Jozef Goetz, 2015 1 2002 Prentice Hall. All rights reserved. Credits: 2011-2014 Pearson Education,...

Jozef Goetz, 2015

1

2002 Prentice Hall. All rights reserved.

Credits: 2011-2014 Pearson Education, Inc. All rights reserved.

Jozef Goetz, 2015

2

Chapter 7 - MethodsOutline

7.1 Introduction7.2 Packaging Code in C#7.3 static Methods, static Variables and Class Math7.4 Declaring Methods with Multiple Parameters7.5 Notes on Declaring and Using Methods7.6 Method Call Stack and Activation Records7.7 Argument Promotion and Casting7.8 The Framework Class Library7.9 Case Study: Random-Number Generation

7.9.1 Scaling and Shifting Random Numbers7.9.2 Random-Number Repeatability for Testing and

Debugging7.10 Case Study: A Game of Chance (Introducing Enumerations)7.11 Scope of Declarations7.10 Case Study: A Game of Chance (Introducing Enumerations)7.11 Scope of Declarations7.12 Method Overloading7.15 Recursion7.16 Passing Arguments: Pass-by-Value vs. Pass-by-Reference

Jozef Goetz, 2015

37.11 Scope of DeclarationsVariable and reference attributes:

1. name, 2. type, 3. size, 4. value, 5. scope, 6. duration

Scope Where an identifier can be referenced/accessed Local variable can only be used in a block declared

Class scope Begins at opening brace, ends at closing brace of class Methods and instance variables

Can be accessed by any method in class Repeated names causes previous to be hidden until scope ends

Block scope Begins at identifier's declaration, ends at terminating brace Have local variables and parameters of methods

When nested blocks and an outer block have an identifier defined; they need unique identifier names otherwise a syntax error

If local variable has same name as instance variable Instance variable "hidden”

Jozef Goetz, 2015

47.11 Scope of Declarations; Duration of Identifiers

Variable and reference attributes: name, type, size, value, duration, scope Duration (lifetime)

the period during which an identifier exists in memory

Automatic duration variables Created when program control reaches their declaration - local variables in a method

or in blocks, They should be initialized before they can be used Exist in block they are declared When block becomes inactive, they are destroyed

Static duration variables Created when defined and loaded into memory for execution Their storage is allocated and initialized when their classes are loaded into memory Exist until program ends

Local variables Created when declared Destroyed when the block exits

Instance variables are initialized by the compiler: Most variables are set to 0 All bool variables are set to false All reference variables are set to null

2002 Prentice Hall.All rights reserved.

Outline5

Scoping.cs

1 // Fig. 6.13 ed1 or 7.9 ed5 Scoping.cs2 // A Scoping example.3 4 using System;5 using System.Drawing;6 using System.Collections;7 using System.ComponentModel;8 using System.Windows.Forms;9 using System.Data;10 11 public class Scoping : System.Windows.Forms.Form12 {13 private System.ComponentModel.Container components = null;14 private System.Windows.Forms.Label outputLabel;15 16 public int x = 1; // instance variable17 18 public Scoping()19 {20 InitializeComponent();21 22 int x = 5; // variable local to constructor23 24 outputLabel.Text = outputLabel.Text +25 "local x in method Scoping is " + x;26 27 MethodA(); // MethodA has automatic local x;28 MethodB(); // MethodB uses instance variable x29 MethodA(); // MethodA creates new automatic local x30 MethodB(); // instance variable x retains its value31 32 outputLabel.Text = outputLabel.Text +33 "\n\nlocal x in method Scoping is " + x;34 }

This variable has class scope and can be used by any method in the class

This variable is local only to Scoping. It hides the value of the global variable

Will output the value of 5

Remains 5 despite changes to global version of x


Outline6

Scoping.cs

35 36 // Visual Studio .NET-generated code37 38 public void MethodA()39 {40 int x = 25; // initialized each time a is called41 42 outputLabel.Text = outputLabel.Text +43 "\n\nlocal x in MethodA is " + x +44 " after entering MethodA";45 ++x;46 outputLabel.Text = outputLabel.Text +47 "\nlocal x in MethodA is " + x + 48 " before exiting MethodA";49 }50 51 public void MethodB()52 {53 outputLabel.Text = outputLabel.Text +54 "\n\ninstance variable x is " + x +55 " on entering MethodB";56 x *= 10;57 outputLabel.Text = outputLabel.Text + 58 "\ninstance varable x is " + x +59 " on exiting MethodB";60 }61 62 // main entry point for application63 [STAThread]64 static void Main() 65 {66 Application.Run( new Scoping() );67 }68 69 } // end of class Scoping

Uses the global version of x (1)

Uses a new x variable that hides the value of the global x

Will permanently change the value of x globally

Jozef Goetz, 2015

77.12 Method Overloading

Procedure/Method overloading allows procedure/methods with the same name

but different parameter set for each procedure/method

1.Parameter Types2. Order of parameters3. Number of parameters

Procedure/Methods cannot be distinguished by return type. The same signature and different return types result in a

syntax error.

Usually perform the same or closely related task On different data types


Outline8

MethodOverload.cs

1 // Fig. 7.10: MethodOverload.cs2 // Using overloaded methods.3 4 using System;5 using System.Drawing;6 using System.Collections;7 using System.ComponentModel;8 using System.Windows.Forms;9 using System.Data;10 11 public class MethodOverload : System.Windows.Forms.Form12 {13 private System.ComponentModel.Container components = null;14 15 private System.Windows.Forms.Label outputLabel;16 17 public MethodOverload()18 {19 InitializeComponent();20 21 // call both versions of Square22 outputLabel.Text = 23 "The square of integer 7 is " + Square( 7 ) +24 "\nThe square of double 7.5 is " + Square ( 7.5 );25 }26 27 // Visual Studio .NET-generated code28

Two versions of the square method are called


Outline9

MethodOverload.cs

Program Output

29 // first version, takes one integer30 public int Square ( int x )31 {32 return x * x;33 }34 35 // second version, takes one double36 public double Square ( double y )37 {38 return y * y;39 }40 41 [STAThread]42 static void Main()43 {44 Application.Run( new MethodOverload() );45 }46 47 } // end of class MethodOverload

One method takes an int as parameters

The other version of the method uses a double instead of an integer


Outline10

MethodOverload2.cs

Program Output

1 // Fig. 7.11: MethodOverload2.cs2 // Overloaded methods with identical signatures and3 // different return types.4 5 using System;6 7 class MethodOverload28 {9 public int Square( double x )10 {11 return x * x;12 }13 14 // second Square method takes same number,15 // order and type of arguments, error16 public double Square( double y )17 {18 return y * y;19 }20 21 // main entry point for application22 static void Main()23 {24 int squareValue = 2;25 Square( squareValue );26 }27 28 } // end of class MethodOverload2

This method returns an integer

This method returns a double number

Since the compiler cannot tell which method to use based on passed values an error is generated

Jozef Goetz, 2015

As of Visual C# 2010 or 2012, methods can have optional parameters that allow the calling method to vary the number of arguments to pass.

An optional parameter specifies a default value that’s assigned to the parameter if the optional argument is omitted.

For example, the method headerpublic int Power( int baseValue, int exponentValue = 2)

specifies an optional second parameter.

You can create methods with one or more optional parameters.

All optional parameters must be placed to the right of the method’s non-optional parameters.

7.13 Optional Parameters

Jozef Goetz, 2015

When a parameter has a default value, the caller has the option of passing that particular argument.

public int Power( int baseValue, int exponentValue = 2)

specifies an optional second parameter.

Any call to Power must pass at least an argument for the parameter baseValue, or a compilation error occurs.

7.13 Optional Parameters (Cont.)

Jozef Goetz, 2015

Optionally, a second argument (for the exponentValue parameter) can be passed to Power.

Consider the following calls to Power:

Power()Power(10)Power(10, 3)

The first generates a compilation error because this method requires a minimum of one argument.

The second is valid because one argument (10) is being passed—the optional exponentValue is not specified in the method call.

7.13 Optional Parameters (Cont.)

Jozef Goetz, 2015

7.13 Optional Parameters

The last call Power(10, 3)is also valid -10 is passed as the required argument and 3 is passed as the optional argument.

In the call that passes only one argument (10), parameter exponentValue defaults to 2, which is the default value specified in the method’s header.

Each optional parameter must specify a default value by using an equal (=) sign followed by the value.

Jozef Goetz, 2015

•At least base Value must pass. •exponentValue is an optional parameter

Progr. Error 7.11: Declaring a non-optional parameter to the right of an optional one is a compilation error.

Jozef Goetz, 2015

Visual C# 2010/2012 provides a new feature called named parameters, which enable you to call methods that receive optional parameters by providing only the optional arguments you wish to specify.

Explicitly specify the parameter’s name and value —separated by a colon (:)—in the argument list of the method call. For example:

t.SetTime( hour: 12, second: 22 ); // sets the time to 12:00:22

t.SetTime( hour: 12, , second: 22 );// error

7.14 Named Parameters

Jozef Goetz, 2015

t.SetTime( hour: 12, second: 22 ); // sets the time to 12:00:22

The compiler assigns parameter hour the argument 12 and parameter second the argument 22.

The parameter minute is not specified, so the compiler assigns it the default value 0.

It’s also possible to specify the arguments out of order when using named parameters. The arguments for the required parameters must always be supplied.

t.SetTime(12); // sets the time 12:00:00 PMt.SetTime(12, 30); // sets the time 12:30:00 PMt.SetTime(12, , 30); // compilation error, C# doesn’t allow // skip an argument

7.14 Named Parameters

Jozef Goetz, 2015

187.15 Recursion

Fig. 7.13 Recursive evaluation of 5!, 5 factorial. A recursive def n! = n (n – 1)!

(a) Procession of recursive calls.

5!

5 * 4!

4 * 3!

3 * 2!

2 * 1!

1

(b) Values returned from each recursive call.

Final value = 120

5! = 5 * 24 = 120 is returned

4! = 4 * 6 = 24 is returned

2! = 2 * 1 = 2 is returned

3! = 3 * 2 = 6 is returned

1 returned

5!

5 * 4!

4 * 3!

3 * 2!

2 * 1!

1

•(a) Each time method calls itself with a slightly simple problem until converges on the base case. Continually breaks problem down to simpler forms.

•(b) then returns to the previous problem,

and a sequence of returns follows up the line until the original problem eventually returns the final

result to the caller.

While the number to be processed is greater than 1, the function calls itself

When the number is 1 or less (the base case), 1 is returned, and each function call can now return an answer until each call has been resolved

Jozef Goetz, 2015

19

General format formany recursive functions

if (some condition for which answer is known)

// base case

solution statement

else // general case

recursive function call

SOME EXAMPLES . . .

Jozef Goetz, 2015

20

Recursive Definition is a definition in which something is defined in terms of smaller

version of itself.

int Factorial ( int number ) // Recursive Solution// Pre: number is assigned and number >= 0.{ if ( number == 0) //(1) base case

return 1 ; //(2)else // general case

return number * Factorial ( number - 1 ) ; //(3) //includes the simpler problem}

Jozef Goetz, 2015

217.15 Recursion

Recursive method Calls itself (directly or indirectly) through another method Method knows how to solve only a simples case base case Method divides problem into

1. Base case2. Simpler problem

– Each time method calls itself with a slightly simple problem until converges on the base case,

Continually breaks problem down to simpler forms

Must converge on the base case in order to end recursion

Each method call remains open (unfinished) Finishes each call and then finishes itself

Jozef Goetz, 2015

22

int fact ( int number ) // Recursive Solution// Pre: number is assigned and number >= 0.{ if ( number == 0) //(1) base case return 1 ; //(2) else

// general case return number * fact ( number - 1 ) ; //(3)

//includes the simpler problem

} //(4)

Each method call remains open

Think of a recursive function as having infinitely many copies of itself

Every call to a recursive function has

Its own code

Its own set of parameters and local variables

After completing a particular recursive call

Control goes back to the calling environment, which is the previous call


Outline23

FactorialTest.cs

1 // Fig. 7.14: FactorialTest.cs2 // Recursive Factorial method.3 4 using System;5 using System.Drawing;6 using System.Collections;7 using System.ComponentModel;8 using System.Windows.Forms;9 using System.Data;10 11 public class FactorialTest : System.Windows.Forms.Form12 {13 private System.ComponentModel.Container components = null;14 15 private System.Windows.Forms.Label outputLabel;16 17 public FactorialTest()18 {19 InitializeComponent();20 21 for ( long i = 0; i <= 10; i++ )22 outputLabel.Text += i + "! = " + 23 Factorial( i ) + "\n";24 }25


Outline24

FactorialTest.cs

Program Output

26 // Visual Studio .NET-generated code27 28 public long Factorial( long number )29 {30 if ( number <= 1 ) // base case31 return 1;32 33 else34 return number * Factorial( number - 1 );35 }36 37 [STAThread]38 static void Main() 39 {40 Application.Run( new FactorialTest());41 }42 43 } // end of class FactorialTest

The Factorial method calls itself (recursion)

The recursion ends when the value is less than or equal to 1

Jozef Goetz, 2015

25Iterative Solution

int Factorial ( int number )

// Pre: number is assigned and number >= 0.

{

int fact = 1;

for (int i = 2; i <= number; i++)

fact = fact * i;

return fact ;

}

Jozef Goetz, 2015

26Example Using Recursion: The Fibonacci Sequence

Fibonacci series Each number in the series is sum of two previous numbers

e.g., 0, 1, 1, 2, 3, 5, 8, 13, 21…

Recursive formulafibonacci(0) = 0fibonacci(1) = 1fibonacci(n) = fibonacci(n - 1) + fibonacci( n – 2 )

Each invocation of the method that does not match one of the base cases results in two additional recursive calls to the method

fibonacci(0) and fibonacci(1) are base cases Golden ratio

The ratio of successive fibonacci numbers converges on a constant value near 1.618

Performance Fibonacci-style recursive methods exponentially generate method calls (result in an

exponential “explosion” of calls) – avoid recursive style for some problems Hinders performance

– Fib(30) has over 2.7 million method calls

– Fib(31) has over 4 million method calls

– Fib(32) has over 7 million method calls

Jozef Goetz, 2015

27Example Using Recursion: The Fibonacci Sequence

Fig. Set of recursive calls to method Fibonacci (abbreviated as F).

return 1 return 0

F( 1 ) F( 0 ) return 1

F( 3 )

F( 2 ) F( 1 )+return

return +


Outline28

FibonacciTest.cs

1 // Fig. 6.16 ed1: FibonacciTest.cs2 // Recursive fibonacci method.3 4 using System;5 using System.Drawing;6 using System.Collections;7 using System.ComponentModel;8 using System.Windows.Forms;9 using System.Data;10 11 public class FibonacciTest : System.Windows.Forms.Form12 {13 private System.ComponentModel.Container components = null;14 15 private System.Windows.Forms.Button calculateButton;16 17 private System.Windows.Forms.TextBox inputTextBox;18 19 private System.Windows.Forms.Label displayLabel;20 private System.Windows.Forms.Label promptLabel;21 22 public FibonacciTest()23 {24 InitializeComponent();25 }26 27 // Visual Studio .NET-generated code28


Outline29

FibonacciTest.cs

29 // call Fibonacci and display results30 protected void calculateButton_Click(31 object sender, System.EventArgs e )32 {33 string numberString = ( inputTextBox.Text );34 int number = System.Convert.ToInt32( numberString ); 35 int fibonacciNumber = Fibonacci( number );36 displayLabel.Text = "Fibonacci Value is " + fibonacciNumber;37 }38 39 // calculates Fibonacci number40 public int Fibonacci( int number )41 {42 if ( number == 0 || number == 1 )43 return number;44 else45 return Fibonacci( number - 1 ) + Fibonacci( number - 2 );46 }47 48 [STAThread]49 static void Main() 50 {51 Application.Run( new FibonacciTest() );52 }53 54 } // end of class FibonacciTest

The number uses the Fibonacci method to get its result

Calls itself twice, to get the result of the the two previous numbers

The recursion ends when the number is 0 or 1


Outline30

FibonacciTest.cs Program Output

Exercise: Expand by displaying all generated numbers.

Exercise: Each number in the series is sum of the last one and the third from the last one.

e.g., 0, 1, 1, 1, 2, 3, 4, 6, 9, 13,…

First draw the sequence diagram for F(4).

Jozef Goetz, 2015

317.15 Recursion vs. Iteration

Iteration1. Uses repetition structures (for, while or do/while)2. Repetition through explicitly use of repetition structure3. Terminates when loop-continuation condition fails4. Controls repetition by using a counter5. No extra calls so it doesn’t consume additional memory

Recursion1. Uses selection structures (if, if/else or switch)2. Repetition through repeated method calls3. Terminates when base case is satisfied4. Controls repetition by dividing problem into simpler one5. Recursive calls take time and consume additional

memory

Both can have infinite loops

Jozef Goetz, 2015

32Recursion vs. Iteration (cont.)

Recursion

negatives: More overhead than iteration (time expensive) More memory intensive than iteration (the overhead of repeated method

calls) – memory space expensive Difficult to test and debug

positives: Recursion more naturally mirrors some problems Often can be implemented with only a few lines of code Can also be solved iteratively but may take large amount of code

Balance Choice between performance (iteration) and good software

engineering (recursion) Recursion usually more natural for some problems Modularizing programs in a neat, Hierarchical manner promotes good software engineering but it has a prize.

Jozef Goetz, 2015

337.16 Passing Arguments: Call-By-Value vs. Call-By-Reference

Passing by value Send a method a copy of the object

Changes to the called method’s copy don’t effect the original variable’s value When returned are always returned by value Set by value by default for value-type variables

Passing by reference Send a method the actual reference point (to the original object in

memory) Causes the variable to be changed throughout the program

When returned are always returned by reference No overhead of copying large data Weaken security, b/c the called function can corrupt the caller’s data

Note: The references themselves passed by value

The ref keyword specifies by reference The out keyword means a called method will initialize the reference variable


Outline34

RefOutTest.cs

1 // Fig. 6.8 ed1 or 7.18 ed3 7.15 ed4 and ed5: RefOutTest.cs2 // Demonstrating ref and out parameters.3 // 3 methods to calculate the square of an integer 4 using System;5 using System.Windows.Forms;6 7 class RefOutTest8 {9 // x is passed as a ref int (original value will change)10 static void SquareRef( ref int x )11 {12 x = x * x;13 }14 15 // original value can be changed and initialized16 static void SquareOut( out int x )17 {18 x = 6;19 x = x * x;20 }21 22 // x is passed by value (original value not changed)23 static void Square( int x )24 {25 x = x * x;26 }27 28 static void Main( string[] args )29 {30 // create a new integer value, set it to 531 int y = 5;32 int z; // declare z, but do not initialize it33

When passing a value by reference the value will be altered in the rest of the program as well

Since x is passed as out the variable can then be initiated in the method

Since not specified, this value is defaulted to being passed by value. The value of x will not be changed elsewhere in the program because a duplicate of the variable is created.

Since the methods are void they do not need a return value.


Outline35

RefOutTest.cs

34 // display original values of y and z35 string output1 = "The value of y begins as " 36 + y + ", z begins uninitialized.\n\n\n";37 38 // values of y and z are passed by reference39 RefOutTest.SquareRef( ref y ); // must use ref40 RefOutTest.SquareOut( out z ); // must use out41 42 // display values of y and z after modified by methods43 // SquareRef and SquareOut44 string output2 = "After calling SquareRef with y as an " +45 "argument and SquareOut with z as an argument,\n" +46 "the values of y and z are:\n\n" + 47 "y: " + y + "\nz: " + z + "\n\n\n";48 49 // values of y and z are passed by value50 RefOutTest.Square( y );51 RefOutTest.Square( z );52 53 // values of y and z will be same as before because Square54 // did not modify variables directly55 string output3 = "After calling Square on both x and y, " +56 "the values of y and z are:\n\n" +57 "y: " + y + "\nz: " + z + "\n\n";58 59 MessageBox.Show( output1 + output2 + output3, 60 "Using ref and out Parameters", MessageBoxButtons.OK,61 MessageBoxIcon.Information );62 63 } // end method Main64 65 } // end class RefOutTest

The calling of the SquareRef and SquareOut methods

The calling of the Square method by passing the variables by value

Jozef Goetz, 2015

Find Maximum (and Minimum) 36

Jozef Goetz, 2015

Extra: Find Maximum and Minimum 37

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Class to find MAX 38

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Driver to test CalculateMax()39

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Jozef Goetz, 2015 1 2002 Prentice Hall. All rights reserved. Credits: 2011-2014 Pearson Education,...

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