CSCI 1301 Book Labs Slides Exercises

Exercises (with solutions)

https://csci-1301.github.io/about#authors

April 5, 2024 (12:47:56 PM)

Homework #1

Part I — Questions

  1. List five pieces of software, and three hardware components of a computer.
    Solution Accept operating system as a software, cellphone is not a component of a computer. Examples of hardware components are random access memory (RAM), central processing unit (CPU), keyboard, mouse, graphical processing unit (GPU). A comprehensive list can be found at https://www.wikiwand.com/en/Computer_hardware.
  2. List four programming languages.
    Solution C, C++, C#, Java, JavaScript, Python, Haskell, Swift, RWBY, etc (Note: HTML is a “Markup Language” and SQL is a “Query Language”, which would technically be incorrect)
  3. What is a GUI?
    Solution Graphical User Interface.
  4. What is a CLI?
    Solution Command-Line Interface.
  5. What, if any, is the difference between a compiler and an assembler?
    Solution The compiler goes from High-level language to Machine language, the assembler goes from Assembly language to Machine language.
  6. Is machine code (or the machine language program) is made of circuits, binary numbers, classes, or compilers?
    Solution Binary numbers.
  7. Give a specific characteristic of C# compared to other programming languages.
    Solution It’s a compiled language, it has automatic garbage collection, it’s object-oriented, it enforces .NET common language specification, etc.
  8. What happens when the source code you are giving to the compiler has a syntax error?
    Solution The compiler returns an error and fails to compile your code.
  9. Is the C# compiler case-sensitive?
    Solution Yes, the C# compiler is case-sensitive (meaning that the IDE differentiates between capital and lowercase letters).
  10. Suppose I replace every empty space in your source code with two empty spaces. Will your program still compile? Why, or why not?
    Solution Yes, because compilers do not care about empty spaces.
  11. Give three keywords.
    Solution You can consult the “official” list of keywords as https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/. Are valid answers: int, using, static, public, etc. Note that main or System are not keywords.
  12. Write a statement that would display, “Hi Mom!” (without the quotes) followed by a new line on the screen, once inserted in a proper method, compiled, and executed.
    Solution Console.WriteLine("Hi Mom!");
  13. Write a statement that would display, “Hello!” (without the quotes) followed by a new line on the screen, once inserted in a proper method, compiled, and executed.
    Solution Console.WriteLine("Hello!");
  14. What is the limitation, if any, to the number of methods you can have per class? Why is the method called Main special?
    Solution A class usually has one or more methods. No limitation, really. It is special because it is the first one to be executed, it is the entry point.
  15. What is the difference between a “rule” and a “convention” in programming language?
    Solution A rule is required by the programming language and enforced by the compiler. A convention is only a recommendation to make your program easier for other humans to read; you can break it and your code will still work.
  16. What is a namespace?
    Solution A named collection of related library code.
  17. In a C# program, can comments start with \\ (double backslash) or with // (double (forward) slash)? Do they have to end with a ; (semicolon)?
    Solution With //, no.
  18. Which of the following, if any, are keywords? Welcome1 public apples int "I’m a string"
    Solution public, int
  19. Which of the following are programmer-defined names (or identifiers)? BankAccount class apples int itemPerCapita statement
    Solution BankAccount, apples, itemPerCapita,statement.
  20. Why are variables called “variables”?
    Solution Because their value may change during the program’s execution.
  21. What is string interpolation?
    Solution The action of inserting a variable into a string that is displayed at the screen.
  22. What is the difference, if any, between 12, and "12"?
    Solution The first one is an integer, the second is a string.
  23. What is the difference, if any, between the WriteLine and Write methods?
    Solution The Write method does not advance the cursor to the next line after the message is displayed at the screen.
  24. Write a statement that would display the following on the screen: Hi Mom!↵ How are you doing?
    Solution Console.Write("Hi Mom!\nHow are you doing?");
  25. Assume we have a variable whose name is myVariable, type is string, and value is "My message". What would be displayed on the screen by the following statement? Console.WriteLine($"Here is my variable: {myVariable}");
    Solution {text} Here is my variable: My message
  26. Assume we have a variable whose name is level, whose type is string, and whose value is "Easy". What would be displayed at the screen by the following statement? Console.WriteLine($"You set the difficulty to {level}.");
    Solution {text} You set the difficulty to Easy.
  27. Which of the following are correct identifier names? $myHome3 class my%variable ANewHope _train _ThisIsAVariable statement Is the name myVariable the same as myvariable? If not, why?
    Solution $myHome3 is correct, but not recommended, ANewHope, _ThisIsAVariable, statement are also correct (but, of course, this latter is weird). The names myVariable and myvariable are treated differently by C#, because it is case-sensitive.
  28. Which of the following are correct identifier names? myClass _Exo_1 Lab3-Exo1 My.Lab.Variable using Lab3_Part1
    Solution Identifiers: myClass, Lab3_Part1
  29. Which of the following are keywords? myClass static Lab3-Exo1 “Hello World” using Lab3_Part1
    Solution Keywords: static, using
  30. Which of the following are correct identifier names? 12_Dec_2019 Lab3-Exo1 MyClass2 My.Lab.Variable string My_Var
    Solution OK: MyClass2, My_Var Not OK: 12_Dec_2019, Lab3-Exo1, My.Lab.Variable, string
  31. Which one(s) of the following, if any, is a correct assignment (assuming that variable, x and apples have been declared as int variables)? 5 => variable; x=5; apples= 23 x <= 23; variable =1,890;
    Solution Only x=5; is correct. For instance, apples= 23 is not correct because a ; is missing: since an assignment is a statement, it must ends with a ;
  32. Write a statement that assigns the value 23 to a variable myAge of type int. You do not need to re-declare that variable.
    Solution myAge = 23;
  33. Cross out the wrong answer in the following sentences, [ like this (incorrect)  |  like this (correct) ]:
    • “If the code does not obey the [  rules  |  conventions ] of C#, then the compiler will complain.”
    • “Every statement needs to end with [  a forward slash /  |  a semi-colon ;  ].”
    • “C# is a [  object-oriented  |  functional ] programming language.”
    • “A class is made up of [  a body and a header  |  multiple using statements ].”
    • “An identifier can contain [  only lower-case letters  |  letters and digits  ].”
    Solution
    • If the code does not obey the rules of C#, then the compiler will complain.
    • Every statement needs to end with a semi-colon ;.
    • C# is an object-oriented programming language.
    • A class is made up of a body and a header.
    • An identifier can contain letters and digits.

Part II – Problems

  1. There are 4 errors in the following code that will prevent it from compiling. Can you spot them all?

    // My first attempt.
using System
class Wel
{
    static void Main();
    {
        ConsoleWriteLine("Welcome \n to the lab!");
    }

    Solution
    1. Missing ; after using System
    2. superfluous ; after static void Main()
    3. missing . between Console and WriteLine
    4. missing closing } for class Wel

Homework #2

Part I — Questions

  1. In C#, what is the “escape character”? Why is it useful?
    Solution \, to print “special characters”, like new line, tabulation, quotations mark, etc.
  2. Write a statement that initializes a variable named myHeightInMeters to your height in meters. What should be the datatype of myHeightInMeters?
    Solution float myHeightInMeters = 1.68f The datatype should be any floating-point datatype.
  3. Suppose you replace every * in your program with the ! symbol. Are you completely sure that your program would still compile? Why or why not?
    Solution No, because /* will be transformed into /!.
  4. Give the values of a and b after the following four instructions have been executed. int a, b; a = 2; b = a * 2 + 1; a -= 1;
    Solution a is 1, b is 5.
  5. Give the values of a and b after the following four instructions have been executed. int a, b; a = 4; b = a * 3 + 1; a /= 2;
    Solution a is 2, b is 13.
  6. Give the values of c and d after the following four instructions have been executed. int c = 3, d; d = 2 + c; c = d * 2; d += 2;
    Solution c is 10, d is 7.
  7. Is there an error in the following code? Explain the error or give the value of b after the second statement is executed. float a = 3.7f; int b = (int)a;
    Solution No error; 3
  8. Is there an error in the following code? Explain the error or give the value of b after the second statement is executed. decimal a = 1.6M; int b = (int)a + a;
    Solution Error, the result type of the operation is a decimal, and cannot be stored in an int.
  9. There is an error in the following code, at the second line. Explain the error, and how you could fix this second line using a cast operator, without changing the datatype of the b variable. decimal a = 2.5M; int b = a / 2;
    Solution The result of the expression a / 2 is of type decimal, so it cannot be stored in an int. This can be fixed by adding an (int) cast to the variable a before dividing it by 2, so that the expression’s result type is int, like this: int b = (int)a / 2;
  10. If one of the operator’s operands is of type float and the other is of type int, what will be the type of the result of the operation?
    Solution float
  11. What is the return type of the operation 12.4 * 3?
    Solution double
  12. Write an explicit conversion from a double variable myDoubleVar to an int variable called myIntVar. You do not need to re-declare those variables. Assuming myDoubleVar’s value is 5.89, what value would be stored in myIntVar?
    Solution myIntVar = (int)myDoubleVar;, the value stored in myIntVar would be 5.
  13. Write a statement that performs an implicit conversion between two different numeric datatypes.
    Solution float m = 3;
  14. Assuming that myLastName and myFirstName are two string variables that have been initialized, write a statement that concatenates them with a space and a comma in-between, and assign the resulting string to a variable named fullName. For instance, if the value of myLastName is "Holbertonand", and the value of myFirstName is "Betty", then the value of fullName after your operation should be "Holbertonand, Betty".
    Solution string fullName = myLastName + ", " + myFirstName;
  15. In C#, what is the name of the method used to read input from the user?
    Solution ReadLine().
  16. What is wrong with the following code? Will the error(s) appear at compilation time or at execution time? int age; Console.WriteLine("Please enter your age:"); age = Console.ReadLine();
    Solution The problem is that we are trying to store a string value (what the Console.ReadLine() will return) into an int variable. This will trigger an error at compilation time.
  17. Will those statements, if placed in a proper Main method, compile? Could this program crash at execution time? Justify your answer. int myAge; Console.WriteLine("Please enter your age:"); myAge = int.Parse(Console.ReadLine());
    Solution The code will compile, but can crash at execution time if the end user inputs a non-integer value (for instance, “This is a test”).
  18. Write a series of statements that: a) Declare an int variable named userAge, b) Display on the screen a message asking the user to enter his or her age, c) Read the value entered by the user and store it in the userAge variable. You can add statement(s) performing intermediate steps if you want.
    Solution int userAge; Console.WriteLine("Please enter your age."); userAge = int.Parse(Console.ReadLine()); Note: This is not the only solution, but the most optimal.
  19. Write a series of statements that: a) Declare an string variable named favoriteColor; b) Display on the screen a message asking the user to enter his or her favorite color; c) Read the value entered by the user and store it in the favoriteColor variable. You can combine some of the statement(s) if you want, but do not display at the screen any information that was not explicitely asked.
    Solution string favoriteColor; Console.WriteLine("Please enter your favorite color."); favoriteColor = Console.ReadLine(); Note: This is not the only solution, but the most optimal.

Part II – Problems

The following three exercises do not require a computer. Make sure you feel ready before starting them, try to do them with limited time and without notes, and, if you want, check your answer using your IDE.

  1. This problem restates the content of the Order of Operations section of the lecture notes and ask you to answer various problems.

    There are 5 different arithmetic operations available in C#:

    Operation Arithmetic Operator Algebraic Expression Expression
    Addition + x + 7 myVar + 7
    Subtraction - x − 7 myVar - 7
    Multiplication * x × 7 myVar * 7
    Division / x/7, or x ÷ 7 myVar / 7
    Remainder (a.k.a. modulo) % x mod  7 myVar % 7

    Computing operations involving one of them is straightforward:

    Operation Result
    3 + 4 7
    3 - 4 -1
    3 * 4 12
    6 / 2 3
    6 % 4 2

    But things can get complicated when multiple operators are used, but no parenthesis are indicated. For instance, should

    7 / 2 - 4 * 8 % 3

    be read as

    (7 ÷ 2) − ((4 × 8) mod  3)  = 3.5 − (32 mod  3)
     = 3.5 − 2
     = 1.5

    or as

    (7 ÷ (2 − 4)) × (8 mod  3)  = (7 ÷ (−2)) × 2
     = (−3.5) × 2
     = −7

    Certainly, the result is not the same and there are other possibile ways this calculation may be performed!

    Actually, C# uses the following three rules:

    1. *, /, and %, called the “multiplicative operations,” are always evaluated before + and -, called the “additive operations.” So that, for instance,

      2 - 4 * 8

      will be evaluated as 2 − (4 * 8) = −30.

    2. If there are multiple operations of the same type, they are evaluated from left to right. For instance,

      4 / 2 * 8

      will be evaluated as (4 ÷ 2) × 8 = 16 and

      4 - 2 + 8

      will be evaluated as (4 − 2) + 8 = 10.

    3. Parenthesis can be used to force a particular order of evaluation, so that 2 * (3 + 4) will be evaluated as 2 × (3 + 4) = 2 × 7 = 14, not as (2 × 3) + 4 = 6 + 4 = 10 as it would without the parenthesis.

    Answer the following:

    1. Which of the following operation(s) compute the arithmetic expression (x × (3 mod  5)) − (y × 7)?

      1. x * 3 % 5 - y * 7
      2. x * (3 % 5) - y * 7
      3. (x * 3) % 5 - y * 7
      4. x * 3 % (5 - y * 7)
      5. (x * 3 % 5) - (y * 7)
      6. (x * ((3 % 5) - (y * 7))

    2. State the order of evaluation of the operators in each of the following operations, and compute the resulting value:

      1. 8 - 39 * 1 / 12 + 5
      2. 12 + -23 / 12 % 3
      3. 90 * 23 / 34 - 12 - 13
      4. 12 % 83 - 2 * 3

    3. (Optional) Check your answers using your IDE. You can use a statement of the form:

      Console.WriteLine($"8 - 39 * 1 / 12 + 5 is {8 - 39 * 1 / 12 + 5}");

    Solution

    No solution is offered for this problem: please, try to process the information and to figure out the answers on your own. As suggested, using your IDE will be an excellent way of making sure that your calculations were correct.

  2. Write down, on a piece of paper, a fully compilable program that initializes an int variable named persons with the value 5, an int variable named bottles with the value 3, and a double variable named literPerBottle with the value 1.5. What should be the type of the literPerPerson variable to be able to be assigned the number of liters every person is going to get, if split equitably? Write the correct initialization of that variable and a statement that displays its value.

    Place a delimited comment with a your name and the time at which you wrote the program at the top of the program.

    Solution

     
     /* This is my code, by Clément Aubert, 31st of January 2024, around 13:15.*/
 using System;

 public class Program
 {
 public static void Main(string[] args)
 {

     int persons = 5;
     int bottles = 3;
     double literPerBottle = 1.5;
     double literPerPerson = (bottles * literPerBottle) / persons;
     // Note that since literPerBottle is a double, we are using double product and division.
     Console.WriteLine(bottles +" bottles of " + literPerBottle + " liter,  split equitably between "
                     + persons +" persons gives " 
                     + literPerPerson + " liter / person.");
     }
 }

  3. Write down, on a piece of paper, a program that:

    1. Declares a string variable named userName.
    2. Displays on the screen “Please enter your name, followed by enter:”.
    3. Reads a string value from the keyboard and assigns the value to the userName variable.
    4. Declares an int variable named number.
    5. Displays on the screen “Please enter your number:”.
    6. Reads an int value from the keyboard and assigns the value to the number variable.
    7. Declares a string variable named id and initializes it with the string referenced by the userName variable, followed by the number entered by the user (you can concatenate a string and an int using the + sign).
    8. Displays on the screen, “Your id is” and the content of the id variable.

    Here is an example of execution, where the user input is underlined, and hitting “enter” is represented by ↵:

    {text} Please enter your name, followed by enter. J͟a͟y͟l͟a͟h͟↵͟ Please enter your area code, followed by enter. 4͟9͟3͟9͟1͟↵͟ Your id is Jaylah49391 Press any key to continue . . .

Homework #3

Part I – Questions

  1. What is “an instance of a class”?

    Solution

    An object.

  2. Fill in the blanks: “A class asserts that every objects created using it should have ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ (i.e.,”data”) and ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ ͟ (i.e., “operations”).”

    Solution

    Attributes and methods.

  3. Give two access modifiers.

    Solution

    public and private.

  4. What, if any, is the difference between a parameter and an argument?

    Solution

    “Parameters” are called “formal parameters” and are used when defining the method, while “arguments” are called “actual parameters” and are used when calling the method. A method has parameters and takes arguments.

  5. Write a statement that creates a new object from the Rectangle class.

    Solution

    Rectangle myRec = new Rectangle();

  6. What is the purpose of the keyword new?

    Solution

    To create objects, to instantiate classes.

  7. Do different objects from the same class share their instance variables?

    Solution

    No, there is one instance variable per object.

  8. Briefly explain the difference between a local variable and an instance variable.

    Solution

    A local variable exists only within a single method; instance variables are available/accessible to all methods in a class.

  9. Suppose we have a Circle class containing

    public void SetRadius(double radiusArgument)
{
    radius = radiusArgument;
}

    Write a statement that create a Circle object, and one statement that sets its radius to 3.5.

    Solution

     
    Circle theCircle = new Circle();
theCircle.SetRadius(3.5);

  10. Indicate the order of evaluation of the operators in each of the following C# operations by adding parenthesis or developping the expression one step at a time, and compute the resulting value:

    1. 3 * 4 - 2
    2. 3 % 2 + 3
    3. 2 - 3 + 3 * 2
    4. 2 + 2 * 1 - 4

  11. Write the complete implementation of a class that contains two attributes (with different data types), a setter for one attribute, a getter for the other attribute, a custom constructor, and a ToString method. You can re-use an example from a lecture or a lab, as long as it satisfies those conditions, or you can invent one. No need to write an application program.

    Solution

    Note: In this solution, the Rectangle class from the lab was used, but any class fulfilling the requirements will work.

    class Rectangle
{
    private int length;
    private int width;

    public Rectangle(int lengthP, int widthP)
    {
        length = lengthP;
        width = widthP;
    }
    public void SetLength(int lengthParameter)
    {
        length = lengthParameter;
    }
    public int GetWidth()
    {
        return width;
    }
    public override string ToString()
    {
        return "Length: " + length + ", Width: " + width;
    }
}

  12. What does the keyword return do?

    Solution

    It is used by a method to return a value to the environment that called it.

  13. Write a get method for an instance variable named total of type int.

    Solution

    public int GetTotal() { return total; }

  14. Write a getter for an attribute of type string named myName.

    Solution

    public string GetMyName() { return myName; }

  15. Write a setter for an attribute of type int named myAge.

    Solution

    public void SetMyAge(int paramMyAge) { myAge = paramMyAge; }

  16. Assuming name is a string instance variable, there is problem with the following setter. Fix it.

    public int SetName(string val){
    name = val;
}

    Solution

    It has the wrong return type, because it does not (and should not) return a value, i.e., replace int with void.

  17. Assume we have an instance of the Rectangle class named myRect and an instance of the Circle class named myCircle. Write statement(s) that will make the radius of myCircle equal to the width of myRect.

    Solution

    myCircle.SetRadius(myRect.GetWidth());

  18. Briefly describe what a format specifier is. Write a statement that uses one.

    Solution

    An indication to format a numerical value in a special way in a string. For example, Console.WriteLine($"{65536:N");} will display “65,536.00” on the screen after applying the :N format specifier to the value 65536.

  19. Write a statement that uses a format specifier.

    Solution

    Console.WriteLine($"myInt:C");

  20. Write a method for the Rectangle class that divides the length and width of the calling object by a factor given as a parameter.

    Solution

    public void DivideBy(int factor) { length /= factor; width /= factor; }

  21. Draw the UML diagram of a class named “Student” with a single attribute, “name”, of type string, and two methods, SetName and GetName.

    Solution
    Student
    - name: string
    + SetName(arg: string)
    + GetName(): string

  22. Write a ToString method for a Account class with two attributes, a string attribute called name and a decimal attribute called amount.

    Solution

    public override string ToString() { return $"Account Name: {name}\nAccount Balance: {amount}"; }

  23. Consider the following UML diagram:

    Circle
    - radius : float
    + setRadius(radiusParam : float) : void
    + getRadius(): float
    + getArea(): float

    What is the name of the class, what are the methods and attributes of the class?

    Solution

    The class is named Circle, it has a single attribute (radius) and three methods (setRadius, getRadius and getArea).

  24. What does it mean to say that instance variables have a default initial value? How is that different from the variables we have been manipulating in the Main method?

    Solution

    When we create an object, the instance variable get a default value. When we declare a variable, it is unassigned.

  25. Is it possible to have more than one constructor defined for a class? If yes, how can C# know which one is called?

    Solution

    Yes, by looking at the signature.

  26. What is the name of a constructor method? What is the return type of a constructor?

    Solution

    The name of the class. It does not have a return type, not even void.

  27. Write a constructor for a Soda class with one string attribute called name.

    Solution

    public Soda() { name = "Generic"; }

  28. Assume we have a Polygon class, that have only one attribute, an int called numberOfSides. Write a constructor for that class.

    Solution

    public Polygon (int numberOfSidesParam) { numberOfSides = numberOfSidesParam; }

  29. What is the “default” constructor? Do we always have the possibility of using it?

    Solution

    The constructor provided with the class by default, that set all the attributes to their default values. If we define our own constructor, this one disappears.

  30. What is the return type of a ToString method? How many arguments does it usually take?

    Solution

    string; 0.

  31. Consider the following partial class definition:

    public class Book
{
    private string title;
    private string author;
    private string publisher;
    private int copiesSold;
}
    1. Write a statement that would create a Book object.
    2. Write a “getter” and a “setter” for the title attribute.
    3. Write a constructor for the Book class taking at least one argument (you are free to decide which one(s)).

    Solution
    1. Book myBook = new Book(); 2 and 3.
    public string GetTitle() 
{ 
    return title; 
}

public void SetTitle(string titleP) 
{ 
    title = titleP; 
}

public Book (string titleP, string authorP, string pubP, int copiesP) 
{ 
    title = titleP; 
    author = authorP; 
    publisher = pubP; 
    copiesSold = copiesP 
}

  32. Consider the following partial class definition:

    class DVD
{
    private string title;
    private decimal price;
}
    1. Write a “setter” for the title attribute.
    2. Write a constructor for the DVD class that takes two arguments.
    3. Write a method called Discount that decreases the price attribute by 20.55%.
    4. Write a (good, informative) ToString method for the class.
    5. Write statements that ask the user to enter a price and then create a DVD object with a price attribute equal to the price the user entered. (The object’s title attribute can be anything you choose).
    6. Draw the UML class diagram for the class you obtained by adding the above four methods to our original class definition.

    Solution

     
     class DVD
 {
     private string title;
     private decimal price;

     public void SetTitle(string titleP)
     {
         title = titleP;
     }

     public DVD(string titleP, decimal priceP)
     {
         title = titleP;
         price = priceP;
     }

     public void Discount()
     {
         price -= price * 0.2055
     }

     public override string ToString()
     {
         return $"DVD Title: {title}\nDVD Price: {price}"
     }
 }

 //program.cs
 Console.WriteLine("What should the price be for the movie "Ender's Game?");
 DVD myDVD = new DVD("Ender's Game", decimal.Parse(Console.ReadLine()));
    DVD
    - title : string
    - price : decimal
    + SetTitle(titleP : string) : void
    + <> DVD(titleP : string, priceP : double)
    + Discount() : void
    + ToString() : string

    Note: This is not the only answer, but is the most optimal.

  33. Consider the following partial class definition:

    class Book
{
    private string title;
    private decimal price;
}
    1. Write a “getter” for the title attribute.
    2. Write a constructor for the Book class that takes two arguments.
    3. Write a method called AddTaxes that increases the price attribute by 6.35%.
    4. Write a (good, informative) ToString method for that class.
    5. Write statements that ask the user to enter a price and then create a Book object with a price attribute equal to the price the user entered. (The object’s title attribute can be anything you choose).
    6. Draw the UML class diagram for the class you obtained by adding the above four methods to our original class definition.

    Solution

     
     class Book
 {
     private string title;
     private decimal price;

     public string GetTitle() { return title; }

     public Book(string titleP, decimal priceP)
     {
         title = titleP;
         price = priceP;
     }

     public void AddTaxes()
     {
         price *= 1.0635m;
     }

     public override string ToString()
     {
         return $"Book Title: {title}, Price: {price}.";
     }
 }

 //program.cs
 Console.WriteLine("What should the price be for the book "Ender's Game?");
 Book myBook = new Book("Ender's Game", decimal.Parse(Console.ReadLine()));
    Book
    - title : string
    - price : decimal
    + GetTitle() : string
    + <> Book(titleP : string, priceP : double
    + AddTaxes() : void
    + ToString() : string

    Note: This is not the only answer, but is the most optimal.

  34. Assume that my Pet class contains one custom constructor:

    public Pet(string nameP, char genderP){
    name = nameP;
    gender = genderP;
}

    What is the problem with the following statement?

    Pet myPet = new Pet('M', "Bob");

    Solution

    The method has for parameters a string and a char, but the problematic statement calls it with a char and a string: the order of the arguments is wrong, it should be Pet myPet = new Pet("Bob", 'M');.

  35. Why would one want to define a constructor for a class?

    Solution

    To be able to set the instance variables directly when creating the objects.

Part II – Problems

There is only one problem this time, and it is harder than what you’ll be asked to do during the exam. Being able to solve it is an excellent sign that you are ready.

  1. You are going to design a class named Triangle. A triangle has three angles, but knowing the value of only two angles is sufficient to determine the value of the third, since they always add up to 180°. Hence, it is sufficient to have only two double attributes, angle1 and angle2. We want to define several methods:

    • a no-arg constructor that sets the value of angle1 to 60.0 and the value of angle2 to 60.0,
    • another constructor, that takes two arguments, and assigns to angle1 the value of the first argument, and assigns to angle2 the value of the second argument,
    • getters for angle1 and angle2,
    • a method that computes and returns the value of the third angle, that we name ComputeAngle3,
    • a method that rotate the triangle: the value of the first angle should be replaced with the value of the second angle, and the value of the second angle should be replaced with the value of the third angle.
    1. Write the UML diagram for the Triangle class.
    2. Write the full, compilable implementation of the Triangle class.

    Solution
    Triangle
    - angle1: double
    - angle2: double
    + Triangle()
    + Triangle(angle1P: double, angle2P : double)
    + GetAngle1(): double
    + GetAngle2(): double
    + ComputeAngle3(): double
    + Rotate(): void 
    class Triangle
{
    private double angle1;
    private double angle2;
    public Triangle()
    {
        angle1 = 60;
        angle2 = 60;
    }
    public Triangle(double angle1Param, double angle2Param)
    {
        angle1 = angle1Param;
        angle2 = angle2Param;
    }
    public double GetAngle1()
    {
        return angle1;
    }
    public double GetAngle2()
    {
        return angle2;
    }
    public double ComputeAngle3()
    {
        return 180 - angle1 - angle2;
    }
    public void Rotate()
    {
        double oldAngle3 = ComputeAngle3();
        angle1 = angle2;
        angle2 = oldAngle3;
    }
}

Homework #4

Part I — Questions

  1. What is sequential processing?

    Solution

    When the code is executed sequentially, without any branching. It implies that the code is processed in the order in which it is presented in the source code: the statement at line n will always be executed after the statement at line n − 1 and before the statement at line n + 1.

  2. What is a decision structure?

    Solution

    A decision structure is a test and one or multiple statement blocks that may or may not be executed based on the outcome of the test. Selection and iteration are two examples of decision structures: in the first one, a statement block can be “skipped over” if the test evaluates to false, in the second one, a statement block can be repeated multiple times, as long as the test evaluates to true. A decision structure makes it possible to have portions of the code executed conditionally.

  3. Decide if the following boolean expressions will evaluate to true or false:

    1. 3 > 2.0 && false
    2. (4 != 3) || false
    3. 'A' == 'b' && ! false
    4. (! false) == (true || 4 == 3)
      Solution
      1. false
      2. true
      3. false
      4. true

  4. Decide if the following Boolean expressions will evaluate to true or false:

    1. 3 > 2.0 || true
    2. (4 != 3) && false
    3. 'A' == 'b' || ! false
    4. (! true) == (true || 4 != 3)
      Solution
      1. true
      2. false
      3. true
      4. false

  5. For each of the following Boolean expressions, decide if it will evaluate to true or false:

    1. ('y' == 'Y') && true
    2. 6 + 2 < 8 || 3 > 4
    3. (true && 4 == 3) == false
    4. 4 > 4 && !false
      Solution
      1. false
      2. false
      3. true
      4. false

  6. For each of the following Boolean expressions, decide if it will evaluate to true or false:

    1. ('y' != 'Y') && true
    2. 6 + 2 < 12 || 3 > 4
    3. (true && 4 >= 3) == false
    4. 13 <= 4 * 3 || !false
      Solution
      1. true
      2. true
      3. false
      4. true

  7. What relational operator is used to determine whenever two values are different?

    Solution

    !=

  8. How do you store the result of a Boolean expression?

    Solution

    In a variable of type bool. Its two possible values are true and false.

  9. Give three relational operators, and then two logical operators.

    Solution

    Possible answers: <=, >=, ==, !=, >, < and !, &&, ||

  10. What would be displayed on the screen by the following code?

    if (false)
{
    Console.WriteLine("Hello!");
}
Console.WriteLine("Hi!");

    Solution

    “Hi!”

  11. Is there a simpler way to write the expression over21 == true, assuming that over21 is a Boolean variable?

    Solution

    We can simply write over21, which will always evaluate to the same value as over21 == true.

  12. Assume that x and y are two int variables that have already been initialized (i.e., declared and assigned), write an if statement that assigns 10 to x if y is (strictly) greater than 5.

    Solution

    if (y > 5) x = 10;

  13. In C#, is there a difference between = and ==? Write a statement that uses =.

    Solution

    Yes, one equal sign serves to write assignment operator, and two equal signs serve to compare. An example of statement that uses comparison first and assignment second could be: if (x == 9)x = 12;

  14. Assuming a name string was declared and initialized with a value given by the user, write an if statement that displays “I have the same name!” if name contains your first name.

    Solution

     
    if(name=="Clément") 
    Console.WriteLine("I have the same name!");

  15. Is the following statement correct, i.e., would it compile, assuming myFlag is a bool variable, and myAge is an initialized int variable?

    if ( myAge > 20 )
{
    myFlag = true
};

    Solution

    No, the semi-colon should come before the closing brace.

  16. If we write a statement that begins with if(false), then the IDE returns a warning, “Unreachable code detected”. What does it mean?

    Solution

    The IDE is warning us that the statements in the block will never be executed.

  17. Write an if statement that prints “Bonjour !” if the value of the char variable lang is 'f'.

    Solution

     
    if (lang == 'f') 
    Console.WriteLine("Bonjour !");

  18. For each of the following boolean expressions, decide if it will evaluate to true or false when the boolean variables x, y and z are all set to true:

    • x || y && z
    • !x || y && z
    • !(x || y) && (z && y)
    • (!x && x) || (!x || x)

    Do the same when they are all set to false.

    Solution

    For each expression, we give first the value if all the variables are set to true, then the value if all the variables are set to false.

    • true, false
    • true, true
    • false, false
    • true, true

    You can check the answer using for instance the following code.

    using System;
class MainClass {
    public static void Main (string[] args) {
        bool x, y, z;
        x = true;
        y = true;
        z = true;
        Console.WriteLine(x || y && z);
        Console.WriteLine(!x || y && z);
        Console.WriteLine(!(x || y) && (z && y));
        Console.WriteLine ((!x && x) || (!x || x));
        x = false;
        y = false;
        z = false;
        Console.WriteLine(x || y && z);
        Console.WriteLine(!x || y && z);
        Console.WriteLine(!(x || y) && (z && y));
        Console.WriteLine ((!x && x) || (!x || x));
    }
}

  19. Write a boolean expression that evaluates to true if a variable x is between 3 (excluded) and 5 (included).

    Solution

    x>3 && 5>=x

  20. Write an if-else statement that assigns 0.1 to z if y is greater or equal than 0, and that assigns −0.1 to z otherwise.

    Solution

    if(y >= 0){ z = 0.1; } else{ z = -0.1; }

  21. Write an if-else statement that assigns "Minor" to an already declared string variable legalStatus if age is strictly less than 18, and that assigns "Major" to legalStatus otherwise.

    Solution

    if(age < 18){ legalStatus = "Minor"; } else{ legalStatus = "Major"; }

  22. Write an if-else statement that displays “It’s free for you!” if an int variable age is between 0 and 18, and “It’s $5.00.” otherwise.

    Solution

    if(age <= 18 && age >= 0){ Console.WriteLine("It's free for you!"); } else{ Console.WriteLine($"It's {5M:C}."); }

  23. Assume we initialized an int variable called courseNumber and a string variable called courseCode. Write a series of statements that will display:

    1. “I’m taking this class!” if courseNumber is 1301 and courseCode is CSCI;
    2. “That’s my major!” if courseCode is CSCI;
    3. “Is that an elective?” if courseNumber is greater than 3000; or
    4. “Is it a good class?” otherwise.

    Your program should display exactly one message.

    Solution

     
    if(courseNumber == 1301 && courseCode == "CSCI")
{
    Console.WriteLine("I'm taking this class!");
}
else if(courseCode == "CSCI")
{
    Console.WriteLine("That's my major!");
}
else if(courseNumber > 3000)
{
    Console.WriteLine("Is that an elective?");
}
else
{
    Console.WriteLine("Is it a good class?");
}

  24. Assume we previously initialized an int variable called graduationYear and a string variable called graduationSemester. Write a series of statements that will display:

    1. “I will graduate at the same time!” if graduationYear is 2023 and graduationSemester is Fall;
    2. “I love this season.” if graduationSemester is Spring;
    3. “That is in a long time!” if graduationYear is greater than 2025; or
    4. “I hope you’ll have an in-person ceremony!” otherwise.

    Your program should display exactly one message.

    Solution

     
    if(graduationYear == 2023 && graduationSemester == "Fall")
{
    Console.WriteLine("I will graduate at the same time!");
}
else if(graduationSemester == "Spring")
{
    Console.WriteLine("I love this season.");
}
else if(graduationSemester > 2025)
{
    Console.WriteLine("That's in a long time!");
}
else
{
    Console.WriteLine("I hope you'll have an in-person ceremony!");
}

  25. Assume we previously initialized a char variable called myChar. Write a series of statements that will display if the character is…

    1. Uppercase
    2. Lowercase
    3. A number
    4. or none of those.

    Your program should display exactly one message. Bonus: Make your message also display the ASCII value of the character.

    Solution

     
    if ((myChar >= 'a') && (myChar <= 'z')) 
{
    Console.WriteLine($"Your character ({(int)myChar}) is lower-case!");
}
else if ((myChar >= 'A') && (myChar <= 'Z')) 
{
    Console.WriteLine($"Your character ({(int)myChar}) is upper-case!");
}
else if ((myChar >= '0') && (myChar <= '9')) 
{
    Console.WriteLine($"Your character ({(int)myChar}) is a number!");
}
else
{
    Console.WriteLine($"Your character ({(int)myChar}) is not a letter or number!");
}

  26. What will be displayed on the screen by the following program?

    int x = 3, y = 2, z = 4;
if (x > y) {z += y;}
if (x > z) {y -= 4;}
Console.WriteLine($"x is {x}, y is {y}, and z is {z}.");

    Solution

    “x is 3, y is 2, and z is 6.”

  27. What will be displayed on the screen by the following program?

    int x = 3, y = 2, z = 4;
if (x >= z) {z += y;} else if (x != y) {z *= y;}
y -= 4;
Console.WriteLine($"x is {x}, y is {y}, and z is {z}.");

    Solution

    “x is 3, y is -2, and z is 8.”

  28. (We’ll use the 24-hour clock, sometimes called “military time”.) Assuming that an int variable hour has been initialized, write part of a program that would display on the screen “Good morning” if hours is less than or equal to 12, and “Hello” otherwise.

    Solution

     
    if (hours <= 12) 
{
    Console.WriteLine("Good morning!");
}
else 
{
    Console.WriteLine("Hello");
}

  29. Assuming that myString is a string variable, write a statement that prints “Hello, Melody!” if the value of myString is equal to Melody, and nothing otherwise.

    Solution

     
    if (myString == "Melody") 
{
    Console.WriteLine("Hello, Melody!");
}

  30. What will be displayed on the screen by the following program?

    int x = 3, y = 2, z = 4;
if (y >= z) {z += y;}
else if (x != y) { if (false) {z -= 3;} else {z += x;}}
Console.WriteLine($"x is {x}, y is {y}, and z is {z}.");

    Solution

    x is 3, y is 2, and z is 7.

  31. Rewrite, if possible, the three following if-else-if statements as switch statements:

    if (myLang == 'f')
{
    Console.WriteLine("Vous parlez Français ?");
}
else if (myLang == 'e')
{
    Console.WriteLine("Do you speak English?");
}
else if (myLang == 'd')
{
    Console.WriteLine("Sprechen Sie Deutsch?");
}
else
{
    Console.WriteLine("I do not know your language!");
}
    if (myCity == "Augusta")
{
    Console.WriteLine("I also live here!");
}
else if (myCity == "Ithaca" || myCity == "Providence")
{
    Console.WriteLine("I used to live there!");
    }
else
{
    Console.WriteLine("I never lived there.");
}
    if (temp == 100.0)
{
    Console.WriteLine("It's ready!");
}
else if (temp >= 90.0)
{
    Console.WriteLine("Almost ready!");
}
else
{
    Console.WriteLine("You have to wait.");
}

    If you think it is not possible or not feasible, explain why.

    Solution

     
    switch (myLang)
{
    case 'f':
        Console.WriteLine("Vous parlez Français ?");
    break;
    case 'e':
        Console.WriteLine("Do you speak English?");
    break;
    case 'd':
        Console.WriteLine("Sprechen Sie Deutsch?");
    break;
    default:
        Console.WriteLine("I do not know your language!");
    break;
}
    switch (myCity)
{
    case "Augusta":
        Console.WriteLine("I also live here!");
    break;
    case "Ithaca":
    case "Providence":
        Console.WriteLine("I used to live there!");
    break;
    default:
        Console.WriteLine("I never lived there.");
    break;
}

    The last one is impossible, since we cannot write ‘switch’ statements comparing all the possible float values!

  32. Give an example of an if statement that could not be rewritten as a switch.

    Solution

    Any condition not making a simple comparison is a good attempt. For instance, trying to convert if(age % 2 == 0){Console.WriteLine("Your age is even.");. into a switch would require to list all the even values, which is not realistic.

  33. Write a switch statement that sets a double discount variable to 0.5 if a string day variable contains "Monday" or "Wednesday", 0.25 if day contains "Saturday", and 0.5 otherwise.

    Solution

     
    switch (day)
{
    case "Saturday":
    {
        discount = 0.25;
        break;
    }
    default:
    {
        discount = 0.5;
        break;
    }
}

Part II – Problems

This time, the two exercises do not require a computer, and are here to craft on your problem-solving skills. Make sure you feel ready before starting them, try to do them with a limited amount of time and without notes, and check your answer using your IDE.

  1. Write a program that asks the user to write a country name and stores the user’s input into a string variable. Then, compare that string with "france": if it is equal, then display at the screen "Bienvenue en France !". Then, compare that string with "usa": if it is equal, then display at the screen "Welcome to the US!". If the string is different from both "france" and "usa", then display at the screen "Welcome to" followed by the name of the country the user typed in. Can you think of two ways to implement this program, one using if-else-if statements, the other using switch?

    Solution

    A possible solution using switch is:

    Console.WriteLine("Please, enter a country name?");
string country_answered = Console.ReadLine();
switch (country_answered)
{
    case "usa":
        Console.WriteLine("Welcome to the US!");
    break;
    case "fr":
        Console.Write("Bienvenue en France!");
    break;
    default:
        Console.WriteLine($"Welcome to {country_answered}");
    break;
}

  2. You want to write a small program for an on-line printing company. Your program should ask the user to chose a format (10 × 15 centimeters, or 8 × 11 inches), ask if it is the first time the customer order through your company, and a number of copies. Then, calculate the total cost of printing those pictures, knowing that

    • Printing a 10 × 15 centimeters picture costs $0.20, printing a 8 × 11 inches picture costs $0.25,
    • A new customer gets a $3 coupon if the order is more than $5,
    • A 10% discount is given if more than 50 copies were ordered,
    • The two previous offers can be cumulated.

    Display on the screen a message starting by “Welcome!”, then a new line, then “We cherish our new customers” if it is the first time the user uses your company, “, so we’re giving you a $3 discount!” if the user is allowed to get the coupon, then print the total and “You had a 10% discount!” if the user ordered more than 50 copies. See below for examples of execution, where the user input is underlined, and hitting carriage return is represented by ↵.

    Enter 'c' for 10x15cm, anything else for 8x11in.
c͟ ↵
Is this your first time here? Type 'y' for 'yes'.
y͟ ↵
Enter a number of copies.
9͟0͟ ↵
Welcome!
We cherish our new customers, so we are giving you a $3 discount!
Your total is $13.50. You had a 10% discount!
    Enter 'c' for 10x15cm, anything else for 8x11in.
p͟ ↵
Is this your first time here? Type 'y' for 'yes'.
N͟o͟t͟ ͟a͟t͟ ͟a͟l͟l͟↵
Enter a number of copies.
1͟2͟0͟ ↵
Your total is $27.00. You had a 10% discount!

    Solution

     
    Console.WriteLine("Enter 'c' for 10x15cm, anything else for 8x11in");
char sizeChoice = char.Parse(Console.ReadLine());
Console.WriteLine("Is this your first time here? Type 'y' for 'yes'.");
char firstTime = char.Parse(Console.ReadLine());
Console.WriteLine("Enter a number of copies");
int copies = int.Parse(Console.ReadLine());
// The following statement is explained below.
decimal totalCost = sizeChoice == 'c' ? copies * 0.2m : copies * 0.25m;
if(firstTime == 'y')
{
    Console.WriteLine("Welcome!");
    string message = "We cherish our new customers";
    if(totalCost > 5m)
    {
        totalCost -= 3m;
        message += ", so we're giving you a $3 discount!";
    }
    Console.WriteLine(message);
}
if(copies > 50)
{
    totalCost -= totalCost * 0.1m;
    Console.WriteLine($"Your total is {totalCost:C}. You had a 10% discount!");
}
else
{
    Console.WriteLine($"Your total is {totalCost:C}.");
}

    The decimal totalCost = sizeChoice == 'c' ? copies * 0.2m : copies * 0.25m; statement uses the conditional operator. This particular statement is a shorter way of writing

    decimal totalCost;
if(sizeChoice == 'c'){
    totalCost = copies  * 0.2m;
}
else{
    totalCost = copies * 0.25m;
}

Homework #5

Part I — Questions

  1. Assume you are given an un-assigned string variable letterGrade, and an already assigned float variable numberGrade. Write a small program that assigns "A" to letterGrade if numberGrade is between 100 and 90 (both included), "B" to letterGrade if numberGrade is between 90 (excluded) and 80 (included), etc., and "Invalid data" if numberGrade is strictly lower than 0 or strictly greater than 100. Should you use a switch statement or a if …else if …else?

    Solution

    An if …else if …else is the right structure for this task:

    float numberGrade;
string letterGrade;
numberGrade = -60; // This is just an example, feel free to change it.
if(numberGrade > 100 || numberGrade < 0)
{
// It's actually easier to get rid of the "invalid" cases first.
    letterGrade = "Invalid Data";
}
else if (numberGrade >= 90)
{
    letterGrade = "A";
}
else if(numberGrade >= 80)
{
    letterGrade = "B";
}
else if(numberGrade >= 70)
{
    letterGrade = "C";
}
else if(numberGrade >= 60)
{
    letterGrade = "D";
}
else
{
// We know the value is greater than 0 but strictly lower than 60.
    letterGrade = "F";
}
Console.WriteLine(numberGrade + " corresponds to " + letterGrade);

  2. Given an int variable counter, write three statements to decrement its value by 1.

    Solution

    We actually know four ways to do that:

    counter = counter - 1;
counter -= 1;
counter--;
--counter;

  3. What will be displayed on the screen?

    int x = 3, y = 7;
Console.WriteLine (x++ +" and "+ --y);

    Solution

    “3 and 6”

  4. What will be displayed on the screen by the following program?

    int counter = 2;
while (counter != 5)
{
    Console.Write(counter + "\n");
    counter++;
}

    Solution

     
    2
3
4

  5. What will be displayed on the screen by the following program? Write the spaces and new line explicitly.

    int counter = 10;
while (counter > 5)
{
    counter--;
    Console.Write(counter + "\n");
    if (counter == 7) {
        Console.WriteLine("Bingo");
    }
}

    Solution

     
    9
8
7
Bingo
6
5

  6. What will be displayed on the screen by the following program?

    int counter = 10;
while (counter != 5) ;
Console.Write(counter + "\n");
counter--;

    Solution

    Nothing, and it will loop forever.

  7. What will be displayed on the screen by the following program?

    int counter = 7;
while (counter != 2)
    Console.Write(counter + "\n");
counter--;

    Solution

    7 infinitely many times.

  8. What is input validation? Name a control structure that can be used to perform it. Why is it important?

    Solution

    Making sure the user’s input is valid. The while loop. Because we cannot trust the user.

  9. What do we name a variable that is increased by some value at every iteration of a loop, i.e., that keeps the running total?

    Solution

    An accumulator.

  10. What is a sentinel value?

    Solution

    It’s a value that will trigger an exit from a loop. It is a value that was agreed on, and that signifies “I now want to exit the loop.”.

  11. Write a program that asks the user to enter a value between 0 and 10, and asks again as long as the user enters integers outside that range.

    Solution

     
    int answer;
do{
    Console.WriteLine("Enter a value between 0 and 10 (both included).");
    answer = int.Parse(Console.ReadLine());
}while(answer > 10 || answer < 0);

  12. Write a small program that asks the user for an integer, and displays “It is positive” if the number entered is positive, “It is negative” if the number entered is negative, and “Not a number” if the user entered a string that is not an integer.

    Solution

     
    int answer;
Console.WriteLine("Enter an integer");
if(!int.TryParse(Console.ReadLine(), out answer)){
    Console.WriteLine("Not a number");
}
else if (answer > 0){
    Console.WriteLine("Positive");
}
else{
    Console.WriteLine("Negative");
}

  13. Write a program containing a while loop that would display the numbers between -100 and 100 (both included) with a space between them when executed.

    Solution

     
    int counter = -100;
while(counter <= 100){
    Console.Write(counter++ + " ");
}

  14. Assume you are given an initialized string variable name, and a string variable field. Write a small program that assigns to field

    • “CS” if name is “Turing” or “Liskov”,
    • “Math.” if name is “Aryabhata” or “Noether”,
    • “Unknown” otherwise.
      Solution 
    string name;
name = "Turing"; // Value given as an example, change it to test.
string field;
switch(name){
    case("Turing"):
    case("Liskov"):
        field = "CS";
        break;
    case("Aryabhata"):
    case("Noether"):
        field = "Math.";
        break;
    default:
        field = "Unknown";
        break;
}
Console.WriteLine(name + " worked in " + field + ".");

  15. Write a program that asks the user to enter a value between 1900 and 1999 (both included), and asks again as long as the user enters integers outside that range.

    Solution

     
    int answer;
do{
    Console.WriteLine("Enter a value between 1900 and 1999 (both included).");
    answer = int.Parse(Console.ReadLine());
}while(answer < 1900 || answer > 1999);

Part II – Problems

  1. Write a switch statement that calculates the number of days in a particular month. You should assume that you are given already assigned month and year int variables, and that your program should set an already declared int numberOfDays variable to 28, 29, 30 or 31 depending on the month / year combination. Your program should start with a switch matching month against certain values, and, if month is 2, uses an if statement to decide whenever the number of days is 28 or 29. You can use something like

    switch (month) {
    …
    case (2):
        if …
        …
        break;
    …
}

    Solution

     
    int month = 2;
int year = 2000;
int numDays = 0;
switch (month) {
    case 1: case 3: case 5:
    case 7: case 8: case 10:
    case 12:
        numDays = 31;
    break;
    case 4: case 6:
    case 9: case 11:
        numDays = 30;
    break;
    case 2:
        if (((year % 4 == 0) && !(year % 100 == 0))
            || (year % 400 == 0))
            numDays = 29;
        else
            numDays = 28;
    break;
    default:
        Console.WriteLine("Invalid month.");
    break;
}
Console.WriteLine("Number of Days = " + numDays);

Homework #6

Part I — Questions

  1. Write a statement that creates a 10-element int array named numbers.

    Solution

     
    int[] numbers = new int[10];

  2. In the following, what is the value of the size declarator? What is the value of the index?

    int[] numbers;
numbers = new int[8];
numbers[4] = 9;

    Solution

    The size declarator is 8, the subscript, or index, is 4.

  3. What is wrong with the following array declaration?

    int[] books = new int[-1];

    Solution

    The size declarator cannot be negative.

  4. Draw the content of the scores array once those statements have been executed.

    int[] scores = new int[3];
scores[0] = 13;
scores[2] = 25;

    Solution
    index 0 1 2
    value 13 0 25

  5. What will be displayed on the screen by the following program?

    for (int num = 3 ; num <= 5 ; num++)
    Console.Write(num + " ");

    Solution

     
    3 4 5

  6. Write a ‘for’ loop that displays on the screen the sequence “1,  2,  3,  4,  5,  6,  7,  8,  9,  10, ”.

    Solution

     
    for (int x = 1 ; x <= 10 ; x ++)
    Console.Write(x + ", ");

  7. Write a ‘for’ loop that displays on the screen the sequence “1,  2,  3,  4,  5,  6,  7,  8,  9,  10”, (note that there is no comma after 10).

    Solution

     
    for (int x = 1 ; x <= 10 ; x ++)
{
    Console.Write(x);
    if (x < 10) Console.Write(" ,");
}

  8. Write a ‘for’ loop that displays on the screen the sequence “1 3 5 7 9 ”.

    Solution

     
    for (int x = 1 ; x <= 10 ; x+= 2)
    Console.Write(x + " ");

  9. Given an int variable myVar initialized with a positive value, write a loop that sums the integers between 0 and myVar (i.e., 0 + 1 + ⋯ + (myVar − 1) + myVar).

    Solution

     
    int sum = 0;
for (int x = 1 ; x <= myVar ; x++)
    sum += x;

  10. Consider the following code:

    for (int y = 1; y <= 3; y++)
{
    for (int z = 1; z < 5; z++)
        Console.Write("Scene " + y + ", take " + z + ". " );
    Console.WriteLine();
}

    How many times does the outer loop iterate (i.e., how many scenes are shot)? How many times does the inner loop iterate (i.e., how many takes for each scene)? Finally, what is the total number of iterations of the nested loops (i.e., how many takes are made, total)?

    Solution

    3, 4, 12.

  11. Which of the following are pre-test loops? do while, switch, while, for and if-else-if.

    Solution

    for and while are pretest loops.

  12. What will be displayed on the screen by the following code?

    int[] values = new int[6];
for (int i = 0 ; i < 6 ; i++)
    values[i] = (i*2);
foreach (int j in values)
    Console.WriteLine(j);

    Solution

     
    0
2
4
6
8
10

  13. Suppose we are given an int array dailyPushUp with 7 elements. Write a piece of code that display the value of the elements stored in the array dailyPushUp.

    Solution

     
    for (int j = 0 ; j < 7 ; j++)
    Console.WriteLine(dailyPushUp[j]);

  14. What is “array bounds checking”? When does it happen?

    Solution

    It is C# making sure that you are not using a subscript outside the allowed range. It happens at run-time.

  15. Is there an error with the following code? If you think there is one, explain it, otherwise draw the content of the myIncomes array once those statements have been executed.

    double[] myIncomes = new double[5];
myIncomes[1] = 3.5;
// No income on day two.
myIncomes[3] = 5.8;
myIncomes[4] = 0.5;
myIncomes[5] = 1.5;

    Solution

    The subscripts are off, they should go from 0 to 4: myIncomes[5] = 1.5; will casue an error.

  16. What would be the size of the test array after the following statement has been executed?

    int[] test = {3, 5, 7, 0, 9};

    Solution

    5

  17. What is the difference, if any, between the following two statements?

    int[] num, scores;
int num[], scores;

    Solution

    In the second one, only num is a reference to an int array, and scores is just an int.

  18. Write a statement that creates and initializes a double array with the values 12.5, 89.0 and 3.24.

    Solution

     
    double[] question = {12.5, 89.0, 3.24};

  19. What is the value of count and the content of number once the following has been executed?

    int count=2;
int[] number={3, 5, 7};
number[count--] = 8;
number[count]--;

    Solution

    count is 1. numbers is 3, 4, 8.

  20. Suppose we have an array named temp that has been declared and initialized. How can we get the number of elements in this array?

    Solution

    By using the Length field: temp.Length.

  21. Describe what the following code would do.

    int[] record = { 3, 8, 11 };
int accumulator = 0;
foreach (int i in record)
    accumulator += i;

    Solution

    Declare and initialize an int array with the values 3, 8 and 11, and then sum those values in an accumulator variable.

  22. Assuming we have two int arrays of the same size, firstA and secondA, write a program that copies the content of firstA into secondA.

    Solution

     
    for (int k = 0 ; k < firstA.Length ; k++)
   secondA[k] = firstA[k];

  23. Assuming we are given an int array named arrayF, write a program that adds one to each of its elements. That is, if arrayF contains 3, 5, 7 and -2 before your program is executed, it should then contain 4, 6, 8 and -1 after your program was executed.

    Solution

     
    for (int k = 0 ; k < arrayF.Length ; k++)
    arrayF[k] += 1;

  24. Assuming we are given an int array named arrayF, write a program that displays the product of its elements. That is, if arrayF contains 2, 3 and -1, then your program should display -6.

    Solution

     
    int prod = 1;
for (int k = 0 ; k < arrayF.Length ; k++)
    prod *= arrayF[k];
Console.WriteLine(prod);

  25. Write a static method (header included) that takes as argument an int array, and display on the screen the value of each element of that array.

    Solution

     
    public static void p(int[] a){
    foreach (int k in a)Console.WriteLine(k);
}

  26. Write a static method (header included) that takes as argument an int array, and stores the value 10 in each element of that array.

    Solution

     
    public static void z(int[] a){
    for (int j = 0 ; j < a.length ; j++) a[j]=10;
}

Quizzes

Those quizzes are given as examples, to help you practise. They were given at week 4 and 7.

Quiz 1

  1. (3 pts) List three keywords.

    Solution

    Keywords are words that have special meanings and cannot be used as identifiers in your program. Examples of C# Keywords include int, string, if, while, void, else, bool etc.

  2. (4pts) Circle the correct identifiers:

    • %Rate
    • static
    • my-variable
    • User.Input
    • YoUrNaMe21
    • test_train
    • _myIdentifier

    Solution

    Valid identifiers can not include reserved words, can not start with a number, and must contain only numbers, letters, and/or the underscore character. The identifiers below follow these rules. my-variable YoUrNaMe21 test_train _myIdentifier

  3. (4 pts) For each of the following, indicate if they are a “rule” of C# or a “convention” between programers by ticking the appropriate column. The first answer is given as an example.

    Statement Rule Convention
    Code should be commented.                     ✓       
    Case matters.
    Variable names should be descriptive.
    Keywords cannot be used as identifiers.
    Each “.cs” file should contain exactly one class.

    Solution
    Statement Rule Convention
    Case matters. ✓
    Variable names should be descriptive. ✓
    Keywords cannot be used as identifiers. ✓
    Each “.cs” file should contain exactly one class. ✓

  4. (4 pts) Write a statement that would display, “Hi Mom!” (with the quotes) followed by a new line on the screen.

    Solution

    The key is to escpae the " character: Console.Write("\"Hi Mom!\"\n"). Note that we could have used Console.WriteLine instead of inserting the new line using the escpae sequence \n.

  5. (5 pts) Write a series of statements that would

    1. declare an int variable called “myAge”,
    2. assign your age to that variable,
    3. display “My age is”, the value of the “myAge” variable, a period, and finally a new line.

    Solution

     
    int myAge;
myAge = 21;
Console.Write("My age is " + myAge + ".\n");

  6. (Bonus) Give examples of situations where the adage “Spaces and new lines do not matter in programs” is actually erroneous.

    Solution

    Spaces and newlines matter when they are used in string data, as blank space in strings is formatted exactly how it is typed. Blank spaces also matters in between words: words in C# must have at least one space between them in order to be compiled correctly (e.g., static void Main() and int days = 7;). If there were no spaces in either of the examples, neither of them would compile. They also matter for in-line comments.

    // My comment
int x;
x = 10;

    If you remove the first newline, the program would not compile. Last but not least, new lines and space matter for readability purposes.

Quiz 2

  1. (2 pts) What is the relational operator used to determine whenever two values are equal?

    Solution

    The operator is two equal signs, ==, not to be confused with a single equal sign, =, used for assignment.

  2. (2 pts) Write a boolean expression that evaluates to true if a variable yourName is different from "Marc" and "Mark".

    Solution

    A possible solution is: yourName != "Marc" && yourName != "Mark". Note that there was no need to write an if statement or a complete statement: only the expression was required.

  3. (5pts) Write a boolean expression that evaluates to true if a variable x is between -10 (excluded) and 10 (included).

    Solution

    A possible solution is: x > -10 && x < 10

    Note that there was no need to write an if statement or a complete statement: onyl the expression was required.

  4. (3 pts) What will be displayed on the screen by the following program?

    int x = 5, y = 1, z = 2;
if (x != z || y < z ) {x += y;}
if (z > y) {if (z % 2 == 0) {z -= 3;} else {z += x;}} else {x = y + z;}
Console.WriteLine($"x is {x}, y is {y}, and z is {z}.");

    Solution

    This program would display:

    ```{text} x is 6, y is 1, and z is -1.

    Press any key to continue… ```

  5. (8 pts) Assume we initialized a string variable named month and a double variable named temperature. Write a series of statements that will display exactly one of the following messages: “What a nice summer day!” if month is “July” and temperature is less than 90 (included); “Better wear a jacket” if temperature is between 45 and 60 (both included); “Happy holidays!” if month is “December”; or “Have a nice day” otherwise.

    Solution

    if(month=="July" && temperature<=90) Console.WriteLine("What a nice summer day!"); else if (temperature >= 45 && temperature <= 60) Console.WriteLine("Better wear a jacket!"); else if (month == "December") Console.WriteLine("Happy Holidays!"); else Console.WriteLine("Have a nice day!")

  6. (Bonus) Give a program that displays “Leap year” if a year variable is

    1. divisible by 4; and
    2. not divisible by 100, unless it is also divisible by 400.

    Your program should correctly identify 2000 and 2400 as leap years, and 1800, 1900, 2100, 2200, 2300, or 2500 as not leap years.

    Solution

     
    if (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0))
    {
        Console.WriteLine("The year " + year + " is a leap year.");
    }
else
    {
        Console.WriteLine("The year " + year + " is not a leap year.");
    }

Quiz 3

  1. (7 pts) Write a switch statement that sets a decimal price variable to 1.50 if a string variable iceCreamFlavor contains "vanilla", 1.75 if it contains "chocolate" or "mint", and -1 otherwise.

    Solution

     
    switch(iceCreamFlavor)
{
    case "vanilla":
        price = 1.50m;
        break;
    case "chocolate":
    case "mint":
        price = 1.75m;
        break;
    default:
        price = -1;
        break;
}

  2. (2 pts) Write a statement that applies the increment operator in prefix position to a variable test.

    Solution

    The increment operator (++) in prefix position (that is, applied before anything else) applied to the variable test gives: ++test;.

  3. (3 pts) What will be displayed on the screen by the following program?

    int x = 10;
x--;
while(x > 0){
    Console.Write(x);
}

    Solution

    This would display 9 (without new line or space) forever.

  4. (8 pts) Write a while loop that displays “1 2 3 4 5 6 7 8 9 10 11 12 13” (spaces included!) at the screen.

    Solution

    There are numerous different ways of writing such a program. A compact one could be:

    int num = 1;
while(num <= 13)Console.Write($"{num++} ");

    while a more verbose could be:

    int num = 1;
while(num <= 13){
    Console.Write($"{num} ");
    num++;
}

  5. (+3+2 pt, bonus) Give a program that displays every multiple of 3 between 0 and 10,000 (that is, “0 3 6 9 12 … 9999”). Bonus (again!): display how many such numbers there are in total.

    Solution

    We can either

    1. Display the counter only if it is a multiple of three:

      int counter = 0;
while(counter <= 1000){
    if (counter % 3 == 0){Console.Write(counter + " ");}
    counter++;
}

    2. Increment the counter 3 by 3:

      int counter = 0;
while(counter <= 1000){
    Console.Write(counter + " ");
    counter+=3;
}

      To answer the “bonus within the bonus”, we need two counters:

    int threeCtr = 0; // Counter for the multiple of 3.
int multipleCtr = 0; // Counter that sum the number of values displayed
while(threeCtr < 10000)
{
    Console.Write($"{threeCtr} ");
    threeCtr += 3; // We move to the next multiple of 3.
    multipleCtr++; // We increment the number of values displayed.
}

Console.WriteLine($"\nThere are {multipleCtr} multiples of 3 from 0 to 10,000.");