What are variables and constants?

Variables store data. Value can change.

Constants hold fixed values. Value stays same.

Variable is a whiteboard. Constant is stone.

Why do we need variables and constants?

• Store information for later use
• Perform calculations
• Accept input from users
• Improve code readability
• Prevent accidental modification of fixed values

Variables and constants overview

                  Variables & Constants
                          │
          ┌───────────────┴───────────────┐
          │                               │
          ▼                               ▼
      Variables                      Constants
          │                               │
   Value Can Change              Value Cannot Change
          │                               │
          ▼                               ▼
    int age = 20;          const float PI = 3.14159;

Variables in C++

What is a variable?

Named memory spot. Holds data that can change.

Variable declaration

data_type variable_name;

int age;

Variable initialization

Initialization sets a value at declaration.

int age = 20;

Variable naming rules

• Letters, digits, underscores allowed
• Must start with letter or underscore
• Names are case-sensitive
• No spaces allowed
• No reserved keywords allowed

// Valid
age
studentName
_marks
totalScore
number1

// Invalid
1age          // Starts with a digit
student name  // Contains a space
float         // Reserved keyword
total-score   // Contains a hyphen

Example 1: Declaring variables

#include <iostream>
using namespace std;

int main()
{
    int age = 20;
    float salary = 55000.50f;
    char grade = 'A';
    bool isStudent = true;

    cout << "Age: " << age << endl;
    cout << "Salary: " << salary << endl;
    cout << "Grade: " << grade << endl;
    cout << "Student: " << isStudent << endl;

    return 0;
}

Modifying a variable

Variable value can change anytime.

#include <iostream>
using namespace std;

int main()
{
    int score = 50;
    score = 80;
    cout << score << endl;
    return 0;
}

Constants in C++

What is a constant?

Value stays fixed throughout the program.

• Pi (π)
• Speed of light
• Tax rates
• Maximum number of users
• Days in a week

Using the const keyword

const keyword creates a constant value.

const double PI = 3.141592653589793;

Example 2: Using constants

#include <iostream>
using namespace std;

int main()
{
    const double PI = 3.14159;
    double radius = 5.0;
    double area = PI * radius * radius;

    cout << "Area = " << area << endl;

    return 0;
}

Example 3: Variable vs constant

#include <iostream>
using namespace std;

int main()
{
    int marks = 80;
    const int passingMarks = 40;

    marks = 90;  // Allowed

    // passingMarks = 50;  // Error

    cout << "Marks: " << marks << endl;
    cout << "Passing Marks: " << passingMarks << endl;

    return 0;
}

Variables vs constants

Feature      | Variable             | Constant
------------ | -------------------- | --------------------
Value        | Can change           | Cannot change
Keyword      | No special keyword   | const
Memory       | Allocated at runtime | Allocated at runtime
Modification | Allowed              | Not allowed
Example      | int age = 20;        | const int DAYS = 7;

Common mistakes and pitfalls

• Uninitialized variable. Fix: initialize before use
• Modifying a constant. Fix: keep value fixed
• Reserved keyword as name. Fix: pick valid names
• Unclear names like a or x1. Fix: use descriptive names

// Incorrect
const int MAX = 100;
MAX = 200;

// Correct
const int MAX = 100;
cout << MAX;

Best practices

• Use descriptive variable names
• Initialize variables when declaring
• Use const for fixed values
• Follow consistent naming convention
• Avoid unnecessary global variables
• Prefer const over magic numbers

When not to use variables or constants

• Variable for fixed value. Use a constant
• Repeated hard-coded values. Use named constant
• Unneeded global variables. Use local scope

Summary

• Variable holds changeable data
• Constant holds fixed data
• Initialize variables before use
• Use meaningful names always
• Use const for fixed values
• Good naming aids maintenance

Frequently asked questions

What is the difference between a variable and a constant?

Variable changes. Constant stays fixed after init.

Why should I use const instead of a regular variable?

Prevents accidental changes. Keeps code safer.

Can I declare a variable without assigning a value?

Yes. But initialize before use to avoid bugs.

What are the rules for naming variables?

Start with letter or underscore. No spaces or keywords.

What is a literal in C++?

Fixed value written directly in code, like 10 or 'A'.

What should I learn after variables and constants?

• Literals
• Operators
• User Input (cin)
• Type Casting
• Expressions
• Control Statements (if, switch)
• Loops (for, while, do-while)