Module3

SELECTIONSTRUCTURES

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MotivationIntheprogramswehavewrittenthusfar,statementsareexecutedoneaftertheother,intheorderinwhichtheyappear.

Programsoftenneedmorethanonepath ofexecution,however.Manyalgorithmsrequireaprogramtoexecutesomestatementsonlyundercertaincircumstances.Thiscanbeaccomplishedwithdecisionstructures.

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Topics

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1. FlowofControl2. Boolean definition &ConditionalStatements3. RelationalOperators4. LogicalOperators5. OperatorPrecedence6. if statements7. if-else statements8. if-else-if statements9. switch statements

1.FlowofControlTheorderofstatementexecutioniscalledtheflowofcontrol

Bydefault,executionislinear:onestatementafteranother

However,wecan:

◦ decide whetherornottoexecuteaparticularstatement◦ executeastatementoverandover, repetitively

Theseselection(decision)statementsarebasedonboolean expressions(orconditions)thatevaluatetotrue orfalse

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AFlowDiagram

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2.TheBoolean• A Boolean resolves toeithertrue or false• You canget Booleanvaluesseveraldifferentways– Simple– Complex

• These Booleans will beused (later) to makedecisions

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2.SelectionStatementsASelection(conditional)statement allowsustochoosewhichstatementswillbeexecutednext:

◦ if – blockofcodeexecutesifaBooleanexpressionistrue.

◦ if-else – willexecuteoneblockofcodeiftheBooleanexpressionistrue,oranotherifitisfalse.

◦ if-else-if – testsaseriesofBooleanexpressionsandexecutecorrespondingblockwhenitfindsonethatistrue.

◦ switch – letsthevalueofavariabledeterminewheretheprogramwillbranchto.

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3.RelationalOperatorsCreate a true or false using a relational operator:

> greater than

< less than

== equals

! not

!= not equal

>= greater than or equal

<= less than or equal

Note the difference between the equality operator (==) and the assignment operator (=)

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RelationalOperatorsExampleLiteralExample:5 > 3 // true6 != 6 // falsetrue == (4 <=2) // false‘c’ != ‘b’ // true!false // true

VariableExample:Num1 ← 5Num2 ← 7Result ← Num2 > Num1 // Result is now true

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4.LogicalOperatorsBooleanexpressionscanalsousethefollowinglogicaloperators:◦ LogicalNOT◦ LogicalAND◦ LogicalOR◦ ExclusiveOR

TheyalltakeBooleanoperandsandproduceBooleanresults

LogicalNOTisaunaryoperator(itoperatesononeoperand)

LogicalANDand ORarebinaryoperators(eachoperatesontwooperands)

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LogicalOperators(NOT)ThelogicalNOT operationisalsocalledlogicalnegation orlogicalcomplement

IfsomeBooleanconditiona istrue,thenNOT a isfalse;ifa isfalse,thenNOT a istrue

Logicalexpressionscanbeshownusingatruthtable

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LogicalOperators(ANDandOR)ThelogicalAND expression

a AND bistrue ifbotha andb aretrue,andfalse otherwise

Thelogical OR expression

a OR bistrue ifeithera orb oristrue,orbotharetrue,andfalseotherwise

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LogicalOperators

LogicalOperatorsinBooleanExpressionsExpressionscanformcomplexconditions

IF (total < MAX + 5 AND NOT found) THENPrint ("Processing…")

ENDIF

Mathematicaloperatorshavehigherprecedence thantheRelationalandLogicaloperators

Relationaloperatorshavehigherprecedence thanLogicaloperators

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BooleanExpressionsSpecificexpressionscanbeevaluatedusingtruthtables

GivenX = total < MAX + 5 AND NOT found

WhatisthevaluesofX ?

5.OrderofPrecedence ofArithmetic, Comparison, andLogicalOperators

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Source: http://www.bouraspage.com/repository/algorithmic-thinking/what-is-the-order-of-precedence-of-arithmetic-comparison-and-logical-operators

5.OrderofPrecedence ofArithmetic, Comparison, andLogicalOperators

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Source: http://www.bouraspage.com/repository/algorithmic-thinking/what-is-the-order-of-precedence-of-arithmetic-comparison-and-logical-operators

OperatorPrecedenceApplyingoperatorprecedenceandassociativityruletotheexpression:

3 + 4 * 4 > 5 * (4 + 3) - 1

(1) Insideparenthesesfirst(2) Multiplications(3) Addition(4) Subtraction(5) Greaterthan

TheexpressionresolvestoFALSE

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NotesonIndentationInJavaandC#

Makesnodifference,butcrucialforreadabilityanddebugging.

CodeinsidetheIF statement isindentedandshouldalsobeenclosed incurlybraces{}.

InPython

IndentationmattersinPython!

Leadingwhitespaceatthebeginningofalogicalline isusedtodeterminethegroupingofstatements.

Nocurlybracesrequired.

Note! Codesamples inthisslideshowmayhavetobere-formattedifcopiedandpasted.

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6.LogicofanIFStatement

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NowLet’sDoSomething!• Using the IF statement, we can decide

whether or not to execute some code

• Has format:

IF (condition) THEN// all the code that’s here will only execute// IF and only IF the condition above is true

ENDIF

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ProblemStatement

WriteaprogramtoconvertuserinputofCelsiustemperaturetoFahrenheit.Displaytheconvertedtemperaturetotheuser.IssueaheatwarningIF

temperatureisover90degreesFahrenheit.

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Pseudocode - IFStatementMAINFahrenheit←0,Celsius←0

PRINT“EnterCelsiustemperature:”

READuserinput

Celsius←userinput

Fahrenheit←9.0/5.0*Celsius+32

PRINTFahrenheit

IF (Fahrenheit>=90)THENPRINT“heatwarning”

ENDIF

ENDMAIN

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JavaExample - if Statement

import java.util.Scanner;public class CelsiusToFahrenheit {

public static void main (String[] args){

double celsius= 0.0, fahrenheit = 0.0;Scanner scan = new Scanner (System.in);System.out.print (“What is the Celsius temperature? ");celsius = scan.nextDouble();fahrenheit = 9.0 / 5.0 * celsius + 32;System.out.println (“The temperature is " + fahrenheit +

“ degrees Fahrenheit”);if (fahrenheit >= 90) {

System.out.println(“It is really hot out there!”);}

}}

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C#Example- ifStatementusing System;class Program{

public static void Main(string[] args){

double celsius = 0.0, fahrenheit = 0.0;Console.Write("What is the Celsius temperature? ");celsius = Convert.ToDouble(Console.ReadLine());fahrenheit = 9.0 / 5.0 * celsius + 32;Console.WriteLine("The temperature is " + fahrenheit

+ " degrees Fahrenheit");if (fahrenheit >= 90){

Console.WriteLine("It is really hot out there!");}

}}

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PythonExample– if Statementdef main():

celsius = float(input("What is the Celsius temperature? "))

fahrenheit = 9.0 / 5.0 * celsius + 32.0

print ("The temperature is", fahrenheit, "degrees Fahrenheit.")

if (fahrenheit >= 90):

print ("It's really hot out there!")

main()

# From: mcsp.wartburg.edu/zelle/python/ppics1/slides/Chapter07.ppt

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7.StructureofanIF-ELSEStatementIF hasacounterpart– theELSE statement

Hasformat:IF (condition) THEN

statementBlock that executes if condition is trueELSE

statementBlock that executes if condition is falseENDIF

IFtheconditionistrue,statementBlock1 isexecuted;IFthecondition isfalse,statementBlock2 isexecuted

NoticethereisnoconditionaftertheELSEstatement.

Oneortheotherwillbeexecuted,butnotboth

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7.LogicofanIF-ELSEstatement

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ProblemRedefined

OriginalProblemStatement:WriteaprogramtoconvertuserinputofCelsiustemperaturetoFahrenheit.Displaytheconvertedtemperaturetotheuser.IssueaheatwarningIFtemperatureisover90degreesFahrenheit.

Add:IFthetemperatureisnotthathigh,outputamessagetotheuser.

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Pseudocode – IF-ELSEStatementMAIN

Fahrenheit ← 0, Celsius ← 0

PRINT “Enter Celsius temperature: ”

READ user input

Celsius ← user input

Fahrenheit ← 9.0 / 5.0 * Celsius + 32PRINT Fahrenheit

IF (Fahrenheit > = 90) THENPRINT “heat warning”

ELSE

PRINT “there is no extreme heat”

ENDIF

END MAIN

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JavaExample– if-else(codesnippet)double celsius= 0.0, fahrenheit = 0.0;Scanner scan = new Scanner (System.in);System.out.print (“What is the Celsius temperature? ");celsius = scan.nextDouble();fahrenheit = 9.0 / 5.0 * celsius + 32;System.out.println (“The temperature is " + fahrenheit + “degrees Fahrenheit”);

if(fahrenheit >= 90) {System.out.println(“It is really hot out there!”);

}else{

System.out.println(“There is no extreme heat today.”);}

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C#Example– if-else(CodeSnippet)double celsius = 0.0, fahrenheit = 0.0;Console.Write("What is the Celsius temperature? ");celsius = Convert.ToDouble(Console.ReadLine());fahrenheit = 9.0 / 5.0 * celsius + 32;Console.WriteLine("The temperature is " + fahrenheit

+ " degrees Fahrenheit");

if (fahrenheit >= 90){

Console.WriteLine("It is really hot out there, be careful!");}else{

Console.WriteLine(“There is no extreme heat today.”);}

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Pythonif-else examplecelsius = float(input("What is the Celsius temperature? "))

fahrenheit = 9.0 / 5.0 * Celsius + 32

print ("The temperature is", fahrenheit, "degrees Fahrenheit.”)

if (fahrenheit >= 90):

print (“It's really hot out there, be careful!”)

else:

print (“There is no extreme heat today.”)

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8.StructureofanIF-ELSE-IF• Selecting one frommany• Assoon as one is true, the rest are not considered• Has format:

IF (condition) THEN//statementBlock that executes if the above boolean is true

ELSE IF (condition) THEN//statementBlock that executes if the above boolean is true

ELSE IF (condition) THEN//statementBlock that executes if the above boolean is true

ELSE//statementBlock that executes if the nothing matched above

ENDIF

Note: only one set of statements will ever execute. Trailing else is optional

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8.LogicofanIF-ELSE-IF

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ProblemRedefined

OriginalProblemStatement:WriteaprogramtoconvertuserinputofCelsiustemperaturetoFahrenheit.Displaytheconvertedtemperaturetotheuser.Issueaheatwarningiftemperatureisover90degreesFahrenheit.

Add:twoadditionalmessagestorecommendanactionbasedontemperature,andatrailingelse.

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Pseudocode – IF-ELSE-IFMAIN

Fahrenheit ← 0, Celsius ← 0PRINT “Enter Celsius temperature: ”READ user inputCelsius ← user inputFahrenheit ← 9.0 / 5.0 * Celsius + 32PRINT Fahrenheit

IF (Fahrenheit > = 90) THENPRINT “heat warning”

ELSE IF (Fahrenheit >= 80) THENPRINT “it is warm, but there is no extreme heat”

ELSE IF (Fahrenheit >= 70) THENPRINT “the temperature is pleasant and suggest a picnic”

ELSEPRINT “a suggestion to take a jacket”

END IFEND MAIN

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JavaExample– if-else-ifdouble celsius= 0.0, fahrenheit = 0.0;Scanner scan = new Scanner (System.in);

System.out.print (“What is the Celsius temperature? ");celsius = scan.nextDouble();fahrenheit = 9.0 / 5.0 * celsius + 32;System.out.println (“The temperature is " + fahrenheit +

“ degrees Fahrenheit”);

if (fahrenheit >= 90){System.out.println(“It is really hot out there!”);

}else if (fahrenheit >= 80){

System.out.println(“It is very warm...”);}// CONTINUED ON NEXT SLIDE

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JavaExample– if-else-if (con’t)else if (fahrenheit >= 70){

System.out.println(“It is very pleasant today!”)}else{

System.out.println(“It is cool today. Take a jacket.”);}

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C#Example– if-else-ifdouble celsius = 0.0, fahrenheit = 0.0;Console.Write("What is the Celsius temperature? ");celsius = Convert.ToDouble(Console.ReadLine());fahrenheit = 9.0 / 5.0 * celsius + 32;Console.WriteLine("The temperature is " + fahrenheit + "degrees Fahrenheit");

if(fahrenheit >= 90){Console.WriteLine(“It is really hot!”);

}else if (fahrenheit >= 80){

Console.WriteLine(“It is very warm!”);}else if (fahrenheit >= 70){

Console.WriteLine(“It is very pleasant!”)}else {

Console.WriteLine(“It is cool today”);}

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Pythonif-else-if examplecelsius = float(input("What is the Celsius temperature? "))

fahrenheit = 9.0 / 5.0 * celsius + 32

print ("The temperature is", fahrenheit, " degrees Fahrenheit.")

if (fahrenheit >= 90):

print ("It's really hot out there, be careful!")

elif (fahrenheit >= 80):

print ("It is warm, but there is no extreme heat today.")

elif (fahrenheit >= 70):print ("It is very pleasant today. You should pack a picnic!")

else:print ("It is cool today. You should take a jacket.")

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9.Theswitch StatementTheswitchstatement providesanotherwaytodecidewhichstatementtoexecutenext

Theswitchstatementevaluatesanexpression,thenattemptstomatchtheresulttooneofseveralpossiblecases(cases)

Eachcasecontainsavalueandalistofstatements

Theflowofcontroltransferstostatementassociatedwiththefirstcasevaluethatmatches

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9.StructureofaswitchThegeneralsyntaxofaswitch statementis:SWITCH variableBEGIN

CASE 1:statementBlockbreak // SUPER IMPORTANT BREAKS!

CASE 2:statementBlockbreak // If we don’t include breaks, it goes to next condition

CASE 3:statementBlockbreak

CASE ...

DEFAULT:statementBlock

END

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7.Logicofswitch statement

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break StatementOftenabreak statementisusedasthelaststatementineachcase'sstatementlist

Abreak statementcausescontroltotransfertotheendoftheswitchstatement

Ifabreak statementisnotused,theflowofcontrolwillcontinueintothenextcase

Sometimesthismaybeappropriate,butoftennot…

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DefaultCaseAswitch statementcanhaveanoptionaldefaultcase

Thedefaultcasehasnoassociatedvalueandsimplyusesthereservedworddefault

Ifthedefaultcaseispresent,controlwilltransfertothedefaultcaseifnoothercasevaluematches

Ifthereisnodefaultcase,andnoothervaluematches,controlfallsthroughtothestatementaftertheswitchstatement

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Switch StatementExpressionTheexpressionofaswitch statementmustresultinanintegertype(byte,short,int,long)orachar type.

Itcannotbeaboolean valueorafloatingpointvalue(floatordouble)

Youcannotperformrelationalcheckswithaswitch statement

InJavaandC#,youcanalsoswitchonastring

InPython,it’sabitcontrived!

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ProblemRedefined

OriginalProblemStatement:WriteaprogramtoconvertuserinputofCelsiustemperaturetoFahrenheit.Displaytheconvertedtemperaturetotheuser.

Add:UsingtheFahrenheittemperatureanddivision,determinetheconditionsoutside(i.e.,inthe100’s,90’s,etc.).Useaswitchstatementtorecommenduseractionbasedontheresult.Ifitisinthe60’sorbelow,suggesttheusertakeajacket.

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Pseudocode – switchStatement

// Read user input like before

conditions ← compute using Fahrenheit variable / 10

SWITCH variable CASE 10 : PRINT “stay inside”, BREAKCASE 9 : PRINT “be careful due to heat”, BREAKCASE 8 : PRINT “it is hot, but not extreme”, BREAKCASE 7 : PRINT “pack a picnic”, BREAK

DEFAULT : PRINT “take a jacket”, BREAK

ENDCASE

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Javaswitch Exampleint condition = (int)fahrenheit / 10;System.out.println("It is in the " + condition + ”0's.");

switch (condition){

case 10:System.out.println("Stay inside!");break;

case 9:System.out.println("It is really hot out there!");break;

case 8:System.out.println("It’s warm, but no extreme heat!");break;

case 7:System.out.println("It is very pleasant today!");break;

default:System.out.println("Take a jacket!");break;

}

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C#switch Exampleint conditions = (int)fahrenheit / 10;Console.WriteLine("It is in the " + conditions + "0's.");

switch (conditions){

case 10:Console.WriteLine("It’s hot! Stay inside!");break;

case 9:Console.WriteLine("It is really hot out there!");break;

case 8:Console.WriteLine("It is warm, but no extreme heat!");break;

case 7:Console.WriteLine("It is very pleasant today!");break;

default:Console.WriteLine("Take a jacket!");break;

}

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Pythonswitch exampledef case_10_handler():

print('It is too hot to go outside today. Stay inside!')def case_9_handler():

print('It is really hot out there, be careful!')def case_8_handler():

print('It is very warm, but no extreme heat today!')def case_7_handler():

print('It is very pleasant today. You should pack a picnic!')def default_handler():

print('It is cool today. You should take a jacket.')def switch_function(switch):

handler = {10: case_10_handler,9: case_9_handler,8: case_8_handler,7: case_7_handler,}

return handler.get(switch)() #extra parentheses for executing function

#Continued on next slide

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Note: Python does not provide a switch statement, but we can implement with dictionary mapping. At this point, you may or maynot be ready to try this. Remember you can accomplish the same tasks with if/elif

Pythonswitch example#Continued from slide 44def main():

celsius = float(input("What is the Celsius temperature? "))fahrenheit = 9.0 / 5.0 * celsius + 32conditions = int(fahrenheit/10)print("It is in the ", conditions,"0's.")switch_function(conditions)

if __name__== "__main__":main()

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In-classProblem

WriteapseudocodeprogramthatreadsinallgradesfromCSE1321lecture(quizzesandtests)andcalculatesafinalgrade.Then,basedonthatfinalgrade,theprogramprintsoutthecorrectlettergradethestudentearned(e.g.90-100

isan‘A’,80-89isa‘B’,andsoon)

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BEGINMAINCREATE/READ studentName //Note:weuseashortcutforthisexampleonlyCREATE/READ quizgrades1-12CREATE/READ testgrades1-4CREATE letterGradeCREATE quizAvg =(quiz1+quiz2+...quiz12)/12CREATE testAvg =(test1+test2+test3+test4)CREATE finalGrade =testAvg *0.80+quizAvg *0.20SWITCH (finalGrade)CASE 90to100

letterGrade =A,BREAKCASE 80to89

letterGrade =B,BREAKCASE 70to79

letterGrade =C,BREAKCASE 60to69

letterGrade =D,BREAKCASE default

letterGrade =F,BREAKENDSWITCHPrintstudentName,finalGrade, letterGradeENDMAIN

Summary• Boolean values can be generatedseveral ways

• Use the if statement to decidewhichpath to take

• Use the else to take the alternate path

• The if-else-if statements allowforselectingonefrommany

• Usetheswitch statementtoallowavariableorexpressiontodetermineprogrampath

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