C++Logical OperatorsLogical NOT

Logical NOT (!) Operator in C++

The Logical NOT operator in C++ is a unary operator (operates on one operand) that reverses the boolean value of an expression.

  • If the condition is true, ! makes it false.
  • If the condition is false, ! makes it true.

This operator is commonly used to negate conditions and write more concise control structures.

Syntax

!condition

Truth Table

Condition!Condition
falsetrue
truefalse

Example 1: Basic Usage

not_basic.cpp
bool isAvailable = true;
std::cout << !isAvailable;  // Output: 0

Explanation:

  • isAvailable is true
  • !isAvailable becomes false → 0 is printed

Example 2: Negating a Comparison

not_comparison.cpp
int age = 15;
std::cout << !(age >= 18);  // Output: 1

Explanation:

  • age >= 18 → false
  • !false → true → Output: 1

Example 3: In an If Statement

not_if.cpp
bool isLoggedIn = false;
 
if (!isLoggedIn) {
    std::cout << "Please log in first.";
}

Explanation:

  • Since isLoggedIn is false, !isLoggedIn becomes true → message is shown

Example 4: With Integers

In C++, integer values can be used in logical expressions:

  • 0 is treated as false
  • Any non-zero value is true
not_integer.cpp
int x = 0;
std::cout << !x;  // Output: 1

Explanation:

  • x is 0 (false), so !x becomes true → Output: 1

Example 5: With Characters

not_char.cpp
char c = 'A';
std::cout << !c;  // Output: 0

Explanation:

  • 'A' has an ASCII value of 65 (non-zero, true)
  • !c → false → Output: 0

Example 6: Complex Condition

not_complex.cpp
int score = 90;
std::cout << !(score < 40 || score > 100);  // Output: 1

Explanation:

  • score < 40 || score > 100 → false
  • !false → true → Output: 1

Example 7: Not in Loops

not_loop.cpp
bool isDone = false;
 
while (!isDone) {
    std::cout << "Processing...\n";
    isDone = true;
}

Explanation:

  • The loop runs once because !isDone is initially true

Summary

  • The ! operator is used to reverse a Boolean value.
  • It’s most useful in if, while, and conditional expressions.
  • Can be used on bool, integers, characters, and expressions that evaluate to Boolean logic.
  • Keep your expressions readable when using ! with complex conditions.

Practice Exercises

  1. Write a program to print “Access Denied” if a user is not authenticated (isAuthenticated = false).
  2. Create a program that prints “Outside range” if a number is not between 10 and 20.
  3. Use the ! operator in a while loop to keep prompting the user until a condition becomes true.
Logical OREqual Operator