Variables in C

Variables are one of the most fundamental concepts in programming. Every program needs a way to store data such as numbers, characters, names, marks, or calculations. In C, this is done using variables.

Think of a variable as a labeled container in memory where data is stored. You can store information in it, update it, and use it whenever required.

🎯 Learning Objectives

After completing this lesson, you will be able to:

• Understand what a variable is.
• Declare and initialize variables.
• Learn variable naming rules.
• Differentiate between declaration and initialization.
• Understand local and global variables.
• Write programs using variables.
• Avoid common mistakes.

📖 What is a Variable?

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

Every variable has:

• A name
• A data type
• A value
• A memory location

For example,

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Student Name → Rahul
Age → 20
Marks → 95

Here, Student Name, Age, and Marks are variables.

🧠 Why Do We Need Variables?

Without variables, programs cannot remember information.

Variables allow programs to:

• Store user input
• Perform calculations
• Save intermediate results
• Display dynamic output
• Reuse data

📦 Memory Representation

When a variable is created, memory is allocated.

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+----------------------+
| age = 20             |
+----------------------+

+----------------------+
| marks = 95           |
+----------------------+

+----------------------+
| grade = 'A'          |
+----------------------+

Each variable occupies memory depending on its data type.

Variable Declaration

Declaration tells the compiler the type and name of a variable.

Syntax

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datatype variableName;

Example

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int age;

float salary;

char grade;

Variable Initialization

Initialization means assigning a value when declaring the variable.

Syntax

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datatype variable = value;

Example

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int age = 20;

float salary = 35000.50;

char grade = 'A';

Declaration vs Initialization

Complete Example

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#include <stdio.h>

int main()
{
    int age = 20;

    printf("Age = %d", age);

    return 0;
}

Output

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Age = 20

Multiple Variables

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#include <stdio.h>

int main()
{
    int age = 20;
    int marks = 95;
    float cgpa = 9.45;

    printf("Age = %d\n", age);
    printf("Marks = %d\n", marks);
    printf("CGPA = %.2f", cgpa);

    return 0;
}

Variable Naming Rules

A variable name:

✅ Can contain letters.

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marks

✅ Can contain digits.

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student1

✅ Can contain underscore.

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student_marks

Cannot

❌ Start with a number

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1marks

❌ Contain spaces

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student marks

❌ Use keywords

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int int;

Naming Conventions

Good

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studentName

totalMarks

averageMarks

Bad

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a

x

abc12345

Local Variables

Variables declared inside a function are called local variables.

Example

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int main()
{
    int age = 20;

    printf("%d", age);

    return 0;
}

The variable exists only inside the main() function.

Global Variables

Variables declared outside all functions are called global variables.

Example

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#include <stdio.h>

int collegeCode = 101;

int main()
{
    printf("%d", collegeCode);

    return 0;
}

Global variables can be accessed by multiple functions.

Real-World Example

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#include <stdio.h>

int main()
{
    char course[] = "KauraX";
    int students = 250;
    float rating = 4.9;

    printf("Platform : %s\n", course);
    printf("Students : %d\n", students);
    printf("Rating   : %.1f", rating);

    return 0;
}

Output

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Platform : KauraX
Students : 250
Rating   : 4.9

Common Beginner Mistakes

❌ Using undeclared variables

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age = 20;

Correct

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int age = 20;

❌ Using spaces

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student name

Correct

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studentName

❌ Starting with numbers

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1age

Correct

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age1

Best Practices

• Use meaningful names.
• Initialize variables whenever possible.
• Avoid single-letter names except in loops.
• Follow consistent naming conventions.
• Keep variables close to where they are used.

🧠 Interview Questions

1. What is a variable?
2. Why are variables required in programming?
3. What is the difference between declaration and initialization?
4. What are the rules for naming variables?
5. Differentiate between local and global variables.

✍ Practice Questions

1. Declare an integer variable and print it.
2. Declare three variables of different data types.
3. Write a program to display your name, age, and CGPA.
4. Write a program using both local and global variables.
5. List five valid and five invalid variable names.

📚 Lesson Summary

In this lesson, you learned:

• What variables are.
• Why variables are important.
• Declaration and initialization.
• Variable naming rules.
• Local and global variables.
• Best coding practices.
• Common beginner mistakes.

Variables are the foundation of programming. In the next lesson, you will learn about the different Data Types available in C and how they determine the type of data a variable can store.

🚀 What's Next?

Next lesson: Data Types in C

You will learn:

• Primitive data types
• Size of each data type
• Range of values
• Modifiers
• Format specifiers
• Practical examples
• Memory usage