Logical Operators

In the previous lesson, you learned to check a single condition with if. But in the real world, decisions often depend on more than one condition. For example: “You can ride this attraction if your age is above 12 AND your height is above 140 cm.” Both conditions must be met! For that, we need logical operators.

Analogy: Amusement Park Entrance Gate

Imagine you want to ride a roller coaster. There’s a guard who checks:

• AND (&&): “Age over 12 AND height over 140 cm?” — both must be met.
• OR (||): “Have a regular ticket OR a VIP ticket?” — just one is enough.
• NOT (!): “NOT a holiday?” — flips the condition.

The Three Logical Operators

The && (AND) Operator

&& produces true only if both conditions are true. If even one is false, the result is false.

AND Truth Table

Easy to remember: AND is strict — everything must be true.

Example: Ride Eligibility Check

#include <iostream>

int main() {
    int age = 14;
    int height = 145;

    std::cout << "Age: " << age << " years" << std::endl;
    std::cout << "Height: " << height << " cm" << std::endl;

    if (age >= 12 && height >= 140) {
        std::cout << "You can ride the roller coaster!" << std::endl;
    } else {
        std::cout << "Sorry, you can't ride yet." << std::endl;
    }

    return 0;
}

Output:

Age: 14 years
Height: 145 cm
You can ride the roller coaster!

What if age is 14 but height is 130? The result is “Sorry, you can’t ride yet.” because 130 >= 140 is false, and true && false = false.

Example: Checking a Number in a Range

One of the most common uses of && is checking whether a number falls within a certain range:

#include <iostream>

int main() {
    int score = 85;

    // Check if score is between 80 and 89
    if (score >= 80 && score <= 89) {
        std::cout << "Score " << score << " is grade B." << std::endl;
    }

    // Check if body temperature is normal (36.1 - 37.2)
    double temperature = 36.8;
    if (temperature >= 36.1 && temperature <= 37.2) {
        std::cout << "Body temperature is normal." << std::endl;
    }

    return 0;
}

Output:

Score 85 is grade B.
Body temperature is normal.

The || (OR) Operator

|| produces true if at least one condition is true. It’s only false when both are false.

OR Truth Table

Easy to remember: OR is lenient — just one being true is enough.

Example: Check for a Day Off

#include <iostream>
#include <string>

int main() {
    std::string day = "Sunday";

    if (day == "Saturday" || day == "Sunday") {
        std::cout << "Awesome, it's a day off! Time to play games." << std::endl;
    } else {
        std::cout << "It's a school day. Keep up the studying!" << std::endl;
    }

    return 0;
}

Output:

Awesome, it's a day off! Time to play games.

Example: Voting Eligibility Check

#include <iostream>

int main() {
    int age = 16;
    bool isMarried = true;

    std::cout << "Age: " << age << " years" << std::endl;
    std::cout << "Is married: " << isMarried << std::endl;

    // In Indonesia, you can vote if age >= 17 OR already married
    if (age >= 17 || isMarried) {
        std::cout << "Eligible to vote in the election." << std::endl;
    } else {
        std::cout << "Not yet eligible to vote." << std::endl;
    }

    return 0;
}

Output:

Age: 16 years
Is married: 1
Eligible to vote in the election.

Even though the age is only 16 (not yet 17), because isMarried is true, false || true = true.

The ! (NOT) Operator

! flips a boolean value. true becomes false, false becomes true.

NOT Truth Table

Example: Check If It’s Not Raining

#include <iostream>

int main() {
    bool raining = false;

    if (!raining) {
        std::cout << "It's not raining. Let's play soccer!" << std::endl;
    } else {
        std::cout << "It's raining. Let's play indoors." << std::endl;
    }

    return 0;
}

Output:

It's not raining. Let's play soccer!

!false produces true, so the if block runs.

Example: Check If Not Done Yet

#include <iostream>

int main() {
    bool assignmentDone = false;
    int pagesLeft = 15;

    if (!assignmentDone) {
        std::cout << "Assignment not finished yet!" << std::endl;
        std::cout << "Still " << pagesLeft << " pages to go." << std::endl;
    }

    return 0;
}

Output:

Assignment not finished yet!
Still 15 pages to go.

Combining Multiple Operators

You can combine &&, ||, and ! in a single condition:

#include <iostream>

int main() {
    int age = 15;
    bool hasParentPermission = true;
    bool isRegistered = true;

    std::cout << "=== Club Registration ===" << std::endl;
    std::cout << "Age: " << age << std::endl;
    std::cout << "Parent permission: " << hasParentPermission << std::endl;
    std::cout << "Registered: " << isRegistered << std::endl;
    std::cout << std::endl;

    // Can join if: (age 13-17) AND (has parent permission) AND (is registered)
    if (age >= 13 && age <= 17 && hasParentPermission && isRegistered) {
        std::cout << "Congratulations! You've been accepted into the club." << std::endl;
    } else {
        std::cout << "Sorry, requirements not met." << std::endl;
    }

    return 0;
}

Output:

=== Club Registration ===
Age: 15
Parent permission: 1
Registered: 1

Congratulations! You've been accepted into the club.

Example: Store Discount

#include <iostream>

int main() {
    bool isMember = false;
    double totalPurchase = 250000;
    bool specialDay = true;

    std::cout << "Member: " << isMember << std::endl;
    std::cout << "Total purchase: Rp" << totalPurchase << std::endl;
    std::cout << "Special day: " << specialDay << std::endl;
    std::cout << std::endl;

    // Gets discount if: member OR (purchase >= 200000 AND special day)
    if (isMember || (totalPurchase >= 200000 && specialDay)) {
        std::cout << "Congratulations! You get a 20% discount!" << std::endl;
        double discount = totalPurchase * 0.2;
        std::cout << "You save: Rp" << discount << std::endl;
    } else {
        std::cout << "No discount today." << std::endl;
    }

    return 0;
}

Output:

Member: 0
Total purchase: Rp250000
Special day: 1

Congratulations! You get a 20% discount!
You save: Rp50000

Precedence Order

When combining operators, C++ has a precedence order:

This means: ! is processed first, then &&, and finally ||.

// Without parentheses:
// a || b && c  is read as  a || (b && c)
// Because && has higher priority than ||

// Example:
bool a = true;
bool b = false;
bool c = true;

// a || b && c
// = true || (false && true)
// = true || false
// = true

Short-Circuit Evaluation

C++ has a clever feature: short-circuit evaluation. This means C++ can be “lazy” and stop checking early:

• AND (&&): If the first condition is false, the second condition is not checked — because the result is guaranteed to be false.
• OR (||): If the first condition is true, the second condition is not checked — because the result is guaranteed to be true.

int x = 0;

// The second condition won't be checked because the first is already false
if (x != 0 && 10 / x > 2) {
    std::cout << "Safe" << std::endl;
}
// This is useful! Without short-circuit, 10/0 would cause an error.

Common Mistakes

1. Using & instead of &&

// WRONG -- & is a bitwise operator (different function!)
if (age > 12 & height > 140) { }

// CORRECT -- && is the logical AND operator
if (age > 12 && height > 140) { }

2. Using | instead of ||

// WRONG -- | is a bitwise operator
if (day == "Saturday" | day == "Sunday") { }

// CORRECT -- || is the logical OR operator
if (day == "Saturday" || day == "Sunday") { }

3. Forgetting to repeat the variable in each comparison

int x = 5;

// WRONG -- you can't write it like this
// if (x == 3 || 4 || 5) { }

// CORRECT -- must write out each comparison fully
if (x == 3 || x == 4 || x == 5) { }

4. Forgetting parentheses when mixing && and ||

// This might not work as you expect
// if (a || b && c) -- read as if (a || (b && c))

// Safer and clearer with parentheses
if ((a || b) && c) { }   // if this is what you mean

Exercises

Exercise 1: Write a program that checks whether someone can get a driver’s license. Requirements: minimum age 17 AND has passed the written test (bool passedTest). Display “Can get a driver’s license!” or “Requirements not met yet.”

Exercise 2: Write a program that checks whether a number is outside the range 1-100. Use the || operator to check if the number is < 1 OR the number is > 100. Display “Number is out of range!” or “Number is valid.”

Exercise 3: Write a program for a bookstore discount system:

• 30% discount if: member AND purchase >= Rp100,000
• 15% discount if: member OR purchase >= Rp200,000
• 0% discount if no conditions are met Display the total before and after the discount.

Summary

You now have the “full arsenal” for making decisions in programs: comparison operators, if-else, and logical operators. With these three tools, your programs can handle various scenarios and make smart decisions!