Methods Version 1.1

Topics Built-in methods Methods that return a value Methods that do not return a value (void methods) Random number generators Programmer defined methods return values of void/int/double/etc. Scope Programmer defined methods 2 Local (automatic) variables

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. 3

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? 4

We often write a program as a series of pieces or blocks 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 5

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

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 7

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

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

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

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

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

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. In software engineering we say that it lacks “COHESION.” 13 REMEMBER, every line of code you write has the potential of 1 to n errors! REMEMBER, other than dating and marriage, programming is the MOST error-prone activity know to mankind.

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

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

So … let’s take each of these pieces and write each as a separate, stand-alone block of code called a method. 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 Each method will have one well defined thing that it does. (Provides a service.) We will have the ability to give each method any data that it needs to do its job. Each method can return to us the results of its work. 16

Method Syntax static int DetermineWinner( int compChoice, int userChoice) { // statements } The type of data returned by this method. The method’s name (for address) These are parameters. Each parameter has a data type and a name. The body of the method is made up of valid C# statements (providing a service), enclosed in curly braces. method header method block (body) 17 Denotes a class level method.

Just as a reminder … Main( ) is a method which satisfies all the conditions specified earlier. Header static void Main( ) Block { (body) } 18 class level specifier return data type Method identifier (address) comma delimited parameter list

General Format of C# Methods Header ( ) { Block (body) } 19

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 (built-in) and tested methods that do common programming tasks. In C#, these libraries are in the.Net library accessed via using statements. 20

Built-In Methods We have already used a number of built-in methods –WriteLine(…), Write(…) –ReadLine(…) –Sqrt(…) –Others? 21

Methods that return a value As an example of a method that returns a value, consider the Sqrt method 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 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. this is called a method invocation. It can be used anywhere an expression can be used. The Sqrt method belongs to the Math class. 22

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

Methods that don’t return a value methods that don’t return a value 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. 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

Prompt the user to make a choice (r-p-s) Get the users input Is the input valid ? no Here is the activity diagram for the method we need to write to get the user’s choice. What is it’s job (service provided)? What data does it need? What should it return? 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 properly * What data the method returns 28

The Method Prologue // The GetUserChoice method // Purpose: gets a valid user choice (1-3) // Parameters: none // Returns: the user choice as an int 29

Prompt the user to make a choice (r-p-s) Get the users input Is the input valid ? no static int GetUserChoice( ) { int choice; do { 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 SCISSORS) Console.WriteLine("Invalid selection."); } while (choice SCISSORS); return choice; } 30

Generate a Random number Between 1 and 3 Here is the activity diagram for the method we need to write to get the computer’s choice. What is it’s job (service it provides)? What data does it need? What should it return? 31

The Method Prologue // The GetComputerChoice function // 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 instantiate it as an object, so we write Random randoms = new Random( ); This is the reference to the Random object. This creates the Random object in the Heap. This initializes the Random object. Know as a constructor. 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); Start at 1 Generate values up to, but not including 4 (1-3) 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

Generate a Random number Between 1 and 3 static int GetComputerChoice( ) { int choice; choice = randoms.Next(1,4); return choice; } We created the Random object randoms elsewhere. 37

user = computer ? yes tie computer = rock ? yes user = paper ? yes winner is user no winner is computer no computer = paper ? yes user = rock ? yes winner is computer winner is user no user = rock ? no yes winner is user winner is computer Here is the activity diagram for the method we need to write to get the computer’s choice. What is it’s job (service)? What data does it need? What should it return? 38

The Method Prologue // 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 int PickWinner(int userCh, int computerCh) { int winner = 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

Display user choice Display computer choice Display the winner Here is the activity diagram for the method we need to display the winner. What is it’s job (service)? What data does it need? What should it return? 41

The Method Prologue // 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

Display user choice Display computer choice Display the winner static void DisplayResults(int userCh, int computerCh, int winR) { 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() 43

Prompt the user to make a choice (y/n) Get the users input Is the input valid ? no Here is the activity diagram for the method we need to decide if the user wants to Play again.. 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) Get the users input Is the input valid ? no 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() Return the choice 46

Now with these methods, our Main( ) method just looks like … static void Main() { // declarations int userChoice = 0, computerChoice = 0; int winner = 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 floor method. 48

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

static int Round (double number) { return (int)(Math.Floor(number + 0.5)); } So, we can write the method Round( ) as follows: 50

Scope Scope has to do with where a variable can be seen. global variables (class level variables) local variables (method level variables) 51

A related term is storage class or lifetime, which defines how long a variable exists within a program. automatic variables – come into existence when they Are declared, and exist until the block in which they are declared is left.. static variables – exist for the lifetime of the program Class level variables – exist for the lifetime of the program (const’s at the class level) 52

Example using System; class Program { static string globalValue = "I was declared outside any method"; static void Main() { Console.WriteLine("Entering main( )..."); string localValue = "I was declared in Main( )"; SomeMethod( ); Console.WriteLine("Local value = {0}", localValue); Console.ReadLine( ); }//End Main() static void SomeMethod( ) { Console.WriteLine("Entering SomeMethod( )..."); string localValue = "I was declared in SomeMethod( )"; Console.WriteLine("Global value = {0}", globalValue); Console.WriteLine("Local value = {0}", localValue); }//End SomeMethod() }//End class Program global variables must be declared outside of any method. They need to be declared within a class as static. Constants are automatically static. the name localValue is used twice. In this case the scope of localValue is inside of Main( ). It is a local variable. localValue is also declared in this method, but its scope is just inside the method. It cannot be seen outside of the method. It is a different variable than the one declared in Main( ). It is a local variable. 53

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Blocks Anytime we use curly braces to delineate a piece of code, that code is called a block. We can declare variables that are local to a block and have block scope. Local variables declared in a nested block are only known to the block that they are declared in. When we declare a variable as part of a loop, for example for (int j = 0; j< MAX; j++) … the variable j will have the block of the loop as its scope. 55

Static Variables A static variable comes into existence when it is declared and it lives until the program ends. A static variable has class scope – that is, it is visible to all of the methods in the class. Static variables live in the data segment. 56

Practice Write the prologue for a method named CalcRatio that takes two integer parameters and returns a double. 57

Practice Write the code for this method. The ratio is found by dividing the first parameter by the second. 58

Practice Write a complete program that (1) gets two input values from the user (2) passes those values to the CalcRatio method (3) displays the result 59

Practice Write a program that converts dollar values into another currency. The program should work as follows: (1) Prints an introduction to the program (2) Gets a currency conversion factor and currency name from user (3) Gets a dollar value (4) Calls a method CalcConvertedValue( ) and displays answer (5) Asks if the user wants to do another conversion (6) If the answer is yes, go back to step 3 (7) Ask if the user wants to do a different conversion (8) If the answer is yes, go back to step 2 60