Boolean logic

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Boolean logic#

The questions we had previously are in fact Boolean expressions.

Boolean expression

an expression that produces a Boolean value. Such a value is either true or false.

Type is bool, which stands for Boolean.

These operators create a Boolean expression from other datatypes. Technically they input other datatypes and output a bool

  • >, >=

  • <, <=

  • ==, !=

These operators combine Boolean expressions. Technically they input bool variables and output a bool.

  • &&

  • ||

We use Boolean logic in branching (if-else, switch) and loops (while, for).

Examples#

// Defining a Boolean variable
var iAmHavingFun = true;

// Simple boolean comparisons
var isEqual = 5 == 5; // true
var isNotEqual = 5 != 5; // false
var isGreater = 10 > 5; // true
var isLess = 5 < 10; // true
var isGreaterThanEqual = 5 >= 5; // true
var isLessThanEqual = 10 <= 5; // false

// Combination operators
var andCondition = 5 > 3 && 10 < 20; // true (both must be true)
var orCondition = 5 > 10 || 3 < 4; // true (at least one must be true)
var notCondition = !(5 == 5); // false (negation)

// Functions can also return Boolean values
List<string> fruits = ["apple", "banana", "cherry"];
var containsApple = fruits.Contains("apple"); // true
var containsOrange = fruits.Contains("orange"); // false

// Practical example: Check if a number is within a range
var number = 7;
var isInRange = number > 5 && number < 10; // true
// The last compound expression beautified using the `is` operator:
var isInRange2 = number is > 5 and < 10;

Activity 12

Which of the following are Boolean expressions and evaluate to false, assuming the following initialization code? The statements in the choices do not affect others.

var t = true;
var f = false;
var c = 3;
var i = 9;
var k = 11;

Choices:

  1. t == f

  2. t = f

  3. i >= c && k > i

  4. !t

  5. k != 11

  6. 10 < c || c <= 20

Activity 13 (Creating Boolean variables for user access check)

You are implementing the user access logic for a program. Create Boolean expressions for the following sentences:

  • The user is an administrator if the user id is larger than 65536

  • the username has at least three characters.

  • password must contain at least one of the characters $, | and @.

  • password must be at least 20 characters long for an admin and 16 characters long for a non-admin

Use the variables:

// Inputs
string username;
string password;
uint userId; 

// Outputs
var userIsAdmin = false;
var usernameValid = false;
var passwordValid = false;

User the following string functions:

  • if u is a string:

    • u.Length gives the length

    • u.Contains('s') return true if s is in the string

Activity 14

Create a program for Activity 13 that:

  • asks the user to enter the input variables username, password etc.

  • prints a meaningful message if both username and password are valid.

  • prints a meaningful message/s if username or password are not valid.

Appendix#

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