Methods Version 1.1. Topics Built-in methods Methods that return a value Methods that do not return...
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Methods Version 1.1
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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
Transcript of Methods Version 1.1. Topics Built-in methods Methods that return a value Methods that do not return...
MethodsVersion 1.1
TopicsBuilt-in methods
Methods that return a valueMethods that do not return a value (void methods)Random number generatorsProgrammer defined methods return values of void/int/double/etc.Scope
Programmer defined methods
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Local (automatic) variables
ObjectivesAt the end of this topic, students should be able to:
Write programs that use built-in methodsKnow how to use methods in the Math libraryCorrectly write code that generates random numbersCorrectly write and use user defined methods in a programDescribe what scope is and how it affects the executionof a program.
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At this point we have learned to write quitecomplex programs, that contain decisions and loops.
… but most of our programs are still quitesmall and easy to manage.
What if I gave you an assignment to write a program that would contain5,000 lines of code?
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We often write a program as a series of pieces or blocks
because it is easier to understand what goes onin a small block (piece) of code.
because we can re-use the same block (piece) of codemany 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
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As an example, suppose that you have been askedto write a program to play paper-rock-scissors.
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Let’s do a top-down design.
Ask for user’s choiceAnd get the input
Generate computer’s
choiceDetermine winner
Display the results
See if user wantsto play again
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Ask for user’s choice
and get the input
Prompt the user to make a choice (r-p-
s)
Get the users input
Is theinput valid
?
no
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Generate computer’s
choice
Generate aRandom numberBetween 1 and 3
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Determine Winneruser =
computer?
yes tie
computer= rock
?
yes user= paper
?
yes winner isuser
no
winner iscomputernono
computer= paper
?
yes user= rock
?
yes winner iscomputer winner is
usernono
user= rock
?no
yes winner isuser
winner iscomputer
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Display user choice
Display computer choice
Display the winner
Display the results
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See if user wantsTo play again
Prompt the user to make a choice (y/n)
Get the users input
Is theinput valid
?
no
PlayAgain
?
noquit
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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 codethat does many different things, it becomesdifficult to visualize what is happening in thecode, and so much harder to get the codeto work correctly. In software engineering wesay that it lacks “COHESION.”
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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 theinput valid
?
no
This is easy to visualize
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tie
quit
This is much harderto visualize. We reallycan’t even get it on onepage . . . and make itreadable.
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So … let’s take each of these pieces and write each asa separate, stand-alone block of code called a method.
Ask for user’s choiceAnd get the input
Generate computer’s
choiceDetermine winner
Display the results
See if user wantsto play again
Each method will have one well defined thing that it does. (Providesa service.)
We will have the ability to giveeach method any data that itneeds to do its job.
Each method can return to usthe results of its work.
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Method Syntax
static int DetermineWinner( int compChoice, int userChoice){
// statements
}
The type of datareturned by thismethod.
The method’sname (for address)
These are parameters.Each parameter has a data type and a name.
The body of the methodis made up of valid C#statements (providing a service), enclosed in curly braces.
method header
method block (body)
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Denotes a class levelmethod.
Just as a reminder … Main( ) is a method which satisfies all theconditions specified earlier.
Header static void Main( )Block {(body)
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class level specifier
return data type Method identifier (address)
comma delimited parameter list
General Format of C# Methods
Header <return type> <method identifier> ( <comma delimited parameter list> ) {
Block <statements>(body) }
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Built-in MethodsIn 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 timefewer errorstested under all conditions
Most programming languages, including C#, includelibraries of pre-written (built-in) and tested methods that docommon programming tasks. In C#, these librariesare in the .Net library accessed via using statements.
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Built-In MethodsWe have already used a number
of built-in methods– WriteLine(…), Write(…)– ReadLine(…)– Sqrt(…)– Others?
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Methods that return a valueAs an example of a method that returns a value, considerthe Sqrt method in the Math class.
To set a value to the square root of the number 9, we wouldwrite
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 oneargument. If so, they are separated bycommas (a comma delimited list).
the value returned by the functionis called its return value.
A method can only have onereturn value.
this is called a method invocation.It can be used anywhere an expressioncan be used. The Sqrt method belongsto the Math class.
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The Math classThe Sqrt method is found in the Math class.
Other common functions in the Math class:
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
name function (service) return type
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RoundingWhen we round a number, we pick the closest integer value.For example,
if a = 2.7, then the rounded value of a is 3if a = 2.4, then the rounded value of a is 2.
The Ceil and Floor methods given in the previous slidedon’t do rounding. For example,
Math.Floor (2.9) returns 2.0, while Math.Ceil (2.3) returns 3.0.
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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 usedin an expression, as expressions must return a typed value.
void methods can have zero or more parameters.
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Writing a Method
What job will the method do?What data does it need to do it’s work?What will the method return?
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Prompt the user to make a choice (r-p-
s)
Get the users input
Is theinput valid
?
no
Here is the activity diagramfor 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?
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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
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The Method Prologue
// The GetUserChoice method// Purpose: gets a valid user choice (1-3)// Parameters: none// Returns: the user choice as an int
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Prompt the user to make a choice (r-p-
s)
Get the users input
Is theinput 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 < ROCK || choice > SCISSORS) Console.WriteLine("Invalid selection.");
} while (choice < ROCK || choice > SCISSORS);
return choice; }
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Generate aRandom numberBetween 1 and 3
Here is the activity diagramfor 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?
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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
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Random Number GeneratorThe .Net library provides a class that we can use to createa random number generator. To create a random numbergenerator object we instantiate it as an object, so we write
Random randoms = new Random( );
This is the referenceto the Random object. This creates the Random object in the Heap.
This initializes the Random object.Know as a constructor.
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Random Number GeneratorA 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 scalethe 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)
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Random Number GeneratorTo shift the range of the random numbers, for example, to geta random number between 1 and 3, use this form of the Nextmethod:
int n = randoms.Next(1, 4);
Start at 1 Generate values up to, but not including 4 (1-3)
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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 compiler36
Generate aRandom numberBetween 1 and 3
static int GetComputerChoice( ) { int choice;
choice = randoms.Next(1,4);
return choice; }
We created the Random object randoms elsewhere.
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user =computer
?
yes tie
computer= rock
?
yes user= paper
?
yes winner isuser
no
winner iscomputernono
computer= paper
?
yes user= rock
?
yes winner iscomputer winner is
usernono
user= rock
?no
yes winner isuser
winner iscomputer
Here is the activity diagramfor 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?
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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
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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 diagramfor the method we need to display the winner.
What is it’s job (service)?What data does it need?What should it return?
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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
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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()
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Prompt the user to make a choice (y/n)
Get the users input
Is theinput valid
?
no
Here is the activity diagramfor the method we need to decide if the user wants toPlay again..
What is it’s job (service)?What data does it need?What should it return?
Return the choice
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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
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Prompt the user to make a choice (y/n)
Get the users input
Is theinput 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
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Now with these methods, ourMain( ) 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()
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Revisit The Rounding Issue
C# does not provide a method that rounds. Let’s write a method that rounds a double tothe nearest integer, using the floor method.
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3 4
for any value n, in this range Math.Floor (n ) = 3.
3.5
but … in this rangeMath.Floor(n + 0.5) = 4.0
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static int Round (double number){ return (int)(Math.Floor(number + 0.5));}
So, we can write the method Round( ) as follows:
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ScopeScope has to do with where a variable can be seen.
global variables (class level variables)
local variables (method level variables)
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A related term is storage class or lifetime, which defines how longa variable exists within a program.
automatic variables – come into existence when theyAre declared, and exist until the block in which they aredeclared 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)
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Exampleusing 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 outsideof any method. They need to be declared withina class as static. Constants are automatically static.
the name localValueis used twice. In this casethe scope of localValueis inside of Main( ). It isa local variable.
localValue is also declaredin this method, but itsscope is just inside themethod. It cannot be seenoutside of the method. Itis a different variable than the one declared in Main( ).It is a local variable. 53
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BlocksAnytime we use curly braces to delineate a piece of code,that code is called a block. We can declare variables thatare local to a block and have block scope.
Local variables declared in a nested block are onlyknown to the block that they are declared in.
When we declare a variable as part of a loop, for examplefor (int j = 0; j< MAX; j++)…
the variable j will have the block of the loop as its scope.
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Static VariablesA static variable comes into existence when it is declared and itlives 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.
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PracticeWrite the prologue for a method named CalcRatiothat takes two integer parameters and returns a double.
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PracticeWrite the code for this method. The ratiois found by dividing the first parameter by the second.
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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
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PracticeWrite 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
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