Keywords, Identifiers & Constants in C – Understanding the Three Essential Building Blocks

Now that you've learned what Tokens are, it's time to explore three of the most important types of tokens you'll use in almost every C program—Keywords, Identifiers, and Constants.

If you look at the infographic, the statement

int age = 20;

is divided into three different parts.

• int → Keyword

• age → Identifier

• 20 → Constant

Although this is just one line of code, each of these parts has a completely different purpose. Think of them as different members of a team. One tells the compiler what type of data you're working with, another gives that data a name, and the last one provides the actual value.

By the end of this lesson, you'll understand exactly what each of these terms means, how they work together, and why confusing them is one of the most common beginner mistakes.

Understanding the Registration Form Example

Before jumping into code, let's first understand the real-world example shown on the left side of the infographic.

Alex is filling out a registration form that contains fields like:

Role : Student
Name : Alex
Age  : 20

Think about it for a moment.

Every piece of information has a different purpose.

• Student tells us the role.

• Alex identifies the person.

• 20 represents the person's age.

Programming works in exactly the same way.

Every piece of information inside a program has a specific role. Some words are reserved by the C language itself, some are names created by the programmer, and some are fixed values.

That's why C divides them into three categories:

• Keywords

• Identifiers

• Constants

Breaking Down the Statement

Let's look at the statement shown in the center of the infographic.

int age = 20;

Although it's a single statement, it actually contains three different types of information.

int   → Keyword
age   → Identifier
20    → Constant

Let's understand each one individually.

What is a Keyword?

The first part is

int

This is a Keyword.

A keyword is a word that already has a predefined meaning in the C language.

When the compiler reads

int

it immediately understands that you're declaring an integer variable.

Notice something important.

You didn't invent the word int.

It already existed before you started writing the program.

That's why keywords are also called reserved words.

The infographic mentions three important characteristics of keywords.

• Reserved Word

• Predefined Meaning

• Defines the Data Type

These three points summarize keywords perfectly.

Why Are Keywords Reserved?

Imagine a library.

The library already has a rule book.

You cannot walk into the library and decide to rewrite those rules.

The rules already exist.

That's exactly why the infographic compares keywords to a Library Rule Book.

Keywords belong to the C language itself.

The programmer cannot change their meaning.

For example,

int

will always represent the integer data type.

Similarly,

float

will always represent floating-point numbers.

Their meanings never change.

Common Keywords in C

Some of the most frequently used keywords are:

int
float
char
if
else
for
while
return
break
continue
switch
void

Every one of these has a predefined purpose inside the language.

Whenever the compiler encounters one of these words, it immediately knows what operation is being performed.

What is an Identifier?

Now let's move to the second part of the statement.

age

This is called an Identifier.

Unlike keywords, identifiers are not predefined.

They are created by the programmer.

An identifier is simply a name used to identify something inside a program.

In this example,

age

is the name of a variable.

Tomorrow you could write

marks

or

salary

or

studentName

These are all identifiers because they are names chosen by the programmer.

The infographic highlights three important characteristics of identifiers.

• Variable Name

• Created by Programmer

• Used to Access Data

Whenever you want to access the stored value, you use the identifier.

Real-Life Analogy for Identifiers

Look at the name tag shown in the infographic.

Imagine you're attending a conference.

Everyone wears a name tag.

One person has

Alex

Another has

John

The name tag doesn't change who the person is.

It simply helps everyone identify them.

Identifiers work the same way.

They don't store information by themselves.

They simply give a meaningful name to the data stored in memory.

Rules for Naming Identifiers

Although you can choose meaningful names, C follows certain rules.

A valid identifier:

• Can contain letters

• Can contain digits

• Can contain underscores

• Must begin with a letter or underscore

• Cannot contain spaces

• Cannot contain special symbols

• Cannot be a keyword

Valid examples:

age
studentName
total_marks
price1

Invalid examples:

1age
student age
marks@
int
float

A good identifier should also describe the data it stores. Names like age, marks, and studentName are much easier to understand than names like a, x, or temp1.

What is a Constant?

Now let's look at the final part of the statement.

20

This is called a Constant.

A constant is simply a fixed value.

Unlike variables, constants do not change by themselves.

In the statement

int age = 20;

the value

20

is an integer constant.

The infographic describes constants using three points.

• Fixed Value

• Does Not Change

• Stored in Memory

Whenever the compiler sees 20, it understands that this value is fixed.

Different Types of Constants

Constants aren't limited to integers.

The infographic shows another example.

float price = 99.5;

char grade = 'A';

Here,

99.5

is a floating-point constant.

'A'

is a character constant.

Similarly,

"John"

is a string constant.

Some common examples are:

100
250
99.5
3.14
'A'
"Hello"

All of these represent fixed values.

Putting Everything Together

Now let's revisit our original statement.

int age = 20;

Here's what each part represents.

int   → Keyword

age   → Identifier

20    → Constant

The keyword tells the compiler what type of data will be stored.

The identifier gives that data a meaningful name.

The constant provides the actual value.

Without any one of these, the statement would be incomplete.

Why Can't We Use Keywords as Identifiers?

The infographic shows an incorrect example.

int int = 20;

At first glance, this may look okay.

But the compiler immediately reports an error.

Why?

Because

int

already has a predefined meaning.

It cannot simultaneously become the name of a variable.

The compiler gets confused.

Should the first int represent the data type?

Should the second int represent the variable name?

Since keywords are reserved, the compiler rejects the statement.

That's why this example is marked as Invalid in the infographic.

The Correct Way

Instead, we should write

int age = 20;

Now everything becomes clear.

int   → Data Type

age   → Variable Name

20    → Value

The compiler understands the statement without any confusion, and the program compiles successfully.

Another Example

The infographic also includes another declaration.

float price = 99.5;

char grade = 'A';

Let's identify each part.

For the first statement:

float  → Keyword

price  → Identifier

99.5   → Constant

For the second statement:

char   → Keyword

grade  → Identifier

'A'    → Constant

Although the values and variable names change, the pattern remains exactly the same.

How the Compiler Reads These Statements

When the compiler scans your program, it doesn't simply read an entire line.

Instead, it recognizes each token individually.

For example,

float price = 99.5;

The compiler processes it like this:

Keyword

↓

Identifier

↓

Operator

↓

Constant

↓

Special Symbol

Once every token follows the rules of the C language, the compiler successfully converts your program into machine code.

That's why the infographic shows the robot giving a thumbs-up with the message Compilation Successful.

Common Beginner Mistakes

One of the most common mistakes is using a keyword as an identifier.

int int = 20;

This is invalid because int is reserved by the C language.

Another common mistake is choosing meaningless variable names.

Instead of writing

int x = 20;

it's much better to write

int age = 20;

because anyone reading the code immediately understands what the value represents.

Beginners also confuse variables with constants. Remember, the identifier is the name, while the constant is the actual value stored in memory.

Interview Questions

1. What is a keyword in C?

A keyword is a reserved word that has a predefined meaning in the C language.

2. What is an identifier?

An identifier is a programmer-defined name used to identify variables, functions, arrays, and other program elements.

3. What is a constant?

A constant is a fixed value that does not change during program execution.

4. Can a keyword be used as an identifier?

No. Keywords are reserved by the C language and cannot be used as variable names.

5. Identify the keyword, identifier, and constant in the following statement:

float price = 99.5;

• float → Keyword

• price → Identifier

• 99.5 → Constant

Quick Recap

Let's summarize everything you've learned.

• Keywords are reserved words provided by the C language with predefined meanings.

• Identifiers are meaningful names created by the programmer to identify variables and other program elements.

• Constants are fixed values such as numbers, characters, and strings.

• In the statement int age = 20;, int is the keyword, age is the identifier, and 20 is the constant.

• Keywords cannot be used as identifiers because they are already reserved by the language.

• Choosing clear and meaningful identifiers makes your programs easier to read, understand, and maintain.

Now that you understand the difference between Keywords, Identifiers, and Constants, you're ready to learn about the next important type of token in C: Operators, where you'll discover how C performs calculations, comparisons, assignments, and logical operations.