Chapter 4 Decisions and Conditions Programming in C#.NET © 2003 by The McGraw-Hill Companies, Inc....

Chapter 4 Decisions and

Conditions

Programming in C# .NET

© 2003 by The McGraw-Hill Companies, Inc. All rights reserved.

© 2003 by The McGraw-Hill Companies, Inc. All rights reserved.4- 2

Objectives

• Use if statements to control the flow of logic• Understand and use nested if statements• Read and create flowcharts indicating the

logic in a selection process• Evaluate conditions using the relational

operators• Combine conditions using && (and) and || (or)

operators


Objectives cont.

• Test the Checked property of radio buttons and check boxes

• Perform validation on numeric fields using if statements

• Use a case structure for multiple decisions• Use one event handler to respond to the

events for multiple controls and determine which control caused the event


Objectives cont.

• Call event handlers from other methods

• Create message boxes with multiple buttons and choose alternate actions based on the user response

• Debug projects using breakpoints, stepping program execution, and displaying intermediate results


if Statements• A decision made by the computer is formed as a

question: Is a given condition true or false? If it is true, do one thing; if it is false, do something else.

Sun is shining?

Go to class Go to beach

false true


if Statements cont.

• Use block (braces) to include multiple statements in the if or else portion of an if statement

• Only the first statement following an if or else is considered part of statement unless braces are used

• if and else do not have semicolons• Statements under the if and else clauses are

indented


if/else Statement–General Form

if (condition){

statement(s);}

if (condition){

statement(s);}else{

statement(s);}

if (condition){

statement(s);}[else if (condition){

statement(s);}][else{

statement(s);}]


The if/else Statement - Example

decUnits = decimal.Parse(unitsTextBox.Text);

if (decUnits < 32.0M){

freshmanRadioButton.Checked = true;}else{

freshmanRadioButton.Checked = false;}


Flowcharting if Statements

• Flowcharts help programmers organize thoughts and design projects more quickly

• Diamond-shape is a decision symbol

Decision

Process Process

false true


Conditions

• C# provides six relational operators

• Results of a comparison is true or false

• Conditions can be formed with variables, constants, object properties, and expressions

• Comparisons must be made on like types– Compare strings with strings– Compare numeric values with numeric values


Relational OperatorsSymbol Relation

TestedExamples

> Greater than decimal.Parse(amountTextBox.Text)>decLimit

inCorrect > 75

< Less than int.Parse(salesTextBox.Text) < 10000

decSales < decQuota

= = Equal to passwordTextBox.Text = = “101”

decBalance = = decGoal

!= not equal to freshmanRadioButton.Checked != true

nameTextBox.Text != “”

>= greater than

or equal to

int.Parse(quantityTextBox.Text) >= 500

decInterestRate >= decMaxRate

<= less than

or equal to

intTotal <= intLimit

decQuantity <= OM


Comparing Numeric Variables and Constants

• Comparison of numeric values uses the sign of the numbers

• Equals sign (=) is an assignment statement

• Double equal sign (= =) is used to test for equality


Comparing Character Data

• Use relational operators to compare character data

• ANSI establishes an order (collating sequence) for all letters, numbers, and special characters by assigning codes

• Compare a char variable to a literal, to another char variable, an escape sequence, or the code number


Comparing Strings

• Cannot use < or > relational operators when comparing strings

• To be equal, two strings must be exactly the same, including capitalization

• Use String.Compare or String.CompareTo methods to compare strings for less than or greater to


Comparing Uppercase and Lowercase Characters

• C# is case sensitive i.e. “y” is not equal to “Y”• To compare strings, use ToUpper and ToLower

methods of the string classTextString.ToUpper()TextString.ToLower()

• Example: aTextBox.Text value RichardaTextBox.Text.ToUpper() value RICHARD

aTextBox.Text.ToLower() value richard


Testing for true or false

• Use shortcuts when testing for true or false

if (blnSuccessfulOperation = = true)

is equivalent to

if (blnSuccessfulOperation)

if (radBlue.Checked)

is equivalent to

if (radBlue.Checked = = true)


Compound Conditions

• Use compound conditions to test more than one condition

• Use logical operators to join conditions

• Logical operators are || (or), && (and),

and ! (not)

• Each side of the logical operator must be a complete condition


Compound Conditions cont.

Logical Operator

Meaning

|| (or) If one condition or both conditions are true, the entire condition is true

&& (and) Both conditions must be true for the entire condition to be true

! (not) Reverses the condition so that a true condition will evaluate false and vice versa


Compound Condition Examples

if (maleRadioButton.Checked && int.Parse(ageTextBox.Text) < 21)

{

intMinorMaleCount ++:

}

if (juniorRadioButton.Checked || seniorRadioButton.Checked)

{

intUpperClassmanCount ++;

}


Combining && and ||

• If there is an && and an ||, the && is evaluated before the ||

• You can change order of evaluation using parentheses; condition inside parentheses will be evaluated first


Short-Circuit Operations

• When evaluating a compound condition, sometimes the second condition is never evaluated– In an || condition, if the first condition is true,

the second condition is not evaluated– In an && condition, if the first condition is

false, the second condition is not evaluated

• This is called short circuiting the operation


Short-Circuit Operations cont.

And-Short Circuit And-Regular Or-Short Circuit Or-Regular

&&

(Forces comparison to occur)

& ||

(Forces comparison to occur)

|

• Use a single comparison operator to avoid short circuiting


Nested if Statements

• An if statement that contains additional if statements is a nested if statement

• You can nest if statements in the else portion of the statement

• You can nest ifs within ifs as long as you want• Project may become difficult to follow and

may not perform as intended when ifs are too deeply nested


Nested if Exampleif (intTemp > 32){

if (intTemp > 80){

commentLabel.Text = “Hot”;}else{

commentLabel.Text = “Moderate”;}

}else{

commentLabel.Text = “Freezing”;}


Nested if Flowchart Example

maleRadioButton.Checked?

ageTextBox.Text<21? ageTextBox.Text<21?

Add 1 to intMinorMaleCount

Add 1 tointMaleCount

Add 1 tointMinorFemaleCount

Add 1 tointFemaleCount

FalseTrue

FalseTrueFalseTrue


Coding an else if

• The second (nested) if can be placed on a line by itself or on the same line as the else

• The indentation changes if else if is on one line

• Most programmers code else if on one line when testing multiple conditions


Coding an else if exampleif (maleRadioButton.Checked){

if (int.Parse(ageTextBOx.Text) < 21){

intMinorMaleCount ++;}else{

intMalecount ++;}

}else if (int.Parse(ageTextBox.Text) < 21){

intMinorFemaleCount ++;}else{

intFemaleCount ++ ;}


Using if Statements with Radio Buttons and Check Boxes

• Place code in button Click event to check for selected options

• A “Simple Sample”– Test the Value of a Check Box

if (testCheckBox.Checked){

messageLabel.Text = “Check box is checked”;

}


Using if Statements with Radio Buttons and Check Boxes cont.

• A “Simple Sample” cont.– Test the State of Radio Buttons

if (freshmanRadioButton.Checked)

{messageLabel.Text = “Freshman”;

}else{

messageLabel.Text = “Sophomore”;}


Checking the State of a Radio Button Group

if (freshmanRadioButton.Checked){

intFreshmanCount ++;}else if (sophomoreRadioButton.Checked){

intSophomoreCount ++;}else if (juniorRadioButton.Checked){

intJuniorCount ++;}else{

intSeniorCount ++;}


Checking the State of Multiple Check Boxes

if (discountCheckBox.Checked){

//Calculate the discount}if (taxCheckBox.Checked){

//Calculate the tax}if (deliveryCheckBox.Checked){

//Deliver it}


Enhancing Message Boxes

• Control the format of the message

• Display multiple buttons

• Check which button the user clicks

• Perform alternate actions depending on the user selection


Displaying the Message String

• Message string can be a string literal enclosed in quotes or a string variable

• May also concatenate several items

• Create a variable for the message

• Call the Show method to display the message


Combining Values into a Message String

• Concatenate a literal with the value from a variable

• Example:

string strMessage;

strMessage=“Total Sales: “ + decTotalSales.ToString(“C”);

MessageBox.Show(strMessage, “Sales”, MessageBoxButtons.OK);


Creating Multiple Lines of Output

• Control the line length and position of the split by inserting a new line (\n) character into the string message

• Combine multiple new line characters for double spacing

• Example: string strMessage;strMessage=“Total Sales: “ + decTotalSales.ToString(“C”) +

“\nAverage Sale: “ + decAverageSale.ToString(“C”);MessageBox.Show(strMessage, “Sales”,

MessageBoxButtons.OK);


Displaying Multiple Buttons

• Use the MessageBoxConstants to choose buttons to display on the message box

• The Show method returns a DialogResult object

• Click on Show and press F1 for list of return types, or point to Show and pause


Declaring an Object Variable for the Method Return

• Declare a variable to hold an instance of the DialogResult type

• Assign the return value of the Show method to the new variable

• Check the value of the return and compare to the DialogResult constants


Specifying a Default Button and Options

• Specify which button is the default (the accept button) with the MessageBoxDefaultButton argument

• Make a message appear right-aligned with the MessageBoxOptions argument


Input Validation

• It is better to find and reject bad data early in a program

• Validation is checking to verify that appropriate values have been provided as input– Make sure data are numeric– Check for specific values– Check a range of values– Make sure required items are entered


Checking for a Range of Values

• Check the reasonableness of a value

• Example:

if (int.Parse(hoursTextBox.Text) > 10)

MessageBox.Show(“Too many hours”,”Invalid Data”, MessageBoxButtons.OK);


Checking for a Required Field

• Compare a text box value to an empty string literal

• Check separately for blank or nonnumeric data

• Check for blanks first


Performing Multiple Validations

• Use a nested if statement to check several input boxes

• This will check the second box only if the first passes

• You can exit the validation processing as soon as a problem is found with a single field


The Case Structure

• Use the case structure to test a single variable for multiple values

• In C#, code the case structure with the switch statement

• Decisions coded with a switch statement can also be coded with nested if statements although the switch statement is simpler and clearer


Switch Statement – General Form

switch (expression){

case ConstantList:[Statement(s);break;]

[case ConstantList:Statement(s);break;]

.

.

.[default:]

[Statement(s);break;]

}


switch Statement cont.

• The expression in a case structure is a variable or a property to be tested

• The constant list is the value you want to match, and may be a numeric or string constant

• There is no limit to the number of statements following a case statement

• A break must appear after all of the statements for a case statement


The switch Statement - Exampleswitch (intListIndex)

{

case 0:

HandleItemZero();

break;

case 1:

HandleItems();

break;

default:

HandleNoSelection();

break;

}


The switch Statement cont.

• To test for a string value, include quotation marks around the literals

• Capitalization in strings must match exactly• The default clause is optional although

recommended• If no default clause and no case conditions are

true, program continues execution at statement following the switch block

• If more than one case value matches expression, only the first matching case clause executes


Sharing an Event Handler

• C# allows sharing of an event-handling method for several controls

• To share an event-handling method among several radio buttons:

1. Double-click on one radio button to create event handler for that button

2. Select another radio button and click on the Events button in Properties window

3. Select an existing method from the drop-down list


Sharing an Event Handler cont.

• Setup a class-level variable to hold user selection

• Use the sender argument that is passed to the CheckedChanged method

• Cast sender to the object type then use its Name property in a switch statement


Sharing an Event Handler - ExampleColor colorNew; //Declare class-level variableprivate void blueRadioButton_CheckedChanged(object sender, System.EventArgs e){

RadioButton selectedRadioButton = (radioButton) sender;switch (selectedRadioButton.Name){

case “blueRadioButton”:colorNew = Color.Blue;break;

case “redRadioButton”:colorNew = Color.Red;break;

}}private void okButton_Click(object sender, System.EventArgs e){

this.BackColor = colorNew; //Change the color based on selected radio button}


Calling Event Handlers

• Write instructions once, in a method, then call that method from another method

• When you call a method, the entire method is executed then execution returns to the statement following the call

• General form is MethodName();• Must include the parentheses, even if they

are empty


Calling Event Handlers cont.

• Every event handler requires same two arguments (sender, e)

private void summaryButton_Click(object sender, System.EventArgs e)

{

// . . .

newOrderButton_Click(sender, e); //Call the newOrderButton_Click

//event-handling method

}


Your Hands-On Programming Example

Create a project for R ‘n R – for Reading ‘n Refreshment that calculates the amount due for individual orders and maintains accumulated totals for a summary. Have a check box for takeout items, which are taxable at 8 percent; all other orders are nontaxable. Include radio buttons for the five coffee selections – Cappuccino, Espresso, Latte, Iced cappuccino, and Iced Latte. The prices for each will be assigned as constants.


Your Hand-On Programming Example cont.

Use a button for Calculate Selection, which will calculate and display the amount due for each item. Display appropriate error messages for missing or nonnumeric data.

A button for Clear for Next Item will clear the selections and amount for the current item. Additional labels in a separate group box will display the summary information for the current order, including subtotal, tax, and total.


Your Hand-On Programming Example cont.

Buttons at the bottom of the form will be used for New Order, Summary, and Exit. The New Order button will confirm that the user wants to clear the current order. If the user agrees, clear the current order and add to the summary totals. The Summary button should display a message box with the number of order, the total dollar amount, and the average sale amount per order.


Debugging C# Projects

• Visual Studio .NET provides many debugging tools to find and eliminate logic and run-time errors

• Follow program logic in Break mode by single-stepping through code

• Use the WriteLine method of the Console class

• Debugging tools are found on the Debug toolbar and Debug menu


Writing to the Output Window

• Use the Console.WriteLine method in code• The Console.WriteLine argument specifies a

message to write or an object to track• General form

Console.WriteLine(TextString);Console.WriteLine(Object);

• Output of Console.WriteLine method appears in Output window

• Using Console.WriteLine does not have to break program execution


Pausing Execution with a Break

• Click on the Break button to pause execution• Insert a breakpoint in code

– Place mouse pointer in gray margin indicator area at left edge of Editor window and click

– Line will be highlighted in red and large red dot will display in margin indicator

– Start execution; When program reaches breakpoint, it will halt and go into break time

– Remove breakpoint by clicking on red dot in gray margin area or using Debug menu to clear all break points


Checking the Current Values of Expressions

• During break time– Point to name of expression or highlighted area in the

Editor window

• During run time– Break the execution

– Display code in the Editor

– Point to the variable or expression to view

• Current contents of expression appear in pop up label


Stepping through Code

• Step through code at break time

• Stepping commands– Step Into– Step Over– Step Out

• Stepping commands for project execute one line at a time and display the Editor window with the current statement highlighted


Step Into• When selected, the next line of code

executes and the program pauses again in break time

• To continue stepping through program execution, continue choosing Step Into command

• Code will step into any called methods• Choose the Continue command to continue

execution without stepping


Step Over and Step Out

• Step Over executes code one line at a time in break time, similar to Step Into

• Step Over does not display lines of code in any called methods

• Step Out continues rapid execution until called method completes, then returns to break mode at statement following the Call


Continuing and Stopping Program Execution

• Press F5 or choose Continue from Debug toolbar or menu to continue rapid execution of program

• To restart program execution from beginning, choose Restart command

• Stop execution by pressing Shift + F5, or by selecting Stop Debugging from the Debug toolbar or menu


The Locals and Autos Windows

• The Locals window displays all objects and variables within scope at break time

• The Autos window “automatically” displays all variables and control contents that are referenced in the current statement and the three statements on either side of the current one


Debugging Step-by-Step Tutorial

In this exercise you will learn to set a breakpoint; pause program execution; single step through program instructions; display the current values in properties, variables, and conditions; and debug a C# program.1. Test the Project2. Break and Step Program Execution3. View the Contents of Properties, Variables, and Conditions4. Continue Program Execution5. Test the White Total6. Correct the Red Total Error7. Correct the White Total8. Test the Corrections


Summary

• C# uses the if / else statement to make decisions.• Flowcharts can help visualize the logic of an if / else

statement.• The conditions for an if statement are evaluated for

true or false.• Conditions can be composed of the relational

operators, which compare numeric items for equality, greater than, or less than.


Summary cont.

• Conditions can be created using strings and the relational operators = = and != (equal and not equal).

• The ToUpper and ToLower methods of the String class convert a text value to upper- or lowercase.

• The && (and) and || (or) operators may be used to combine multiple conditions.

• A nested if statement contains an if statement within either a true or false action of a previous if statement.


Summary cont.

• The state of radio buttons and check boxes should be tested with if statements in the event handler of a button.

• The MessageBox.Show method can display multiline message with the new line (\n) character.

• You can display multiple buttons on a message box and check selection using DialogResult.

• Data validation checks the reasonableness or appropriateness of the value in a variable or property.


Summary cont.

• You can assign an event-handling method to a control in the Properties window.

• You can use the sender argument in an event-handling method to determine which control caused the method to execute.

• One method can call another method.• A variety of debugging tools are available in Visual

Studio including writing to the Output window, breaking program execution, displaying contents of variables, and stepping through code.

Chapter 4 Decisions and Conditions Programming in C#.NET © 2003 by The McGraw-Hill Companies, Inc....

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