C Language – Basics

1. Character Set

A character set is a collection of characters that can be used while writing a C program.

The commonly used characters in C include:

Category	Characters / Examples
Small alphabets	a - z
Capital alphabets	A - Z
Digits	0 - 9
Special symbols	+, -, *, /, %, #, =, <, >
Whitespace characters	Space, Tab, New Line
Punctuation	;, ,, ( ), { }, [ ]

Example

int marks = 90;

This statement contains letters, a number, an assignment operator, and a semicolon.

2. Tokens

A token is the smallest meaningful unit of a C program that is recognized by the compiler.

In simple words, when the compiler reads a C program, it breaks the source code into meaningful units called tokens.

Major Types of Tokens in C

Token Type	Example
Keywords	int, if, while
Identifiers	marks, studentName
Constants	10, 25.5, 'A'
String literals	"Hello"
Operators	+, -, *, =
Punctuators	;, ,, ( ), { }

Example: Identifying Tokens

int age = 20;

Token	Type
int	Keyword
age	Identifier
=	Operator
20	Constant
;	Punctuator

3. Keywords

Keywords are reserved words that have a predefined meaning in the C programming language.

Because keywords already have a special meaning to the compiler, they cannot normally be used as names of variables, functions, arrays, etc.

32 Keywords in Traditional C (C90)

Example

int age = 20;

Here, int is a keyword. It tells the compiler that age is an integer variable.

4. C is Case-Sensitive

C distinguishes between uppercase and lowercase letters. Therefore, the following identifiers are different:

age
Age
AGE

These are treated as three different identifiers by the compiler.

5. Identifier

An identifier is a name given by the programmer to program elements such as variables, functions, arrays, structures, and other user-defined elements.

Rules for Selecting an Identifier

1. A keyword cannot be used as an identifier.
2. The first character must be an alphabet (A-Z or a-z) or an underscore (_).
3. An identifier cannot begin with a number. Numbers can be used after the first character.
4. Spaces are not allowed in identifiers.
5. Special characters and symbols are not allowed, except the underscore (_).
6. Both uppercase and lowercase letters can be used.
7. C identifiers are case-sensitive.

Valid and Invalid Identifiers

Identifier	Valid?	Reason
int	Invalid	int is a keyword.
1stNo	Invalid	An identifier cannot begin with a number.
first name	Invalid	Spaces are not allowed.
CLang@1	Invalid	@ is not allowed in an identifier.
INT	Valid	It is not the keyword int.
_1stNo	Valid	It starts with an underscore.
No1	Valid	The number occurs after the first character.
first_name	Valid	Underscore is allowed.
firstName	Valid	Uses alphabets only.

Examples of Valid Identifiers

age
studentName
total_marks
_marks
number1
MAX_VALUE

6. Variable

A variable is a named memory location used to store a value of a particular data type.

The value stored in a variable can change during program execution. This is why it is called a variable.

Example

int age = 20;

In the above statement:

• int → Data type
• age → Variable name / identifier
• = → Assignment operator
• 20 → Initial value
• ; → Statement terminator

Changing the Value of a Variable

int age = 20;

age = 21;
age = 22;

The variable age can hold different values at different points during program execution.

7. Variable Declaration

Before using a variable, we normally declare it by specifying its data type and name.

int age;
float salary;
char grade;

We can also declare and initialize a variable at the same time.

int age = 20;
float salary = 25000.50;
char grade = 'A';

Declaration	Meaning
int age;	Declares an integer variable named age.
float salary;	Declares a floating-point variable named salary.
char grade;	Declares a character variable named grade.

8. Complete Example

#include <stdio.h>

int main()
{
    int age = 20;
    float marks = 85.5;
    char grade = 'A';

    printf("Age = %d\n", age);
    printf("Marks = %.1f\n", marks);
    printf("Grade = %c\n", grade);

    return 0;
}

Important Parts

• int, float, and char are keywords.
• age, marks, and grade are identifiers.
• 20 and 85.5 are constants.
• = is an assignment operator.
• ; terminates a statement.

9. Quick Revision

Concept	Remember
Character Set	Characters that can be used to write a C program.
Token	Smallest meaningful unit recognized by the compiler.
Keyword	Reserved word with a predefined meaning.
Identifier	Name given by the programmer to program elements.
Variable	Named memory location used to store a value.
Case Sensitive	Uppercase and lowercase letters are treated differently.

10. Practice Questions

A. Fill in the Blanks

B. True or False

C. Identify Valid and Invalid Identifiers

Classify each of the following as Valid or Invalid:

1. studentName
2. 2marks
3. total_marks
4. float
5. first-name
6. _count
7. marks2
8. student name
9. MAX_VALUE
10. @number

D. Multiple Choice Questions

E. Short Answer Questions

1. What is a character set in C?
2. What is a token?
3. What is a keyword? Give five examples.
4. Why can’t keywords normally be used as identifiers?
5. What is an identifier?
6. Write any five rules for naming identifiers.
7. What is a variable?
8. Differentiate between a keyword and an identifier.
9. Why is C called a case-sensitive language?
10. Explain the statement int age = 20;.

11. Practice Answer Key