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Methods Version 1.1

Methods Version 1.1. Topics. Built-in methods. Methods that return a value. void methods. Random number generators. Programmer defined methods. Scope. Objectives. At the end of this topic, students should be able to:. Write programs that use built-in methods

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Methods Version 1.1

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  1. MethodsVersion 1.1

  2. Topics Built-in methods Methods that return a value void methods Random number generators Programmer defined methods Scope

  3. Objectives At the end of this topic, students should be able to: Write programs that use built-in methods Know how to use methods in the Math library Correctly write code that generates random numbers Correctly write and use user defined methods in a program Describe what scope is and how it affects the execution of a program.

  4. At this point we have learned to write quite complex programs, that contain decisions and loops. … but most of our programs are still quite small and easy to manage. What if I gave you an assignment to write a program that would contain 5,000 lines of code?

  5. because it is easier to understand what goes on in a small block (piece) of code. because we can re-use the same block (piece) of code many times from within our program we call this functional decomposition -- breaking the program down into more manageable blocks (pieces). in C# these smaller blocks (pieces) are called methods We often write a program as a series of pieces or blocks

  6. As an example, suppose that you have been asked to write a program to play paper-rock-scissors.

  7. Let’s do a top-down design. Ask for user’s choice And get the input Generate computer’s choice Determine winner Display the results See if user wants to play again

  8. Ask for user’s choice and get the input Prompt the user to make a choice (r-p-s) Get the users input no Is the input valid ?

  9. Generate computer’s choice Generate a Random number Between 1 and 3

  10. Determine Winner user = computer ? yes tie no user = paper ? winner is user yes yes computer = rock ? winner is computer no no user = rock ? winner is computer yes yes computer = paper ? winner is user no no user = rock ? winner is user yes winner is computer no

  11. Display the results Display user choice Display computer choice Display the winner

  12. See if user wants To play again Prompt the user to make a choice (y/n) Get the users input no Is the input valid ? no Play Again ? quit

  13. We could now write this program as one long list of statements. It would be very big and quite complex … there would be several loops and lots of decision blocks. Whenever you have a large block of code that does many different things, it becomes difficult to visualize what is happening in the code, and so much harder to get the code to work correctly.

  14. This is easy to visualize Prompt the user to make a choice (r-p-s) Get the users input no Is the input valid ?

  15. This is much harder to visualize. We really can’t even get it on one page . . . and make it readable. tie quit

  16. So … let’s take each of these pieces and write each as a separate, stand-alone block of code called a method. Each method will have one well defined thing that it does. (Provides a service.) Ask for user’s choice And get the input Generate computer’s choice We will have the ability to give each method any data that it needs to do its job. Determine winner Display the results Each method can return to us the results of its work. See if user wants to play again

  17. Method Syntax These are parameters. Each parameter has a data type and a name. The type of data returned by this method. The method’s name method header static intDetermineWinner( intcompChoice, intuserChoice) { // statements } The body of the method is made up of valid C# statements (providing a service), enclosed in curly braces. method block (body)

  18. Just as a reminder … Main( ) is a method which satisfies all the conditions specified earlier. Header static void Main() Block { (body) }

  19. General Format of C# Methods Header static <return type> <method identifer> ( <comma delimited parameter list ) Block { (body) <statements> }

  20. Built-in Methods In general, if you can find some written and tested code that does what you want, it is better to use that already existing code than to re-create the code yourself. saves time fewer errors tested under all conditions Most programming languages, including C#, include libraries of pre-written and tested methods that do common programming tasks. In C#, these libraries are in the .Netlibrary accessed via using statements.

  21. Methods that return a value As an example of a method that returns a value, consider the Sqrtmethod in the Math class. To set a value to the square root of the number 9, we would write result = Math.Sqrt (9); this is called a method invocation. It can be used anywhere an expression can be used. The Sqrt method belongs to the Math class. this is the method’s argument. The argument may be a literal value, a variable, a constant, or an expression. Some methods may take more than one argument. If so, they are separated by commas (a comma delimited list). the value returned by the function is called its return value. A method can only have one return value.

  22. The Math class The Sqrt method is found in the Math class. Other common functions in the Math class: name function (service)return type Pow (int x, int y) calculates xy double Abs (double n) absolute value of n double Ceil (double n ) smallest integer >= n double Floor (double n) largest integer <= n double

  23. Rounding When we round a number, we pick the closest integer value. For example, if a = 2.7, then the rounded value of a is 3 if a = 2.4, then the rounded value of a is 2. The Ceil and Floor methods given in the previous slide don’t do rounding. For example, Math.Floor (2.9) returns 2.0, while Math.Ceil (2.3) returns 3.0.

  24. Methods thatreturn void methods that don’t return a value of void are called void methods. void methods are written as statements. They cannot be used in an expression, as expressions must return a typed value. void methods can have zero or more parameters. void methods can have return; statement.

  25. Writing a Method What job will the method do? What data does it need to do it’s work? What will the method return?

  26. Here is the activity diagram for the method we need to write to get the user’s choice. Prompt the user to make a choice (r-p-s) Get the users input What is it’s job (service provided)? What data does it need? What should it return? no Is the input valid ?

  27. The Method Prologue Every method should have a method prologue. The method prologue tells us * What the purpose of the method is * What data the method needs to do its work * What data the method returns

  28. The Method Prologue // The GetUserChoicemethod // Purpose: gets a valid user choice (1-3) // Parameters: none // Returns: the user choice as an int

  29. static intGetUserChoice( ) • { • intchoice = 0; • do • { • Console.Clear(); • Console.WriteLine("Enter your choice: "); • Console.WriteLine("1 - Rock"); • Console.WriteLine("2 - Paper"); • Console.WriteLine("3 - Scissors: "); • choice = int.Parse(Console.ReadLine( ) ); • choice = char.ToUpper(choice); • if (choice < ROCK || choice > SCISSORS) • Console.WriteLine("Invalid selection."); • } while (choice < ROCK || choice > SCISSORS); • return choice; • } Prompt the user to make a choice (r-p-s) Get the users input no Is the input valid ?

  30. Here is the activity diagram for the method we need to write to get the computer’s choice. Generate a Random number Between 1 and 3 What is it’s job (service it provides)? What data does it need? What should it return?

  31. The Method Prologue // The GetComputerChoicefunction // Purpose: generates a random choice (1-3) // Parameters: none // Returns: the computer choice as an int // Pre-conditions: none // Post-conditions: none

  32. Random Number Generator The .Net library provides a class that we can use to create a random number generator. To create a random number generator object, we write Random randoms = new Random( ); This initializes the Random object. This is the reference to the Random object. This creates the Random object in the Heap.

  33. Random Number Generator A random number generator generates a pseudo-random integer value between zero and 2,147,483,646. To get a random number within a specific range we scale the result …. for example, to get a number between 0 and 2, inclusive int n = randoms.Next( 3 ); generates value up to, but not including 3 (0-2)

  34. Random Number Generator To shift the range of the random numbers, for example, to get a random number between 1 and 3, use this form of the Next method: int n = randoms.Next(1, 4); Generate values up to, but not including 4 (1-3) Start at 1

  35. Random Number Generator * To get a repeatable sequence of pseudo-random numbers, use the same seed when creating the Random object Random randoms = new Random( 3 ); * same machine, same compiler

  36. static intGetComputerChoice( ) { int choice; choice = randoms.Next(1,4); return choice; } Generate a Random number Between 1 and 3 We created the Random object randoms elsewhere.

  37. user = computer ? yes Here is the activity diagram for the method we need to write to get the computer’s choice. tie no user = paper ? winner is user yes yes computer = rock ? winner is computer no no user = rock ? winner is computer yes yes computer = paper ? winner is user no no user = rock ? winner is user yes What is it’s job (service)? What data does it need? What should it return? winner is computer no

  38. The Method Prologue // The PickWinner method // Purpose: decide who wins, computer or user // Parameters: computer choice, user choice // Returns: the winner as an int // (1 – user wins, 2 – computer wins, 0 - tie) // Pre-conditions: none // Post-conditions: none

  39. static intPickWinner(intuserCh, intcomputerCh) { intwinner = 0; if (userCh == computerCh) winner = 0; else if (computerCh == ROCK) { if (userCh == PAPER) winner = USER; else // userCh = scissors winner = COMPUTER; } else if (computerCh == PAPER) { if (userCh == ROCK) winner = COMPUTER; else // userCh = scissors winner = USER; } else // computerCh = scissors { if (userCh == ROCK) winner = USER; else winner = COMPUTER; } return winner; } //End PickWinner()

  40. Here is the activity diagram for the method we need to display the winner. Display user choice Display computer choice What is it’s job (service)? What data does it need? What should it return? Display the winner

  41. The Method Prologue // The DisplayResults method // Purpose: displays each choice and the winner // Parameters: computer choice, user choice, winner // Returns: nothing (void) // Pre-conditions: none // Post-conditions: none

  42. static void DisplayResults(intuserCh, intcomputerCh, intwinR) { Console.Write("You chose "); if (userCh == ROCK) Console.WriteLine("Rock."); else if (userCh == PAPER) Console.WriteLine("Paper"); else Console.WriteLine("Scissors"); Console.Write("I chose "); if (computerCh == ROCK) Console.WriteLine("Rock"); else if (computerCh == PAPER) Console.WriteLine("Paper"); else Console.WriteLine("Scissors"); if (winR == 0) Console.WriteLine("It is a tie."); else if (winR == USER) Console.WriteLine("You win."); else Console.WriteLine("I win."); Console.WriteLine(); }//End DisplayResults() Display user choice Display computer choice Display the winner

  43. Prompt the user to make a choice (y/n) Here is the activity diagram for the method we need to decide if the user wants to Play again.. Get the users input no Is the input valid ? What is it’s job (service)? What data does it need? What should it return? Return the choice

  44. The Method Prologue // The PlayAgain method // Purpose: get user answer to “playa gain?” // Parameters: none // Returns: the user’s choice (y or n) // Pre-conditions: none // Post-conditions: none

  45. Prompt the user to make a choice (y/n) static char PlayAgain() { char answer = ‘N’; do { Console.Write("Do you want to play again? "); answer = char.Parse(Console.ReadLine()); answer = char.ToLower(answer); if (answer != 'y' && answer != 'n') Console.WriteLine("Invalid response."); } while (answer != 'y' && answer != 'n'); return answer; }//End PlayAgain() Get the users input no Is the input valid ? Return the choice

  46. Now with these methods, our Main( ) method just looks like … static void Main() { // declarations intuserChoice = 0, computerChoice = 0; intwinner = 0; char yn = ‘N’; Console.WriteLine("Play Rock, Paper, and Scissors"); do { userChoice = GetUserChoice(); computerChoice = GetComputerChoice(); winner = PickWinner(userChoice, computerChoice); DisplayResults(userChoice, computerChoice, winner); yn = playAgain(); yn = char.ToLower(yn); } while (yn == 'y'); Console.ReadLine(); }//End Main()

  47. Revisit The Rounding Issue C# does not provide a method that rounds. Let’s write a method that rounds a double to the nearest integer, using the floormethod.

  48. for any value n, in this range Math.Floor(n ) = 3. 3.5 4 3 but … in this range Math.Floor(n + 0.5) = 4.0

  49. So, we can write the method Round( ) as follows: static intRound(double number) { return (int)(Math.Floor(number + 0.5)); } Note that the Math class has a Round method That works just this way.

  50. Scope Scope has to do with where a variable can be seen. global variables (class level variables) local variables (method level variables)

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