Theory

1. C Language

• C is a structured, procedural language.
• Created by Dennis Ritchie in 1972.
• Fast and portable; can work close to hardware.
• Used for system software and general apps.

2. Structure of a C Program

Sections:

• Documentation (comments)
• Link section (#include header files)
• Global declarations
• main() function - starting point
• User-defined functions

Syntax:

#include <headerfile.h>  // Link section
// Global declarations

int main() {            // main function
    // code
    return 0;
}

// User-defined function
return_type function_name(parameters) {
    // function body
}

Example:

#include <stdio.h>
int main() {
    printf("Hello World");
    return 0;
}

3. Keywords

• Reserved words in C with predefined meaning.
• Cannot be used as identifiers.
• Examples: int, float, if, while, return

4. Identifiers

• Names for variables, functions, or arrays.

Rules:

• Start with a letter or underscore.
• Do not use keywords.
• Case-sensitive.

5. Constants

• Fixed values that do not change.

Types and examples:

• Integer: 10
• Floating: 3.14
• Character: 'A'
• String: "Hello"

6. Data Types

• Define what kind of data a variable can hold.

Types:

• Basic: int, float, double, char
• Derived: arrays, pointers
• Void: no value

Syntax:

data_type variable_name = value;

Example:

int age = 20;
float pi = 3.14;
char grade = 'A';

7. Operators

• Arithmetic: +, -, *, /, %
• Relational: ==, !=, >, <, >=, <=
• Logical: &&, ||, !
• Assignment: =, +=, -=, *=, /=
• Increment/Decrement: ++, --
• Conditional: ? :

Syntax Example:

int a = 5, b = 3;
int sum = a + b;        // arithmetic
int max = (a > b) ? a : b; // conditional

8. Control Structures

Decision Making:

If:

if(condition) {
    // code
}

If-Else:

if(condition) {
    // code
} else {
    // code
}

Switch:

switch(expression) {
    case value1:
        // code
        break;
    case value2:
        // code
        break;
    default:
        // code
}

Loops:

For:

for(initialization; condition; increment/decrement) {
    // code
}

While:

while(condition) {
    // code
}

Do-While:

do {
    // code
} while(condition);

Jump Statements:

• break - exit loop
• continue - skip current iteration
• goto - jump to label
• return - exit function

9. Functions

• Reusable block of code that performs a task.

Types:

• Library functions: printf(), scanf()
• User-defined functions

Syntax:

return_type function_name(parameters) {
    // code
    return value;  // if return_type is not void
}

Example:

#include <stdio.h>
void greet() {
    printf("Hello!\n");
}
int main() {
    greet();
    return 0;
}

Call Methods:

• Call by value: copy passed; original unchanged
• Call by reference: address passed; original may change

10. Arrays

• Collection of same-type elements stored in order.

Syntax:

• 1D Array: data_type array_name[size];
• 2D Array: data_type array_name[rows][columns];

Examples:

int arr[5] = {1,2,3,4,5};
int matrix[2][2] = {{1,2},{3,4}};

11. Strings

• Character array terminated by a null character \0.

Syntax:

char str[size] = "text";

Example:

char name[10] = "CSE";

Common Functions:

• strlen(str) - returns length
• strcpy(dest, src) - copy string
• strcmp(str1, str2) - compare strings
• strcat(dest, src) - concatenate strings

12. Pointers

• Variable that stores the address of another variable.

Syntax:

data_type *pointer_name;
pointer_name = &variable;

Example:

int x = 10;
int *p = &x;
printf("%d", *p);  // prints 10

Uses:

• Access arrays and strings
• Pass by reference in functions

13. File Handling

• Store and retrieve data permanently in files.

Functions:

• fopen() - open file
• fclose() - close file
• fgetc(), fputc() - read/write characters
• fprintf(), fscanf() - formatted I/O

Syntax:

FILE *fp;
fp = fopen("filename", "mode");
fclose(fp);

Modes:

• "r" - read
• "w" - write
• "a" - append
• "r+" - read and write

Example:

#include <stdio.h>
int main() {
    FILE *fp;
    fp = fopen("data.txt", "w");
    fprintf(fp, "Hello C\n");
    fclose(fp);
    return 0;
}

Key Differences (Plain Text, Bullets)

While loop and Do-while loop

• While: checks condition first; may not run.
• Do-while: runs once, then checks; runs at least once.
• While: entry-controlled. Do-while: exit-controlled.
• Use while when you may skip; use do-while when one run is needed.

For loop and While loop

• For: count known. While: count unknown.
• For: init, condition, update together. While: separate.
• For: compact for counter loops. While: flexible for condition loops.
• Example: For prints 1 to 100; while reads input until 0.

Array and String

• Array: stores same-type elements. String: char array ending with '\0'.
• Array can be numeric/char/etc.; string is always char.
• Array elements stand alone; string characters form text.
• Example: Array 3; String "Hello".

Array and Pointer

• Array: fixed-size values. Pointer: stores an address.
• Array size set at compile time; pointer flexible.
• Array accessed directly; pointer dereferenced with *.
• Example: Array int arr[5]; Pointer int *p = &arr[0].

Call by Value and Call by Reference

• Value: pass a copy; original unchanged.
• Reference: pass address; original may change.
• Value is safer; reference is more efficient but can modify input.
• Example: value swap(a,b); reference swap(&a,&b).

Local and Global Variable

• Local: declared inside a function; accessible only there.
• Global: declared outside; accessible everywhere.
• Local exists during the function call; global for the whole program.
• Example: Local int x inside main; Global int x outside all functions.