C Tokens, Keywords and Identifiers etc

Last Updated : Jan 2026

Let's try to understand the smallest meaningful pieces of C code.

Tokens

A token is the smallest meaningful unit of a C program recognized by the compiler. For example:

int age = 20;
// Don't worry what this is for now,we will study it later,for now just try to understand it

This line can be broken into:

int
age
=
20
;

Each is a token. Think of a sentence:

I am learning C.

It contains individual words and punctuation.

Types of C Tokens

C tokens can broadly be classified as:

C Tokens
│
├── Identifiers
├── Keywords
├── Constants
├── Operators
└── Punctuators / Special Symbols

We'll understand each of them one by one,but we'll learn more about each in upcoming sections.

Identifiers

An identifier is a name given by the programmer to a program element. Examples:

age, price, studentName, total, marks, calculate

These names can be used for things such as:

Variables
Functions
Structures
Other user-defined entities

For example:

int age;

Here:

age → identifier

The programmer chose the name age to store an value so we can easily communicate. It is just like :

In math we write
x=10,
y=20,
z=30

when we need 10,20,30 we just write x,y,z respectively.Here

x,y,z → identifiers
10,20,30 → values/data

Note : when we combine identifiers and values/data we call it variable.

int age = 20;
int months = 5;

Rules for Identifiers

Identifiers must begin with a letter or an underscore.
Never start with a number but number can be after letters or underscores.
Identifiers are case-sensitive.
Spaces are not allowed.
Can not redeclare an identifier with the same name.
Identifiers cannot be keywords.

e.g:

Valid Identifiers: age, studentAge, student_age, marks1, totalMarks, _private

Invalid Identifiers:1age, student age, student-age, int, float

because of:
1age        → starts with digit
student age → contains space
student-age → '-' is not allowed in an identifier
int         → keyword
float       → keyword

Case-Sensitivity: age, Age, AGE, aGe

age ≠ Age ≠ AGE ≠ aGe

Keywords

A keyword is a reserved word that has a predefined meaning in C. For example:

int

means something specific to the compiler. You cannot normally use int as your own identifier name. For example:

int int = 10;

is invalid. Why? Because int already has a special meaning.

Some Common C Keywords

You don't need to memorize all of them right now.

int, char, float, double, void
if, else, switch, case, default
for, while, do
break, continue, return
struct, union, enum
const, static, extern
sizeof, typedef

We'll learn these gradually.

Constants

A constant is a value that does not change during the relevant execution/context of the program. For example:

int age = 10
// 10 is a constant

There are several types of constants:

Integer Constants (Whole Numbers)
Floating-Point Constants (Real/Decimal Numbers)
Character Constants (Single Character)
String Constants (Text) (Sequence of characters)

We'll study constants in much greater detail later. For now:

10, 20, 100, 3.14, 'A',"Hello"

are examples of literal constant values.

Operators

Operators are symbols that tell C to perform operations. For example:

a + b

Here:

a → operand
+ → operator
b → operand

Some common operators:

+, -, *, /, %, =, ==, >, <, &&, ||, !

We'll have an entire section dedicated to operators.

Punctuators / Special Symbols

C uses various symbols to structure programs. Examples:

;
,
( )
{ }
[ ]
#

For example:

int age = 20;

;   → semicolon

Let's Break Down a Complete Statement

Consider:

float price = 99.50;

Tokens:

float
price
=
99.50
;

Classification:

float  → keyword
price  → identifier
=      → operator
99.50  → floating-point constant
;      → punctuator

Whitespace and Tokens

Consider:

int age = 20;

The spaces help us visually separate tokens:

int
age
=
20
;

The compiler uses lexical rules to identify these tokens. This is why:

int age=20;

can still be valid. The spaces aren't always required between every token. But readable formatting is strongly recommended.

A Useful Mental Model

Think of C code like a sentence.

C Program
   ↓
Statements
   ↓
Tokens
   ↓
Characters

For example:

int age = 20;

Statement
    ↓
Tokens
    ↓
int | age | = | 20 | ;
    ↓
Characters

This is roughly how the compiler begins understanding your source code.

Till now: We learned about symbols,variables, keywords, constants, operators, punctuators and whitespace.Remember what is our learning flow:

Symbol  → token → Instruction → Program

So Next Topic is Introduction. we will discuss about it in next section

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