C++ Logical Operators in `if` Statements: &&, ||, and !

1. Learning Objectives

After completing this lesson, you will be able to:

• Use logical operators inside if and if-else statements.
• Combine multiple conditions using &&, ||, and !.
• Understand how changing variable values affects program output.
• Write programs to solve real-life problems.

2. Quick Revision: Logical Operators

&&

AND

Both conditions must be true.

Example: marks >= 50 && attendance >= 75

||

OR

At least one condition must be true.

Example: marks >= 90 || attendance == 100

!

NOT

Reverses a condition from true to false or false to true.

Example: !(marks >= 50)

Example 1: Student Pass or Fail Using AND (&&)

Problem: A student passes only if marks are at least 50 and attendance is at least 75%.

Step 1 — Understand the problem

Suppose a student obtains 65 marks and has 80% attendance.

We need to check two conditions:

1. Are marks greater than or equal to 50?
2. Is attendance greater than or equal to 75%?

Both conditions must be true for the student to pass.

Step 2 — Write the code

 
#include <iostream>
using namespace std;
 
int main() {
    int marks = 65;
    int attendance = 80;
 
    if (marks >= 50 && attendance >= 75) {
        cout << "Pass";
    } else {
        cout << "Fail";
    }
 
    return 0;
}
 

Step 3 — Understand the code

Step 4 — Trace the conditions

Marks >= 50

TRUE

Attendance >= 75

TRUE

TRUE && TRUE

TRUE

Output

Pass

Step 5 — Modify the code

Change only the variable values and predict the output.

Your task: Set marks to 70 and attendance to 60. Explain why the output is Fail.

Example 2: Scholarship Eligibility Using AND (&&)

Problem: A student can receive a scholarship if family income is below Rs. 50,000 per month and marks are at least 80.

Step 1 — Understand the problem

Both conditions must be satisfied:

• Family income is less than 50,000.
• Marks are at least 80.

Step 2 — Write the code

 
#include <iostream>
using namespace std;
 
int main() {
    int income = 40000;
    int marks = 85;
 
    if (income < 50000 && marks >= 80) {
        cout << "Eligible for scholarship";
    } else {
        cout << "Not eligible";
    }
 
    return 0;
}
 

Step 3 — Understand the code

• income < 50000 → True.
• marks >= 80 → True.
• true && true → True.

Output:

Eligible for scholarship

Step 4 — Modify the code

Try these changes one by one.

Learning point: If either condition becomes false, the AND expression becomes false.

Example 3: Bonus Eligibility Using OR (||)

Problem: An employee receives a bonus if sales are at least Rs. 100,000 OR the employee has completed 10 years of service.

Step 1 — Understand the problem

Only one condition needs to be true.

Step 2 — Write the code

 
#include <iostream>
using namespace std;
 
int main() {
    int sales = 80000;
    int serviceYears = 12;
 
    if (sales >= 100000 || serviceYears >= 10) {
        cout << "Bonus approved";
    } else {
        cout << "Bonus not approved";
    }
 
    return 0;
}
 

Step 3 — Understand the code

• sales >= 100000 → False.
• serviceYears >= 10 → True.
• false || true → True.

Output:

Bonus approved

Step 4 — Modify the code

Change the values and predict the output.

Learning point: OR returns false only when both conditions are false.

Example 4: NOT (!) — Checking Registration

Problem: Display a message if a student is not registered.

Step 1 — Write the code

 
#include <iostream>
using namespace std;
 
int main() {
    bool registered = false;
 
    if (!registered) {
        cout << "Please register first";
    } else {
        cout << "Registration complete";
    }
 
    return 0;
}
 

Step 2 — Understand the code

The variable registered is a Boolean variable. It can store true or false.

• registered = false
• !registered becomes true.
• The first block executes.

Output:

Please register first

Step 3 — Modify the code

Change:

bool registered = false;

to:

bool registered = true;

Expected output:

Registration complete

Learning point: The NOT operator reverses the Boolean value.

5. Practice Coding Exercises

Important: When combining && and || in the same condition, use parentheses to make the intended logic clear. For example:

if (marks >= 60 && (hasTestResult || hasExemption))

This means the student needs marks of at least 60 and must have either a valid test result or an approved exemption.