(grade) {
    case 'A': printf("Excellent"); break;
    case 'B': printf("Good");      break;  // This runs
    case 'C': printf("Average");   break;
    default:  printf("Try again");
}

// Ternary operator
int max = (a > b) ? a : b;  // min one-liner

// Loops
// for loop
for (int i = 1; i <= 5; i++) {
    printf("%d ", i);  // 1 2 3 4 5
}

// while loop
int n = 1;
while (n <= 5) {
    printf("%d ", n++);
}

// do-while (runs at least once)
int choice;
do {
    printf("Enter choice (1-3): ");
    scanf("%d", &choice);
} while (choice < 1 || choice > 3);

// break and continue
for (int i = 0; i < 10; i++) {
    if (i == 5) break;     // Exit loop when i=5
    if (i % 2 == 0) continue; // Skip even numbers
    printf("%d ", i);     // 1 3
}

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4. Functions

// Function declaration (prototype)
int add(int a, int b);
void printHello(void);
float calculateArea(float r);

// Function definition
int add(int a, int b) {
    return a + b;
}

void printHello() {  // void = no return value
    printf("Hello from BCA!\n");
}

float calculateArea(float r) {
    return 3.14159 * r * r;
}

// Recursive function — factorial
long factorial(int n) {
    if (n <= 1) return 1;  // Base case
    return n * factorial(n - 1);  // Recursive case
}
// factorial(5) = 5 × 4 × 3 × 2 × 1 = 120

// Call by value — original NOT modified
void increment(int x) {
    x++;  // Only local copy changed
}

// Call by reference (using pointer) — original IS modified
void incrementByRef(int *x) {
    (*x)++;  // Dereference and modify original
}

int main() {
    int num = 10;
    increment(num);      // num still 10
    incrementByRef(&num); // num now 11
    printf("%d\n", num); // 11
}

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5. Arrays & Strings

// 1D Array
int marks[5] = {85, 90, 78, 92, 88};
float avg = 0;
for (int i = 0; i < 5; i++)
    avg += marks[i];
avg /= 5;

// 2D Array (Matrix)
int matrix[3][3] = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};
printf("%d\n", matrix[1][2]); // 6

// Strings — char array ending with '\0'
char name[20] = "BCA Student";
printf("%s\n", name);       // BCA Student
printf("%lu\n", strlen(name)); // 11 (without \0)

// String functions (string.h)
char s1[20] = "Hello";
char s2[] = "World";
strcat(s1, s2);      // s1 = "HelloWorld"
strcpy(s1, "New");   // s1 = "New"
strcmp("abc","abc"); // 0 (equal), <0 (s1<s2), >0 (s1>s2)
strlen("Hello");     // 5

// String input
char city[50];
printf("Enter city: ");
fgets(city, 50, stdin);  // Safe — prevents buffer overflow
// Note: fgets includes '\n', use scanf("%49s", city) for single word

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6. Pointers

int x = 42;
int *p = &x;    // p = address of x (e.g., 0x7ffee4b8)

printf("%d\n",  x);   // 42 — value of x
printf("%p\n", &x);   // 0x7ffee4b8 — address of x
printf("%p\n",  p);   // 0x7ffee4b8 — value of p (= address of x)
printf("%d\n", *p);   // 42 — value at address p (dereference)

*p = 100;   // Modify x through pointer
printf("%d\n", x);  // 100

// Pointer arithmetic
int arr[] = {10, 20, 30, 40, 50};
int *ptr = arr;       // Points to arr[0]
printf("%d\n", *ptr);     // 10
printf("%d\n", *(ptr+1)); // 20 (moves 4 bytes forward)
ptr++;
printf("%d\n", *ptr);     // 20

// Array as pointer
for (int i = 0; i < 5; i++) {
    printf("%d ", *(arr + i));  // Same as arr[i]
}

// Dynamic memory allocation
int *dynArr = (int*)malloc(5 * sizeof(int));  // Heap allocation
if (dynArr == NULL) { printf("Memory error"); exit(1); }
for (int i = 0; i < 5; i++) dynArr[i] = i * 10;
free(dynArr);  // Always free! Memory leak warna
dynArr = NULL; // Good practice

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7. Structures & Unions

// Structure — each member has own memory
struct Student {
    int rollNo;
    char name[50];
    float gpa;
};  // Note: semicolon required

struct Student s1 = {101, "Rahul", 8.5};
printf("%s - GPA: %.2f\n", s1.name, s1.gpa);

// Typedef for easier use
typedef struct {
    int x, y;
} Point;

Point p = {10, 20};
printf("(%d, %d)\n", p.x, p.y);

// Structure with pointer
typedef struct Node {
    int data;
    struct Node *next;  // self-referential
} Node;

// Union — shared memory
union Data {
    int i;      // 4 bytes
    float f;    // 4 bytes
    char c;     // 1 byte
};  // sizeof(union Data) = 4 (max member)

union Data d;
d.i = 42;
printf("%d\n", d.i);  // 42
d.f = 3.14f;          // Now i is corrupted!
printf("%f\n", d.f);  // 3.14

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8. File Handling

#include <stdio.h>

// Write to file
FILE *fp = fopen("data.txt", "w");  // Open for writing
if (fp == NULL) {
    printf("Cannot open file!");
    exit(1);
}
fprintf(fp, "Roll: %d, Name: %s\n", 101, "Alice");
fclose(fp);  // Always close!

// Read from file
fp = fopen("data.txt", "r");
char line[100];
while (fgets(line, 100, fp) != NULL) {
    printf("%s", line);
}
fclose(fp);

// File modes:
// "r" — read, "w" — write (overwrite), "a" — append
// "rb","wb" — binary mode

// fprintf / fscanf
fp = fopen("marks.txt", "w");
for (int i = 0; i < 3; i++) {
    int roll; float gpa;
    scanf("%d %f", &roll, &gpa);
    fprintf(fp, "%d %.2f\n", roll, gpa);
}
fclose(fp);

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Exam Important Questions

1. Data types aur sizeof() explain karo
2. Pointers kya hain? Pointer arithmetic example ke saath explain karo
3. Call by value vs Call by reference difference aur example
4. Recursive function likhein — factorial ya fibonacci
5. Structure aur Union mein difference example ke saath
6. Dynamic memory allocation — malloc, calloc, realloc, free
7. File handling — write aur read program likhein
8. 2D array matrix multiplication program

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Viva Questions

Q: void pointer kya hota hai? A: void *p — kisi bhi data type ka address store kar sakta hai. Generic pointer. Dereference karne se pehle type cast karna padta hai: (int)p.

Q: Stack aur Heap memory mein kya fark hai C mein? A: Stack — local variables, function calls (automatic management, fast, limited size). Heap — malloc/calloc se dynamic allocation, programmer manages, larger, slower.

Q: Dangling pointer kya hota hai? A: Jab pointer ek freed memory location ko point karta rahe — use-after-free bug. free(ptr); ptr = NULL; se prevent karo.