Lecture Notes on Boolean Types and Operators

Selection and Boolean Types

Introduction to Selection

• Selection lets programs change what they do based on data.

• Think of it like a fork in the road: the program chooses which path to take.

• A key part of this is the boolean type, which helps the program make these choices.

Boolean Type

• Booleans are simple: they can only be $true$ or $false$.

• It's like a light switch: either on ($true$) or off ($false$).

• Declaration:

  • In C#, you can use $\texttt{System.Boolean}$

  • Or simply use the $\texttt{bool}$ keyword.

• Storage:

  • Booleans are stored as a byte.

  • Technically, they only need one bit, but they use a full byte.

• Default Value:

  • The default value of a boolean is $false$.

• Examples:

  bool isAtSemesterBreak = false;
  bool areYouBored = true;

• Literals:

  • When you write $true$ or $false$ in code, use lowercase.

• Output:

  • When you print a boolean to the screen, the first letter is capitalized ($True$ or $False$).

Boolean Operators

• These operators help you compare values and get a $true$ or $false$ result.

• Less Than Operator: \,<

  • Returns $true$ if the left side is smaller than the right side.

  • Example:
    csharp 5 < 10 // true

• Greater Than Operator: \,>

  • Returns $true$ if the left side is larger than the right side.

  • Example:
    csharp 10 > 5 // true

• Less Than or Equal To Operator: \,<=

  • Returns $true$ if the left side is smaller than or equal to the right side.

  • Example:
    csharp 5 <= 5 // true

• Greater Than or Equal To Operator: \,>=

  • Returns $true$ if the left side is larger than or equal to the right side.

  • Example:
    csharp 5 >= 5 // true

• Equal To Operator (Equivalence Operator): $\,==$

  • Checks if both sides are the same.

  • Important: Use two equal signs ($==$), not one ($=$), which is for assignment.

  • Example:
    csharp 5 == 5 // true

• Not Equal Operator: $\,!=$

  • Checks if both sides are different.

  • Example:
    csharp 5 != 10 // true

• Two-Character Operators: Make sure the characters are right next to each other (like <= , not < = ).

• Usage with Variables: Booleans are great with variables.

  • Example: A grocery delivery service.

  • A customer spends $80.50.

  • Free delivery starts at $75.
    csharp bool freeDelivery = (amountSpent > threshold); // true

Logical Operators

• These let you combine two boolean expressions.

• Types: AND, OR, EXCLUSIVE OR (XOR).

• AND: Returns $true$ only if both things are $true$.

  • Example: (Is it sunny?) AND (Is it warm?) -> Must be both to be $true$.

• OR: Returns $true$ if either one or both things are $true$.

  • Example: (Is it Saturday?) OR (Is it Sunday?) -> If either is $true$, then it's the weekend.

• XOR: Returns $true$ if only one thing is $true$.

  • Example: (Is the light switch on?) XOR (Is the generator running?) -> If only one is $true$, the power is on.

• Venn Diagram Analogy:

  • Imagine Students (red) and Staff (blue) at QUT.

  • The purple part where they overlap: that's a Tutor who is both a PhD student and staff.

• C# Code Representation:

  booleanExpression1 BOOLEAN_OPERATOR booleanExpression2

• Example: Tutor Status

  • You must be a student AND a staff member to be a tutor.

• Example: Parking Discount

  • You get a discount if you are a student OR a staff member (or both).

Logical Operators in C

||||
|---|---|---|---|
||||
||||

Short Circuit Evaluation

• Using double symbols ($\texttt{\&amp;\&amp;}$, $\texttt{||}$) can speed up your code.

• Double ampersand ($\texttt{\&amp;\&amp;}$):

  • If the first part is $false$, the second part isn't even checked because the whole thing will be $false$.

  • Example:
    csharp (false) && (expensive calculation) // calculation skipped

• Double pipe ($\texttt{||}$):

  • If the first part is $true$, the second part isn't checked because the whole thing will be $true$.

  • Example:
    csharp (true) || (expensive calculation) // calculation skipped

• Efficiency:

  • This can really help if one part takes a long time or happens a lot.

• Best Practice: Always use double symbols ($\texttt{\&amp;\&amp;}$, $\texttt{||}$) for better performance.

• Parentheses:

  • Use parentheses around boolean expressions to avoid mistakes.

Collecting Boolean Input from User

• Input: Users can type $true$ or $false$ (it doesn't matter if it's uppercase or lowercase).

• Invalid Inputs: Typing $yes$ or $no$ will cause an error.

• Code Example:
  1