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Lesson 16

Boolean

Covers bool, expressions, logical operators, boolalpha.

Lesson content

What is boolean in C++?

Bool stores only true or false.

bool isLoggedIn = true;
bool hasLicense = false;

Why does boolean exist?

  • Is the password correct?
  • Is the user an admin?
  • Has payment completed?
  • Is student eligible for admission?

Bool beats raw 1/0 for readability.

Real-world use cases

  • Login and authentication systems
  • Banking applications
  • Online shopping websites
  • Game development
  • Attendance systems
  • Traffic light simulations
  • Access control systems
  • Form validation

Boolean data type

bool variableName;

bool isStudent = true;

Boolean values

Value | Meaning
----- | -------------------
true  | Condition is true
false | Condition is false
bool isPassed = true;
bool isMember = false;

Boolean expressions

Expression evaluating to true or false.

age >= 18
marks >= 50
number == 100

Relational operators

Operator | Meaning                  | Example
-------- | ------------------------ | --------
==       | Equal to                 | a == b
!=       | Not equal to             | a != b
>        | Greater than             | a > b
<        | Less than                | a < b
>=       | Greater than or equal to | a >= b
<=       | Less than or equal to    | a <= b

Logical operators

&&  Logical AND
||  Logical OR
!   Logical NOT
age >= 18 && hasLicense

marks >= 50 || sportsQuota

!isLoggedIn

Boolean output

C++ prints bools as numbers by default.

true  -> 1
false -> 0

Use boolalpha for word output.

cout << boolalpha << true;
// Output: true

Example 1: Boolean variable

#include <iostream>
using namespace std;

int main()
{
    bool isStudent = true;

    cout << boolalpha;
    cout << isStudent;

    return 0;
}

Example 2: Boolean expression

#include <iostream>
using namespace std;

int main()
{
    int age = 20;
    bool canVote = age >= 18;

    cout << boolalpha;
    cout << canVote;

    return 0;
}

Example 3: Boolean with if

#include <iostream>
using namespace std;

int main()
{
    bool isLoggedIn = true;

    if (isLoggedIn)
    {
        cout << "Welcome!";
    }
    else
    {
        cout << "Please log in.";
    }

    return 0;
}

Example 4: Logical AND

#include <iostream>
using namespace std;

int main()
{
    int age = 22;
    bool hasLicense = true;

    if (age >= 18 && hasLicense)
    {
        cout << "You can drive.";
    }
    else
    {
        cout << "You cannot drive.";
    }

    return 0;
}

Example 5: Password verification

#include <iostream>
#include <string>
using namespace std;

int main()
{
    string password;

    cout << "Enter password: ";
    cin >> password;

    bool isCorrect = (password == "admin123");

    if (isCorrect)
    {
        cout << "Access Granted";
    }
    else
    {
        cout << "Access Denied";
    }

    return 0;
}

Common mistakes and pitfalls

  • = instead of ==. Fix: use == to compare
  • Comparing bool to true. Fix: just use if(isReady)
  • Forgetting boolalpha. Fix: add it before output
  • Numbers instead of true/false. Fix: use bool literals
  • Confusing && and ||. Fix: pick correct operator
// Wrong
if (isLoggedIn == true)
{
    cout << "Welcome";
}
// Correct
if (isLoggedIn)
{
    cout << "Welcome";
}

Best practices

  • Use clear names like isValid
  • Prefer true/false over 1/0
  • Use boolalpha for readable output
  • Keep boolean expressions simple
  • Combine conditions only when needed

When not to use this

  • More than two states needed. Use enum
  • Numerical calculations required
  • Multiple related values. Use structs or classes

Summary

  • Boolean uses the bool data type
  • Stores only true or false
  • Expressions evaluate to true or false
  • Relational operators produce booleans
  • Logical operators combine booleans
  • Default output is 1 or 0
  • boolalpha prints true and false
  • Use clear names for readability

Frequently asked questions

What is a boolean in C++?

Data type storing true or false.

Why does true print as 1?

Default is numeric. Use boolalpha for words.

What is the difference between bool and int?

Bool holds true/false. Int holds whole numbers.

What are boolean expressions?

Expressions giving true or false, like age >= 18.

When should I use a boolean variable?

Use for two-state conditions like yes/no.

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