BBM#101# #Introduc/on#to# Programming#I# Fall$2014,$Lecture$4$

Transcription

1 BBM#101# #Introduc/on#to# Programming#I# Fall$2014,$Lecture$4$ Today#! Condi/onal#Branching#! Logical#Expressions! ifandif<statements! switchstatement! gotostatement AykutErdem,ErkutErdem,FuatAkal 1! 2! Rela/onal#Operators#! Take2#expressionsasoperands! e.g., a$<$3 readsas a$is$less$than$3! Yieldeithertheintvalue0#(false)ortheintvalue1#(true). Operator# Meaning# <=# Less#than#or#equal#to# <# Less#than# ># Greater#than# >=# Greater#than#or#equal#to# ==# Equal#!=# Not#equal# Rela/onal#Operators#(Examples)#! ValidExamples(assumea=1andb=2)! a<b "1(true)! a!=b 1 "0(false)! a+1<=b "1(true)! InvalidExamples! a=<b! a=>b! TheMostConfusedCase( = vs. == )! a#=#b isanassignmentexpression! a#==#b isatestforequality! OneofthemostcommonproblemstheCprogrammerfacesismixing themup 3! 4!

2 Logical#Operators#! Theprecedenceofishigherthan #! Bothoperatorsareoflowerprecedencethanallunary, arithmetcandrelatonaloperators.! i.e.,!#>##>># # #! Expressionsconnectedbyor areevaluatedfromleuto right. Operator# # Meaning# Logical#AND# # Logical#OR#!# Logical#Nega/on# Truth#Table# a# b# a##b# a# #b#!a# zero# zero# 0# 0# 1# zero# nonzero# 0# 1# 1# nonzero# zero# 0# 1# 0# nonzero# nonzero# 1# 1# 0# We had covered this before while studying Boolean Algebra in Lecture 2.! 5! 6! Logical#Operators#(Examples)#! NegaTonExamples!!5 "0!!!5 "1!!(6<7) "0!!6<7 "1!!(3[4) "0 Logical#Operators#(Examples)#! GivendeclaraTons int a = 3, b = 3, c = 3; double x = 0.0, y = 2.5; char ch = g! Expressions!!(a < b) c " 1! ch >= a ch <= z " 1! x a b 3 " 0! a < b x < y " 0! a < b x < y " 1 7! 8!

3 Short#Circuit#Evalua/on#! Fortheexpressionsthatcontaintheor #operands,the expressionprocessstopsassoonastheoutcomeisalready known.! Supposeexpr1iszero.! expr1 expr2 = 0(expr2willnotbeevaluated.)! Supposeexpr1isnonzero.! expr1 expr2 = 1 (expr2willnotbeevaluated.) The#if#Statement#! Syntax if (condition)! IfthecondiTonistrue(nonzero)! thestatementwillbeexecuted.! IfthecondiTonisfalse(0)! thestatementwillnotbeexecuted. 9! 10! The#if#Statement#(Example)#! Supposeaprogramwhichwritesamessageifthestudent passestheclass if (grade >= 60)! MulTplestatementsmaybegroupedbypu`ngtheminside curlybraces {. if (grade >= 60) { totalpassed++; Example:#Writeaprogramthatprintsthemaximumof twonumbersenteredbytheuser #include <stdio.h> int main ( ) { int value1, value2, max = 0; printf( Enter two values:\n ); scanf( %d %d, value1, value2); if (value1 > value2) max = value1; if (value1 <= value2) max = value2; printf( %d\n, max); return 0; 11! 12!

4 The#if<#Statement#! Syntax if (condition) statement 1; statement 2;! IfthecondiTonistrue(nonzero)! statement$1willbeexecuted.! IfthecondiTonisfalse(zero)! statement$2willbeexecuted. 13! The#if<#Statement#(Example)#! Supposeaprogramwhichwritesadifferentmessageifthe studentpassesorfailstheclass if (grade >= 60)! MulTplestatementsmaybegroupedbypu`ngtheminside curlybraces {. if (grade >= 60) { totalpassed++; { totalfailed++; 14! Example:#(Re)Writeaprogramthatprintsthemaximum oftwonumbersenteredbytheuser The#Dangling##Problem# #include <stdio.h> int main ( ) { int value1, value2, max = 0; printf( Enter two values:\n ); scanf( %d %d, value1, value2); if (value1 > value2) max = value1; max = value2; printf( %d\n, max); return 0; 15!! Considerthecodebelow.Whichif#doesthe#belongto? if (grade < 60) if (attendance == 100)! Danglingafachestothenearestif.Alwaysusecurlybraces toavoidambiguoussituatons. if (grade < 60) { if (attendance == 100) # Do NOT forget to get the message here, too $ 16!

5 The#?: #Construct#! Syntax (expression)? value1 : value2$! Thestatementreturnsvalue1iftheexpressionevaluatesto true.returnsvalue2otherwise. $! RevisiTngif[statementexample: printf( "%s\n", grade >= 60? "Passed!" : "Failed!");! Or,itcouldbewrifenas: grade >= 60? printf("passed! ) : printf("failed!");! Nested#if<#Statements#! TestsformulTplecasesbyplacingif selectonstatements insideif selectonstatement! Syntax if (condition 1) statement 1; if (condition 2) statement 2;... if (condition n) statement n; default! Oncecondi<on$iismet,restofstatementsskipped! IfnocondiTonismet,default$statement$isexecuted 17! 18! Nested#if<#Statements#(Example)#! Codesegmentforasimplecalculator if (operator == + ) result += value; if (operator == - ) result -= value; if (operator == * ) result *= value; if (operator == / ) result /= value; printf( Unknown operator! ); The#switch#Statement#! Theswitch#statementevaluatesthevalueofanexpressionand branchestooneofthecaselabels.! Syntax# switch ( expression ) { case constant 1 : break ; case constant n : default: break ;! Duplicatelabelsarenotallowed.Theexpressionmustevaluate aninteger,character,orenumeraton. 19! 20!

6 The#switch#Statement#(Example)# if<#vs.#switch#statement#! RevisiTngthecodesegmentforasimplecalculator switch (operator) { case + : result += value; case '-': result -= value; case '*': result *= value; case '/': result /= value; default: printf("unknown operator! ); 21! if (month==1) { printf( Jan. ); if (month==2) { printf( Feb. ); if (month==3) { printf( Mar. ); if (month==4) { printf( Apr. ); if (month==5) { printf( May ); { printf( Summer ); Slidecredit:B.Huang switch(month) { case 1: printf( Jan. ); case 2: printf( Feb. ); case 5: printf( May ); default: printf( Summer ); 22! Dus/er#Corner#of#the#switch#Statement# The#goto#Statement#! breakstatementexitstheswitchstructure.! Ifabreak#statementisnotthere,execuTonwillconTnuewith thenextstatement. switch (control) { case 0: printf("reset\n"); case 1: printf("initializing\n"); case 2: printf("working\n");! Because,itisnotpossibletodetermineiftheprogramis supposedtofallthroughfromcase0tocase1,orifthe programmerforgottoputinabreak#statement. Program prints: Reset Initializing! case$2doesnotneedabreakasitisthelaststatement.but,put abreakthereanyways. 23!! Syntax goto label; label: statement! ProgramflowjumpstothestatementrightaUerthelabel$ $! Thegotostatementiscoveredhereonlyforthesakeof completeness.! DoNOTuseit!! Itmakesthelogicoftheprogramcomplex.! Inmodernprogramming,gotostatementisconsideredaharmful constructandabadprogrammingpractce.! anyprogramcanbeperfectlywrifenwithouttheuseofgotostatement. 24!

7 Summary#! Condi/onal#Branching#! Logical#Expressions! ifandif<statements! switchstatement! gotostatement 25!