Purpose of C

Dennis Ritchie developed C to make system software (like operating systems). UNIX is one of the first operating systems written in C, which made it portable and efficient.

Features of C

C became popular because it:

1. Combined the efficiency of assembly language with the simplicity of a high-level language.
2. Allowed writing programs that can run on different computers portable.
3. Became a foundation for many modern languages, such as C++, Java, and Python.

Versions of C

Version	Year	Description
K&R C	1978	First official version (by Dennis Ritchie).
ANSI C	1989	Standardized the language.
C89 / C90	1989–1990	International Standard (ANSI: American National Standards Institute)
C99	1999	Added new features like inline functions, variable-length arrays
C11	2011	Added multi-threading support
C17 / C18	2017–2018	Minor updates & fixes

Why C is Important?

1. It's called the “Mother of Programming Languages.”
2. Many modern languages like C++, Java, C#, and Python were built on concepts of C.
3. It's still used in:

• System programming
  • Embedded programming
  • Compilers
  • Operating system development.

Summary

**Developed by:** Dennis Ritchie **Year:** 1972 **Place:** Bell Laboratories (A&T) **Purpose:** To develop the UNIX OS **Derived from:** B & BCPL **Known as:** ==Mother of all modern programming languages==

Purpose of Flowcharts

1. To understand the logic of a program easily
2. To plan a program before writing the actual code
3. To debug/explain the process to others

Features of C

1. Simple and easy C uses simple syntax Easy to learn commands
2. Portable C programs can run on different computers without change
3. Structured Languages A program can be divided into functions/blocks.
4. Rich Library C has many built-in functions like printf() and scanf().
5. Middle-Level Language Supports both low-level and high-level programming.
6. Modular Large programs can be broken into smaller functions.

Tokens In C

Tokens are the smallest building blocks of a C program. They are like words in a language.

Types of Tokens

1. Keywords
2. Identifiers
3. Constants
4. Operators
5. Strings
6. Special Symbols

1 Keywords

Keywords are reserved words with special meaning in C.

Rules

1. You cannot use them as variable names.
2. They tell the compiler what to do.

Examples: int, float, if, else, for, while, return, char, const, void, switch.

2 Identifiers

Identifiers are names given by the programmer to things like:

• variables
• functions
• arrays
• structures, etc...

Purpose: They help identify data and functions in a program.

3 Constants

Constants are values that do not change during program execution.

Types of Constants

Type	Example
Integer Const	10, -5, 2, 3
Floating Const	5.55, 7.25, 9.24
Char Const	'A', 'B', 'C'
String Const	"My Name"

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4. Operators in C

An operator is a symbol that tells the compiler to perform an operation such as addition, comparison, assignment (+, <, =.) etc...

Types of Operators:

1. Arithmetic Operators (+, -, *, /, %).
2. Relational Operators (==, !=, >, <, >=).
3. Logical Operators (&&, !).
4. Assignment Operators (=, +=, -=, *=, /=)
5. Increment & Decrement Operators (i++, --i)
6. Ternary Operator (? 😃

1. Arithmetic Operators

Operator	Meaning
+	Addition
-	Subtraction
*	Multiplication
/	Division
%	Modulus (remainder)

2. Relational Operators

Operator	Meaning
==	Equal to
!=	Not equal
>	Greater than
<	Less than
>=	Greater than or equal

3. Logical Operators

Operator	Meaning / Example
&&	Logical AND → (a > 0 && b > 0)
!	Logical NOT → !(a > 0)

4. Assignment Operators

Operator	Meaning / Example
=	Assign value → a = 10
+=	Add & assign → a += 5
-=	Subtract & assign → a -= 5
*=	Multiply & assign → a *= 5
/=	Divide & assign → a /= 5

5. Increment & Decrement Operators

Form	Meaning
i++	Post-increment → Use first, then add 1
++i	Pre-increment → Add 1 first, then use
i--	Post-decrement → Use first, then subtract 1
--i	Pre-decrement → Subtract 1, then use

6. Ternary Operator

Symbol: ? : It can be used instead of a simple if–else.

Example:

a = 5;
b = 6;
a > b ? a : b;

Output:

6

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5. Strings

A string is a sequence of characters enclosed in double quotes.

Properties:

• Always ends with a null character \0 internally
• Stored as an array of characters

Correct Example:

char name[] = "c";
printf("%s", name);

Output:

c

Fixed: strings must use double quotes, not single quotes.*

6. Special Symbols

Symbol	Meaning
;	End of statement
{ }	Start & end of block / function
( )	Used in functions & conditions
[ ]	Arrays
,	Separator between variables
#	Preprocessor directive (#include)
' '	Character constant
" "	String constant
\	Escape character

Variables in C

A variable is a named memory location used to store data that can change.

Why Variables Are Needed? Variables store data during program execution. A variable is a container that stores a value.

Variables are needed to:

• Store input data
• Perform calculations
• Store program results
• Make code readable and reusable

Variable Declaration

Tells the compiler what type of data the variable will store.

Example:

int a;
float b;
char c;

Variable Initialization

Assigning a value to a variable.

Example:

a = 10;
b = 10.5;
c = 'M';
stockerid = "SSzxxc1";

Rules for Naming Variables

Must start with a letter (a–z or A–Z) or underscore (_)

• 1a = 10; → ❌ Invalid
• A = 10; → ✔️ Valid
• _a = 10; → ✔️ Valid
• a = 10; → ✔️ Valid

Examples of valid identifiers:

int age;
int _height;

Can contain only:

• letters
• digits
• underscores _

Examples:

Valid	Invalid
int total_marks;	int total-mark;
int sum1;	int price#;

No spaces allowed

Valid	Invalid
int total_marks;	int total marks;

Cannot use keywords: Examples: int, float, if, else cannot be used as variable names

Case Sensitive C is a case-sensitive language, meaning uppercase and lowercase letters are treated as different variables.

Example:

int age = 10;
Age = 25;
printf("%d", age);   // Output: 10

Note

Here, age and Age are different.

No Special Characters Allowed (Except Underscore _)

You cannot use characters like:

• @
• %
• -
• #
• $

Examples:

Valid	Invalid
total_marks	total-marks
sum1	sum@

Length of Variable Name There is no strict limit, but most compilers recognize only the first 31 characters.

Meaningful and Readable Names Use meaningful names. Avoid:

int a, b, c;   // Meaningless

Data Types

Data types define:

• what kind of data a variable can store
• how much memory it occupies

1 Primitive Data Types

Data Type	Size (bytes)	Format Specifier	Example
int	2 or 4	%d	int a = 10;
float	4	%f	float b = 10.5;
double	8	%lf	double d = 8.54;
char	1	%c	char c = 'M';

2. Derived Data Types

Derived Type	Definition	Example
1. Array	Group of same-type elements	c int a[5]; a = {1, 2, 3, 4, 5};
2. Pointer	A variable that stores the address of another variable	c int *p;
3. Structure	Group of different data types	c struct Student { ... };
4. Union	Shares the same memory location	c union info { ... };
5. Function	Returns a value after execution	c int add(int a, int b) { ... }

3. User-Defined Data Types

Keyword	Definition	Example
typedef	Gives a new name to an existing type	c typedef int num;
enum	Defines a set of named constants	c enum day { Mon, Tue, Wed };

Data Type Modifiers Used to change the size or range of basic data types:

Modifier	Used With	Example
short	int	short int a;
long	int, double	long int b;
signed	int, char	signed char c;
unsigned	int, char	unsigned char c;

Structure of a C Program

A C program is divided into 6 sections:

1. Documentation Section
2. Link Section
3. Definition Section
4. Global Declaration Section
5. main() Function Section
6. Sub-program Section (User-defined functions)

main() Function Section

{
    Declaration Part
    Executable Part
}


// * The **declaration part** contains variable declarations
// * The **executable part** contains statements to be executed



// Subprogram Section
// Contains user-defined functions:


Function 1
Function 2

Function n

Documentation Section

• This part contains information about the program such as name, purpose, author, and date.
• It is written as comments, and the compiler ignores it.

Preprocessor Section

• This section includes header files using the #include statement.
• Header files contain predefined functions like printf() and scanf().

Syntax

#include <stdio.h>   // stdio → standard input/output
#include <math.h>    // math.h → mathematical functions

Definition Section

• Used to define constants or macros before the main program.
• #define keyword is used for constant definitions.

Example:

#define PI 3.14

Note

Whenever the compiler finds `PI`, it replaces it with `3.14`.

Global Declaration Section

• Variables or functions declared here can be used anywhere in the program.
• They are defined outside the main() function.

Example:

int num = 10;

main() Function Section

• This is the starting point of every C program.
• The compiler starts execution from the main() function.
• The main function can return a value (usually int).

Syntax

int main()
{
    // variable declaration
    // logic / process
    
    return 0;
}

Note

Everything that happens in a C program runs inside the `{}` of the **main()** function.

Subprograms (User-Defined Functions)

• These are functions created by the programmer to perform specific tasks.
• They make programs modular and easier to reuse.

I/O Statements in C

In C, input and output are done using certain functions. There are 2 types:

1. Formatted I/O
2. Unformatted I/O

Formatted I/O

These allow formatted input and output using format specifiers. 1) printf() → OutputSyntax:

printf("%d", a);

Common Format Specifiers

Format Specifier	Meaning
%d	int
%f	float
%s	string
%c	char
%lf	double

Example (double output)

int a = 10;
printf("%d", a);

2) scanf() → Input

Syntax

scanf("format", &variable_name);

Example

int a;
scanf("%d", &a);