Module 2: Variables and Data Types in C

Learning Objectives

By the end of this module, you will be able to:

• Understand variables and constants.
• Learn C keywords and identifiers.
• Use different data types in C.
• Work with integers, floating-point numbers, characters, and Boolean values.
• Perform type conversion.
• Use the sizeof operator.
• Take user input using scanf().
• Display output using printf().
• Build beginner C programs using variables and calculations.

Introduction

Every program works with data. Whether you are building a calculator, game, banking application, or operating system, your program needs to store and process information.

In C, data is stored using variables, and the type of data stored is defined by data types.

For example:

• Student age → Integer
• Product price → Float
• Student grade → Character
• Login status → Boolean

Understanding variables and data types is one of the most important foundations of C programming.

Variables

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

Think of a variable as a container that holds a value.

Syntax

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1data_type variable_name;

Example

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1int age;

Here:

• int is the data type.
• age is the variable name.

Declaring Variables

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1int marks;
2float salary;
3char grade;

Variables are declared before they are used.

Initializing Variables

Assigning a value to a variable is called initialization.

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1int age = 20;
2float price = 99.99;
3char grade = 'A';

Multiple Variable Declaration

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1int x, y, z;

Initialization together:

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1int a = 10, b = 20, c = 30;

Variable Naming Rules

A variable name:

• Can contain letters, digits, and underscores.
• Must begin with a letter or underscore.
• Cannot contain spaces.
• Cannot be a keyword.
• Is case-sensitive.

Valid Names

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1age
2studentName
3_marks
4salary2025

Invalid Names

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12age
2student name
3float
4my-name

Constants

A constant is a value that cannot be changed during program execution.

Using const

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1const float PI = 3.14159;

Attempting to change PI results in an error.

Using #define

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1#define MAX_STUDENTS 100

The preprocessor replaces every occurrence of MAX_STUDENTS with 100 before compilation.

Difference Between Variables and Constants

Keywords

Keywords are reserved words in C with predefined meanings.

You cannot use keywords as variable names.

Common C Keywords

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1int
2float
3char
4if
5else
6while
7for
8switch
9case
10return
11void
12break
13continue
14const
15sizeof

Modern C standards define more keywords, but these are the most commonly used by beginners.

Identifiers

Identifiers are names given to:

• Variables
• Functions
• Arrays
• Structures
• Files

Example:

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1int studentAge;

Here, studentAge is an identifier.

Identifier Rules

✔ Begin with a letter or underscore

✔ Use letters, numbers, and underscores

✔ Cannot be keywords

✔ Case-sensitive

Example:

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1Marks
2marks
3MARKS

All three are different identifiers.

Data Types

A data type tells the compiler:

• How much memory to allocate.
• What kind of data will be stored.
• What operations are allowed.

Categories of Data Types

Basic Data Types

• int
• float
• double
• char

Derived Data Types

• Arrays
• Pointers
• Functions

User-Defined Data Types

• Structure
• Union
• Enum
• typedef

int Data Type

The int data type stores whole numbers.

Examples:

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110
2250
3-45
41000

Example:

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1int age = 22;
2
3printf("%d", age);

Output

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122

Format specifier:

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1%d

float Data Type

Stores decimal numbers with single precision.

Example:

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1float price = 199.99;
2
3printf("%f", price);

Output

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1199.990005

Display only two decimal places:

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1printf("%.2f", price);

Output

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1199.99

double Data Type

Stores decimal numbers with higher precision than float.

Example:

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1double pi = 3.1415926535;
2
3printf("%.10lf", pi);

Output

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13.1415926535

Format specifier:

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1%lf

char Data Type

Stores a single character.

Example:

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1char grade = 'A';
2
3printf("%c", grade);

Output

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1A

Characters are enclosed in single quotes.

Correct:

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1'A'

Incorrect:

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1"A"

Boolean Data Type

C does not have a built-in bool type in older standards. Since C99, include <stdbool.h>.

Example:

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1#include <stdio.h>
2#include <stdbool.h>
3
4int main()
5{
6    bool isPassed = true;
7
8    printf("%d", isPassed);
9
10    return 0;
11}

Output

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11

Values:

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1true
2false

Type Conversion

Type conversion means converting one data type into another.

There are two types:

• Implicit Conversion
• Explicit Conversion

Implicit Conversion

The compiler converts automatically.

Example:

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1int a = 10;
2float b = a;
3
4printf("%.2f", b);

Output

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110.00

Explicit Conversion (Type Casting)

Programmer manually converts data.

Example:

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1int a = 5;
2int b = 2;
3
4float result = (float)a / b;
5
6printf("%.2f", result);

Output

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12.50

Without casting:

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12

sizeof Operator

The sizeof operator returns the memory occupied by a data type or variable in bytes.

Example

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1printf("%zu\n", sizeof(int));
2printf("%zu\n", sizeof(float));
3printf("%zu\n", sizeof(char));

Typical output on many systems (implementation-dependent):

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14
24
31

Input Using scanf()

scanf() reads input from the keyboard.

Syntax

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1scanf("format", &variable);

Example

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1int age;
2
3scanf("%d", &age);

Notice the & symbol.

It gives the memory address of the variable.

Reading Multiple Values

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1int a, b;
2
3scanf("%d %d", &a, &b);

Input

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110 20

Reading Float

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1float price;
2
3scanf("%f", &price);

Reading Character

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1char grade;
2
3scanf(" %c", &grade);

The leading space helps skip leftover whitespace.

Output Using printf()

printf() displays output on the screen.

Example

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1printf("Welcome to C Programming");

Printing Variables

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1int age = 22;
2
3printf("Age = %d", age);

Multiple Variables

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1char name[] = "John";
2int age = 20;
3
4printf("Name: %s\nAge: %d", name, age);

Common Format Specifiers

Practice Project 1: Temperature Converter

Convert Celsius to Fahrenheit.

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1#include <stdio.h>
2
3int main()
4{
5    float celsius, fahrenheit;
6
7    printf("Enter temperature in Celsius: ");
8    scanf("%f", &celsius);
9
10    fahrenheit = (celsius * 9 / 5) + 32;
11
12    printf("Temperature in Fahrenheit = %.2f\n", fahrenheit);
13
14    return 0;
15}

Practice Project 2: Area Calculator

Calculate the area of a rectangle.

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1#include <stdio.h>
2
3int main()
4{
5    float length, width, area;
6
7    printf("Enter length: ");
8    scanf("%f", &length);
9
10    printf("Enter width: ");
11    scanf("%f", &width);
12
13    area = length * width;
14
15    printf("Area = %.2f\n", area);
16
17    return 0;
18}

Practice Project 3: Simple Interest Calculator

Calculate simple interest.

Formula:

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1Simple Interest = (Principal × Rate × Time) / 100

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1#include <stdio.h>
2
3int main()
4{
5    float principal, rate, time, interest;
6
7    printf("Enter Principal: ");
8    scanf("%f", &principal);
9
10    printf("Enter Rate: ");
11    scanf("%f", &rate);
12
13    printf("Enter Time (Years): ");
14    scanf("%f", &time);
15
16    interest = (principal * rate * time) / 100;
17
18    printf("Simple Interest = %.2f\n", interest);
19
20    return 0;
21}

Best Practices

• Use meaningful variable names such as studentAge instead of a.
• Initialize variables before using them.
• Use const for values that should not change.
• Choose the appropriate data type to save memory and improve performance.
• Use the correct format specifier with scanf() and printf().
• Prefer double over float when higher precision is required.

Module Summary

In this module, you learned:

• What variables are and how to declare and initialize them.
• The difference between variables and constants.
• Rules for keywords and identifiers.
• The basic C data types: int, float, double, char, and bool.
• How implicit and explicit type conversion works.
• How to use the sizeof operator to determine memory usage.
• How to read user input with scanf() and display output with printf().
• How to apply these concepts through practical programs such as a temperature converter, area calculator, and simple interest calculator.

With these fundamentals, you're ready to move on to the next module, where you'll learn operators, expressions, and decision-making in C.