By: Md Rezaul Huda Reza [email protected]. Decision and iteration statements. Methods. E. I....

UNIVERSITY OFSOUTH ASIA

Lecture 4:

By: Md Rezaul Huda [email protected]

CSE- 629VISUAL PROGRAMMING

WITH INTERNET)

E. I. Teodorescu

Decision and iteration statements. Methods.

UNIVERSITY of South Asia

3Md Rezaul Huda [email protected]

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LAST WEEK: VARIABLES

Area in computer memory where a value of a particular data type can be stored Syntax : type identifier(name);

Assigning a value to a variable Syntax : type identifier = expression;

(Note: read ‘=‘ as ‘becomes’)

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THIS WEEK…

Using decision statements Using repetition statements Methods in general Instance methods and variables Code generated by Visual Studio

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COMMENTS In-Line Comments (//)

// This is traditionally the first program written. Everything to the right of the slashes ignored by the

compiler. Carriage return (Enter) ends the comment Multi-line Comment (/* */) block comments

/* This is the beginning of a block multi-line comment. It can go on for several lines or just be on a single line. */

Example:

int x = 3 // this is z comment stating that a variable of type integer was

//declared and initialized to 3

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Using decision statementsUsing repetition statements



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MAKING DECISIONS Central to both selection and iteration constructs Involves conditional expression - “the test”

Produces a Boolean result Declaring Boolean variable bool

can hold a value that is either true or falseExample

bool someCondition = false;

bool areYouReady = true;

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EQUALITY AND RELATIONAL OPERATORS

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Do not confuse the equality operator == with the assignment operator = .



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LOGICAL OPERATORS && (AND)True if BOTH the conditions are true

|| (OR)True if EITHER of the conditions are

true

! (NOT)Negates a bool – true becomes false,

false becomes true

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IF...ELSE SELECTION STATEMENTS Classified as one-way, two-way, or

nested Alternate paths based on result of

conditional expression Expression must be enclosed in

parentheses Produce a Boolean result

One-wayWhen expression evaluates to false,

statement following expression is skipped or bypassed

No special statement(s) is included for the false result

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SELECTION STATEMENTS (IF STATEMENTS)

if (boolExpression) { statement; }

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Either the true statement(s) executed or the false statement(s) — but not both

if (boolExpression) { statement; }else{ statement; }

One-way Selection Statement Two-way Selection Statement

Curly brackets are required with multiple statements

Try should use them even if you have one statement. Why?



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IF…ELSE EXAMPLEint seconds ;

if (seconds == 59) {seconds =0;

}else {

seconds++;}

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// Note : == is different than =

if (seconds = 59) { // Compile error seconds =0;

}else

…...

//Note : you can use Boolean var as an expresion

bool isReady;if (isReady == true) // OK

…..if (isReady) // OK

…...

What is the difference

between “==“ and “=“ ?



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IF…ELSE EXAMPLE 2int hoursWorked =40 ;double payRate;string resultString;pouble payAmount;

if (hoursWorked > 40) { payAmount = (hoursWorked – 40) * payRate * 1.5 + payRate * 40; resultString= “You worked “ + (hoursWorked – 40) + “ hours overtime.”;

resultString = resultString + “ Your payment is ” + payAmount.}else{ payAmount = hoursWorked * payRate; resultString = “ Your payment is ” + payAmount.} // Next line will be executed, whether the expression evaluates true or false

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NESTED IF-S You can nest if statements inside other if

statements

if(Condition1)

Statement1;

else if(Condition2)

Statement2;

else if(Condition3)

Statement3; else

Statement-n;

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NESTED IF-S.... // some code before

age = int.Parse(textBox1.Text);

if (age<18) {

label1.Text = "to young to drink!";

}

else if(18<=age and age<65)

{

label1.Text = “Work hard, Party Hard";

}

else if(65<=age and age<100)

{

label1.Text = “Easy on Work, Still Party Hard";

}

else

{

label1.Text = “Wow! Tell me your secret";

}

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SWITCH SELECTION STATEMENTS Executes a set of logic depending on the value

of a given parameter Multiple selection structure Also called case statement Works for tests of equality only Single variable or expression tested

Must evaluate to an integral or string value

Requires the break for any case No fall-through available

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Switch Statements General Form

switch (controllingExpression) { case value1:

statement(s); break; . . . case valueN:

statement(s); break; // jump-statement [default:

statement(s); break;]}

Selector expression is an

integral or string type

expression.

Value must be a of the same type

as selector

Optional

The embedded statement(s) is

executed if control is transferred to the

case or the default.

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int n = int.Parse(textBox1.Text);int cost = 10;

switch(n) { case 0: case 1:

cost += 25; break;

case 2: cost += 28; break;

case 3: cost += 50; break;

default: cost+ = 30; break;

}

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Switch Statements Example



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LOOPING (REPITITION, ITERATION) Useful because you can repeat

instructions with many data setsrepetition or iteration structures statements are executed in order, except

when a jump statement is encountered. keywords are used in iteration statements:

whiledo for foreach – later, when we study collections in

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USING THE WHILE STATEMENTwhile (conditional expression){

statement(s); // the while body

}

The while statement executes a statement or a block of statements until a specified expression evaluates to false

No semicolon after the conditional expression

the test of expression takes place before each execution of the loop

a while loop executes zero or more times.

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DO…WHILE STATEMENTS

Post test do statement is executed at least once regardless of

the value of the expression The do statement executes a statement or a

block of statements repeatedly until the expression evaluates to false

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do { statement;}while( conditional

expression);



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WHILE AND DO…WHILE EXAMPLE

int y = 0;string s ;while (y < 3) // No semicolon{ s= “Number: “ + y ;

y++ ; }

What is the value of s at the end of the loop?

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int y = 0;string s ;do // No semicolon on this

line{ s= “Number: “ + y ;

y++ ;

}while (y < 3);

What happens in the do…while loop?



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WHILE AND DO…WHILE EXAMPLE

int y = 3;string s ;while (y < 3) // No semicolon{ s= “Number: “ + y ;

y++ ; }


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int y = 3;string s ;do // No semicolon on this

line{ s= “Number: “ + y ;

y++ ;

}while (y < 3);




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FOR LOOP executes a statement or a block of

statements repeatedly until a specified expression evaluates to false. Usually associated with counter-controlled types

packages initialization, test, and update all on one line

General form is:for (initializers; conditional expression; iterator){

statement; }

Interpreted as: for (initialize; test; update)

statement

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FOR STATEMENT The for statement executes the statement

repeatedly as follows: First, the initializers are evaluated.Then, while the expression evaluates to true, the

statement(s) are executed and the iterators are evaluated.

When the expression becomes false, control is transferred outside the loop.

Example

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for(i = 0; i<5; i++) { s = s + i + “\n”; }



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NESTED LOOPS Loop can be nested inside an outer loop

Inner nested loop is totally completed before the outside loop is tested a second time

Example:

int inner;string s = “ “;for (int outer = 0; outer < 3; outer++){ for(inner = 10; inner > 5; inner --) { s = s + outer + “ “ + inner + “\n”; }}MessageBox.Show(s);

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Defining and Using Methods



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METHODS Also known as functions Used to define behaviour. A method

does something active Question: What is the difference

between a method and a variable?Variables simple store dataMethods: A sequence of statements

with a name; A piece of code that does something (it has a

task)

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ANATOMY OF A METHOD

It has: A name: first line stating important information

about the methodA body: Enclosed in a block { }

Can be called (invoked) one or more times All programs consist of at least one method,

Main( )

Methods are defined inside classes

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DEFINING A METHOD.

[Method access] returnType methodName ( [ParametersList] )

{ //method body statements go here}

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Method structure

Example:

optional

optional

public int CalculateAge(int y)

{

int birthYear = y;

return 2010-y;

}



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CALLING A METHOD

Q: What is the purpose of declaring/creating methods? So you can use them at a later stage! They are “handy” pieces of code that do something

Use a method by calling it from another part of the code.

A method can be called as many times as you wish A method should be called at least once

• otherwise not much use to it!

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Calling a method result = methodName ( [ParametersList] );

optional



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METHOD NAMES Follow the rules for creating an

identifier Starts with capitalSensible name

ExamplesCalculateSalesTax( )AssignSectionNumber( )DisplayResults( ) InputAge( )ConvertInputValue( )

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METHOD BODY Enclosed in curly brackets { }

Include statements ending in semicolonsDeclare variables

Do arithmetic

Call other methods

Value returning methods must include return statement

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METHOD BODY 2 The method header is followed by the method

body

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// Program.cs

public class Hello{

}

public static void Main() {

Application.Run(new Form1());}



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THE RETURN TYPE Indicates what type of value is

returned when the method is completed

Always listed immediately before method name

voidNo value being returned

return statementRequired for all non-void methods Compatible value (with declared type)

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RETURN TYPE A any data type can be a return type

What types do you know?: bool bytechar decimaldouble floatint longobject sbyteShort string

You must write a return statement inside the method

Void = no return type. The return type is set to void by default.

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METHOD EXAMPLE

public void Calculate ( ){

int x = 7; int y = 3; var result = 0;

result = x * y; }

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What is special about it?

Is the Calculate() method returning anything?



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RETURN TYPE

public double CalcSpeed(double m, int h){ double miles = m; int hours = h;

return miles / hours;}

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Return type

Compatible value (double) returned



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WRITING RETURN STATEMENTS

private int AddValues(int x, int y){

return x + y ;}

…….

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The return statement is usually positioned at the end of your method. Why?

If you want a method to return information you must write a return statement inside the method!

If the return type is “void”, the method doesn’t return a statement !



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PARAMETERS (ARGUMENTS) Why do we want to use parameters?

you may want a method to do something, but that method needs some external data

The external data can be different every time you call that method.

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public double CalcSpeed(double m, int h){ double miles = m; int hours = h;

return miles / hours;}double porscheSpeed = CalcSpeed(260.0, 1)double tractorSpeed = CalcSpeed(80.0, 2)



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PARAMETERS (CONTINUED) Declaring a method with parameterspublic int AddValues( int x, int y, int z){ return x+y+z;}

Calling the method

int r = AddValues(1,2,3) ; AddValues ( r, AddValues(3,3,3), 3*r );

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formal parametersWhen the method is declared.

Actual argumentsWhen the method is called.

Exact match of the method’s name you are calling (case sensitive)



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APPLYING SCOPE. The scope of a variable is the region of the

program where the variable is usable Local scope

Local to a method Declared inside the method body Used only inside the method body !! Local variables must be instantiated

Class scope – next lecture Declared inside the class body, but not inside a

method In C# are called fields (class variables in java) Use fields(class variables) to share information

between methods

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QUESTIONvoid method1()

{int myVar;

}void method2()

{ myVar =

22;}

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int myField;void method1()

{myField = 22;}

void method2(){ myField++;}

Will get an error

Is there anything wrong with the given examples?



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OVERLOADING METHODS A method has the ability to behave differently

according to the types and number of parameters that are passed to it.

C# allows you to define different versions of a method, and the compiler will automatically select the most appropriate one based on the parameters supplied. methods with the same name but with a different set

of parameters signature of a method = Each combination of

parameter types

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CALLING OVERLOADED METHODS

public class AddingNumbers

{

int x;

public int Add(int a, int b)

{

return a+b;

}

public int Add(int a, int b, int c)

{

return a+b+c;

}

………..x = Add(3, 2)+ Add(4, 5, 6);

string s = “Result= “ + x;

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2 methods with the same name!

What are the signature of these

methods?



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SPOT THE ERROR(S)!public class AddingNumbers

{

double x;

public int AddNumbers(int a, int b) {

return a+b;

}

public int AddNumbers(int c, int d) {

return d+c;

}

public int AddNumbers(int f, double g) {

return d+c;

}

public int AddNumbers(double f, double g) {

return f+g;

}

………..

x = AddNumbers (3, 2)+ AddNumbers (4, 5) + AddNumbers (3.2, 2.0)+

AddNumbers (4, 5.0); ;

string s = “Result= “ + x;

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SOME METHODS YOU KNOW .NET Framework Class Library (FCL) is in

effect code that can be reused. Inside this there are many reusable

methods Last lecture we used some FCL methods

to convert types. Can you remember any?Parse( ) methodToString( ) methodThe Convert methods

Convert.ToDouble( ) Convert.ToInt32( ) etc

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CODE EXAMPLES IN VS

QUESTIONING SESSION



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IF EXAMPLE

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What is the role of \n?

What is the red “thing”?

MessageBox.Show(

What is this?



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What is s and what is it set to?

What is Parse(s) and what does it do?

Switch Selection



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DO … WHILE EXAMPLE.

private void button1_Click(object sender, EventArgs e) { int y = 0; string s = " ";

do // No semicolon on this line { s = s + y + " " + y++ + " " + ++y + "\n";

}

while (y < 3);

label1.Text = s;

}

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What is the difference between ++y and y++?

Event created by double clicking button1

What would the output be if the condition was y<0 ?

What will be the output in the label?



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WRITING A CURRENCY CONVERTER APPLICATION.

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What is this?

What is this?

What is this?



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DECLARING A METHOD

public double ConvertFahranheitToCelsius(int aNumber)

{ result = (5.0 / 9.0) * (aNumber - 32); return Math.Round(result,2); }

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nameWhat are they? What is this?

What id Round(…)?What is this?



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ADDING THE BEHAVIOUR OF THE BUTTON When the button is clicked : , inputNumber = int.Parse(textBoxInput.Text);

labelResult.Text = inputNumber + " Fahranheit is

" + ConvertFahranheitToCelsius(inputNumber)+ " Celsius";

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Who is this?



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QUESTION

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What would happen if the formula is written :(5/9) * (aNumber – 32)

What will the value of the result variable be ?



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CODE AVAILABLE ON TEACHMATAvailable applications on teachmat for week 2: doWhileEx

do...while and while examples AgeExample

Parse() method, switch, while IfElseExample

If statements ForLoopExample

For loop MethodExample

methods, parameters(arguments) SwitchApp

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ANY QUESTION ? ? ?



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THANK YOU

By: Md Rezaul Huda Reza [email protected]. Decision and iteration statements. Methods. E. I....

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