McGraw-Hill © 2010 The McGraw-Hill Companies, Inc. All rights reserved. Chapter 4 Decisions and Conditions.

McGraw-Hill © 2010 The McGraw-Hill Companies, Inc. All rights reserved.

Chapter 4

Decisions and Conditions

McGraw-Hill © 2010 The McGraw-Hill Companies, Inc. All rights reserved. 4-2

Chapter Objectives - 1

• Use if statements to control the flow of logic• Understand and use nested if statements• Read and create action diagrams that illustrate

the logic in a selection process• Evaluate Boolean expressions using the

relational or comparison operators• Combine expressions using logical operators &&

(and), || (or), and ! (not)• Test the Checked property of radio buttons and

check boxes


Chapter Objectives - 2

• Perform validation on numeric fields using if statements

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

for multiple controls• Call an event handler 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 - 1

• A decision made by the computer formed as a question

• If the condition is true– Do one thing

• If the condition is false– Do something else

if the sun is shining (condition)

go to the beach (action to take if condition is true)

else

go to class (action to take if condition is false)


if Statements - 2

• In an if statement– When the condition is true

• Only the statement following the if is executed

– When the condition is false• Only the statement following the else clause, if present, is

executed

• Use braces to include multiple statements in the if or else portion of statement

• Always use braces– Good programming practice


if Statement – General Form

• Condition to test is placed in parentheses• if and else statements do not have semicolons

– A semicolon terminates the statement– Any statement following the semicolon executes

unconditionally• Indent for readability and clarity

if (condition){ // Statement(s)}[else{ // Statement(s)}]


if Statement - Examples

• Type an if statement– Press Enter

• Editor places insertion point on blank line, indented

• Multiple statements must be in braces– Editor places braces directly under if or else statements

unitsDecimal = decimal.Parse(unitsTextBox.Text);if (unitsDecimal < 32m){ freshmanRadioButton.Checked = true;}else{ freshmanRadioButton.Checked = false;}


Charting if Statements

• A Uniform Modeling Language (UML) activity diagram is a useful tool for showing the logic of an if statement

• Helps to organize thoughts and design projects more quickly

• UML includes several types of diagrams– Activity diagram is a visual planning tool for decisions and

actions for an entire application or a single method


Boolean Expressions - 1

• The test in an if statement is based on a Boolean expression (condition)

• Use relational (comparison) operators– Comparison is evaluated to either true or false

• Formed with variables, constants, object properties and arithmetic expressions

• Must be made on like data types– i.e. Strings to strings or numeric value to

numeric value


Boolean Expressions - 2

• The Relational Operators

Symbol Relation tested Examples

> greater thandecimal.Parse(amountTextBox.Text > limitDecimal correctInteger > 75

< less thanint.Parse(salesTextBox.Text) < 10000saleDecimal < limitDecimal

== equal topasswordTextBox.Text == “101”nameTextBox.Text == nameString

!= not equal tofreshmanRadioButton.Checked != truenameTextBox.Text != “”

>=greater than or equal to

int.Parse(quantityTextBox.Text) >= 500

<= less than or equal to countInteger <= maximumInteger


Comparing Numeric Variablesand Constants

• Algebraic comparisons are used– Sign of the number is taken into account

–2 is less than –1

• Use two equal signs to test for equality (==)if (decimal.Parse(priceTextBox.Text) == maximumDecimal)

• A single equal sign (=) means replacement in an assignment statement– Generates a compiler error if used in a

comparison


Comparing Character Data - 1

• Use relational operators to compare data stored in char data type– C# stores characters using 16-bit Unicode– Latin alphabet, numerals, punctuation have

same values in ANSI code as Unicode– ANSI has established order (collating

sequence)

• A char variable can be compared to– A literal, another char variable, an escape

sequence or the code number


Comparing Character Data - 2

• Compare char variable to a literal– Enclose char literal in single or double quotes– Compares to the value of the literal– Without quotes compares to code number

(from ANSI code chart)(sexChar == ‘F’) //Compare to the uppercase letter ‘F’.

(codeChar != ‘\0’) //Compare to a null character (character 0).

(codeChar == ‘9’) //Compare to the digit 9.

(codeChar == 9) //Compare to the ANSI code 9 (Tab char).


Comparing Strings

• Compare strings with equal to (==) and not equal to (!=) operators

• Comparison starts at leftmost character, proceeds one character at a time

• As soon as one character is not equal to the corresponding character– Comparison is terminated– Boolean expression returns false


The CompareTo Method

• Tests for less than or greater than aString.CompareTo(bString)

– Returns zero if the strings are equal

– Returns a positive number if aString is greater than bString

– Returns a negative number if bString is greater than aString

if (aString.CompareTo(bString) > 0) //Is aString > than bString?


Testing for True or False

• Shortcuts when testing for true or false

• C# evaluates expression in if statement

• If condition is a Boolean variable or property, it holds true or false values

if (blueRadioButton.Checked == true)

is equivalent to

if (blueRadioButton.Checked)


Comparing Uppercase andLowercase Characters

• String comparisons are case sensitive– Uppercase “Y" is not equal to lowercase “y"

• User input may be uppercase, lowercase, or a combination

• To compare uppercase and lowercase characters– Convert user input to all uppercase or all lowercase

• Use ToUpper and ToLower methods of the string class to convert

• Compare to the correct case of the literalif (nameTextBox.Text.ToUpper() ==

"PROGRAMMING"){

// Do something.}


Compound Boolean Expressions

• Test more than one condition by joining conditions with logical operators

Logical Operator

Meaning Example

|| (or)If one expression or both expressions are true, the entire expression is true

int.Parse(numberLabel.Text) == 1 || int.Parse(numberLabel.Text) == 2

&& (and)

Both expressions must be true for the entire expression to be true

int.Parse(numberTextBox.Text > 0 && int.Parse(numberTextBox.Text) < 10

! (not)

Reverses the Boolean expression so that a true expression will evaluate false and vice versa

! foundBool


Combining Logical Operators

• When combining logical operators– Can use multiple && and || operators– If both && and || are used, && is evaluated

before || – Change the order of evaluation with

parenthesesif (saleDecimal > 1000.0m ||

discountRadioButton.Checked && stateTextBox.Text.ToUpper() != "CA")

{//Code here to calculate the discount.

}


Short-Circuit Operations

• When evaluating a compound Boolean expression, the second condition may short-circuit (not be evaluated)– There is no need to evaluate the second expression if

• the first expression has an || and evaluates true• the first expression has an && and evaluates false

• To force the second comparison to be made, use a single comparison operator:– & (and) – | (or)


Nested if Statements

• An if statement that contains additional if statements

• Nest ifs in the true block• Nest ifs in the else block

– Code is simpler when else if is used

• if statements can be nested as deeply as desired– Projects are difficult to follow and may not perform

correctly when ifs are too deeply nested


Coding an else if

• To nest in the else statement, use else if (on one line), or as two separate statements

if (tempInteger <= 32){ commentLabel.Text = "Freezing";}else if (tempInteger > 80){ commentLabel.Text = "Hot";}else{ commentLabel.Text = "Moderate";}

if (tempInteger <= 32){ commentLabel.Text = "Freezing";}else{ if (tempInteger > 80) { commentLabel.Text = "Hot"; } else { commentLabel.Text = "Moderate"; }}Note that the indentation changes

when else if is on one line.


Using if Statements with Radio Buttons and Check Boxes

• Use if statements instead of taking action in CheckedChanged event handlers

• Use if statements to determine which options are selected

• Make programs consistent with standard Windows applications– Place code in Click event handlers of buttons

• i.e. OK or Apply button


Checking the State of a Radio Button Group

• To check the state of multiple radio buttons, use a nested if in a button’s click event handler

if (freshmanRadioButton.Checked) { freshmanCountInteger++; } else if (sophmoreRadioButton.Checked) { sophomoreCountInteger++; } else if (juniorRadioButton.Checked) { juniorCountInteger++; } else if (seniorRadioButton.Checked) { seniorCountInteger; }

Only one radio button can be selected at a time.


Changing and TestingRadio Buttons

• When one button’s checked property is true, all others are automatically set to false– Use this fact to easily clear a set of radio buttons

• Add an extra button to the group– Set Visible property to false– To clear visible buttons, set Checked property of the

invisible radio button to true– Use to verify through code that user has made a

selection• Test Checked property of invisible radio button


Checking the State ofMultiple Check Boxes

• To test the state of multiple check boxes, use separate if statements– Any number of boxes can be selected

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

//Calculate the delivery charges.{


Enhancing Message Boxes

• Control the format of the message

• Display multiple buttons– Check to see which button the user clicked– Perform alternate actions depending on the

user’s selection


Displaying the Message String

• Message string displayed in a message box can be– A string literal (enclosed in quotes)– A string variable

• Create a variable for the message• Format the message• Call the Show method

– Above three steps make code easier to read and follow


Combining Values into aMessage String

• Concatenate a literal and the value from a variable– Include an extra space inside the literal to

separate the variable’s value from the literal

string messageString = “Total Sales: ” + totalDecimalSales.ToString(“C”);

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


Creating Multiple Lines of Output - 1

• Long messages wrap to a second line

• Insert a NewLine (\n) character in the string message

string formattedTotalString = totalSalesDecimal.ToString("N");string formattedAvgString = averageSaleDecimal.ToString("N");string messageString = "Total Sales: " + formattedTotalString + "\n" + "Average Sale: " + formattedAvgString;MessageBox.Show(messageString, "Sales Summary", MessageBoxButtons.OK);


Creating Multiple Lines of Output - 2

• Combine multiple NewLine constants for double spaced or multiple line messages

// Concatenate the text for the messagestring summaryString = "Drinks Sold: " + drinksInteger.ToString() + "\n\n" + "Number of Orders: " + ordersInteger.ToString() + “n\n" + "Total Sales: " + totalSalesDecimal.ToString("C");// Display the message box.MessageBox.Show(summaryString, "Juice Bar Sales Summary", MessageBoxButtons.OK, MessageBoxIcon.Information);


Using the CharacterEscape Sequences

• Other constants are available from the character escape sequence (\) list

Escape Sequence Description

\' Includes a single quote in a character literal

\" Includes a double quote in a string literal

\\ Includes a backslash in a string literal\n New line\r Carriage return\b Backspace character\t Horizontal tab


Displaying Multiple Buttons

• Choose buttons to display in a message box using the MessageBoxButtons constants– Returns a DialogResult object


Declaring an Object Variablefor the Method Return - 1

• Capture information about the Show method’s outcome– Declare a variable to hold an instance of the

DialogResult typeDialogResult whichButtonDialogResult;

– Assign the return value of the Show method to the new variablestring messageString = "Clear the current order figures?";

whichButtonDialogResult = MessageBox.Show(messageString, "Clear Order", MessageBoxButtons.YesNo, MessageBoxIcon.Question);


Declaring an Object Variablefor the Method Return - 2

– Check the value of the return– Compare to the DialogResult constants

• Yes, No, Ok, Retry, Abort and Cancel

if (whichButtonDialogResult == DialogResult.Yes)

{

// Code to clear the order.

}


Specifying a Default Button and Options

• Two additional signatures for the MessageBox.Show method– Display multiple buttons and make one the default (the

Accept button)– Right-Align text by setting MessageBoxOptions

argument

// Make the No button (Button2) the default button // and right-align the message.

whichButtonDialogResult = MessageBox.Show(messageString, "Clear Order", MessageBoxButtons.YesNo, MessageBoxIcon.Question, MessageBoxDefaultButton.Button2, MessageBoxOptions.RightAlign);


Input Validation

• Validation – Checking to verify appropriate values have been input

• Validation may verify:– Input is numeric– A specific value– A range of values– Required items are entered


Checking for a Range of Values

• Check reasonableness of a value– Example – A company does not allow more

than 10 hours to be worked in a single dayif (int.Parse(hoursTextBox.Text) > 10){ MessageBox.Show("Too many hours.", "Invalid

Data", MessageBoxButtons.OK);

}


Checking for a Required Field - 1

• Compare a text box value to an empty string

if (nameTextbox.Text != “”)

• Test in an if statement, invalid entries caught in true portion

if (nameTextBox.Text == “”){

MessageBox.Show(“Required entry.”, “Invalid Data”, MessageBoxButtons.OK);

}else{

//Good data - - Perform some action.}


Checking for a Required Field - 2

• Check separately for blank or nonnumeric data– Display a

meaningful message to the user

– Check for blanks first, a blank field will throw an exception with a parsing method

if (quantityTextBox.Text != "") // Not blank.{ try quantityDecimal = decimal.Parse(quantityTextBox.Text); catch // Nonnumeric data. { messageString = "Nonnumeric data entered for quantity."; MessageBox.Show(messageString, "Data Entry Error"); }}else // Missing data.{ messageString = "The quantity is required."; MessageBox.Show(messageString, "Data entry error");}


Performing Multiple Validations - 1

• Avoid displaying multiple message boxes in a row

• Use a nested if statement

• Check second value only if first value passes test

• Exit processing if a problem is found with any field


Performing Multiple Validations - 2

if (nameTextBox.Text != ""){ try { unitsDecimal = decimal.Parse(unitsTextBox.Text); if (freshmanRadioButton.Checked || sophomoreRadioButton.Checked || juniorRadioButton.Checked || seniorRadioButton.Checked) { // Data valid - - Do calculations or processing here. } else { MessageBox.Show(“Please select grade level.”, “Data Entry Error”, MessageBoxButtons.OK); } } catch(FormatException) { //Display error message }}


The switch Statement - 1

• Provides flexible and powerful alternative to if statements for testing multiple conditions

switch (expression){ case testValue1:

[statement(s);break;]

[ case testValue2:statements(s);break;]

[default:]statements(s);break;]

}


The switch Statement - 2

• The test values must be the same data type as the expression

• There is no limit to the number of case blocks and no limit to the number of statements following each case statement

• default clause is optional, code executes only if no other case is matched

• The break statements for each case, including the default, are required

• Only the statements in the first matched case execute


Sharing an Event Handler - 1

• Multiple controls can share an event-handling method• Code an event-handling method

Click the Events button in the Properties window

Drop down the list for an event and select the previously written method



• Radio button example– Write code in CheckedChanged event of one button

of radio button group– Rename the method to reflect its purpose

• Right-click on method name in Code Editor and select Refractor/Rename

– Set the method for every radio button in the group– Declare a class-level variable to hold selection– In OK button event-handling method, take action

based on selection



• Radio button example (cont.)– sender argument is passed to CheckChanged

method, defined as a generic object– Cast sender to a RadioButton type to use its

Name property– Use a switch statement to determine which

radio button is selected


Calling Event Handlers

• Never duplicate code• Write code once in an event-handling method

– “Call” the method from another method by naming the method

• Entire method is executed• Execution returns to statement following the call

– Include parentheses, leave empty if no arguments– If method requires arguments, place arguments within

parentheses


Debugging C# Projects

• Debugging tools help find and eliminate logic and run-time errors

• Follow program logic in Debug mode– Single-step through code– WriteLine method of Console class

• Debug toolbar and Debug menu contain debugging tools


Writing to the Output Window

• Place Console.WriteLine methods in code to display useful information without breaking program executionConsole.WriteLine(“calculateButton method entered.”);Console.WriteLine(quantityTextBox);Console.WriteLine(“quantityInteger = ” + quantityInteger);


Pausing Execution with theBreak All Button

• Click the Break All button on the Debug toolbar– Project enters Debugging mode at the current

line

• Better to choose location of break– Force a break with a breakpoint


Forcing a Break - 1

• Place mouse pointer in gray margin at left edge of the Editor window on a line of executable code– Click in margin and line is highlighted in red

and a large red dot displays in margin

• Start execution, project will halt at breakpoint and go into debug mode

• Remove breakpoint by clicking on red dot or clear all breakpoints from the Debug menu


Forcing a Break - 2

• Toggle breakpoints on/off by clicking in Editor’s gray left margin area

Breakpoint


Checking the CurrentValues of Expressions

• Quickly check the current value of a variable, a control, a Boolean expression, or an arithmetic expression– Break execution using a breakpoint– Point to the name of the expression– Current contents of expression pop up in a small label

called a DataTip, when the expression is in scope


Stepping through Code - 1

• The best way to debug a project is to trace program execution line by line– Break execution with a breakpoint or choose a

stepping command• Step Into (F11) – Executes next line of code, continue

stepping through code by repeatedly choosing Step Into command

• Step Over (F10) – Executes next line of code, displays only the lines of code in the current method being analyzed

• Step Out (Shift + F11) – Rapidly executes a called method, returns to debug mode at the statement following the call



• Continue Program Execution– Press F5 or choose Continue from the Debug

toolbar or menu– Choose Restart to restart execution from

beginning

• Stopping Execution– Press Shift + F5 or select Stop Debugging

from the Debug toolbar or menu



• Edit and Continue– Stop execution, make minor modifications to

the code, and continue execution without recompiling

– If changes are too major, the debugger does not allow the changes

• Stop program execution, make the changes and recompile the program


The Locals Window

• Displays all objects and variables within scope at debug time

• Display Locals window from the toolbar or the Debug/Windows/Locals menu

• Expand the "this" entry to see state of form’s controls and the values of class-level variables


The Autos Window

• Automatically displays all variables and control contents referenced in current statement– Also displays a few statements on either side of the

current statement

• Autos window available when program stops at a breakpoint

Select Locals or Autos tab in the debugging window

McGraw-Hill © 2010 The McGraw-Hill Companies, Inc. All rights reserved. Chapter 4 Decisions and Conditions.

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