NumericalConversionsStringsinC++

CS16:SolvingProblemswithComputersILecture#8

ZiadMatni

Dept.ofComputerScience,UCSB

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GradeDistributionforMidterm#1CS16,Spring18(Matni)

AVERAGE(MEAN)=84.5%MEDIAN=87%

LectureOutline

•  NumericalConversions– Binary,Octal,Hexadecimal

•  Strings– CStringsvs.C++Strings

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CountingNumbersinDifferentBases•  We“normally”countin10s

–  Base10:decimalnumbers–  Numbersymbolsare0thru9

•  Computerscountin2s–  Base2:binarynumbers–  Numbersymbolsare0and1–  Representedwith1bit(21=2)

•  Otherconvenientbasesincomputerarchitecture:–  Base8:octalnumbers–  Numbersymbolsare0thru7–  Representedwith3bits(23=8)

–  Base16:hexadecimalnumbers–  Numbersymbolsare0thruF

•  A=10,B=11,C=12,D=13,E=14,F=15–  Representedwith4bits(24=16)–  Whyare4bitrepresentationsconvenient???

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PositionalNotationinDecimala.k.a.:HowILearnedNumbersin3rdGrade…

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642 is: 6 hundreds, 4 tens, and 2 units It’s a number in base 10 (aka decimal)

We can write it in positional notation:

= 6 x 100 = 600

= 4 x 10 = + 40 = 2 x 1 = + 2 = 642 in base 10

6x102+4x101+2x100

Whatif“642”isexpressedinthebaseof13?

So,“642”inbase13isequivalentto

“1068”inbase10

6 x 132 = 6 x 169 = 1014 + 4 x 131 = 4 x 13 = 52 + 2 x 13º = 2 x 1 = 2

= 1068 in base 10

PositionalNotationAnythingààDEC

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PositionalNotationinBinary

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11011 in base 2 positional notation is:

1 x 24 = 1 x 16 = 16 + 1 x 23 = 1 x 8 = 8 + 0 x 22 = 0 x 4 = 0 + 1 x 21 = 1 x 2 = 2 + 1 x 20 = 1 x 1 = 1

So, 1011 in base 2 is 16 + 8 + 0 + 2 + 1 = 27 in base 10

Q:Whatisthedecimalequivalentofthebinarynumber1101100?A:Lookforthepositionofthedigitsinthenumber.Thisonehas7digits,thereforepositions0thru6

ConvertingBinarytoDecimal

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1 x 26 = 1 x 64 = 64 + 1 x 25 = 1 x 32 = 32 + 0 x 24 = 0 x 16 = 0 + 1 x 23 = 1 x 8 = 8 + 1 x 22 = 1 x 4 = 4

+ 0 x 21 = 0 x 2 = 0 + 0 x 2º = 0 x 1 = 0

= 108 in base 10

1 1 0 1 1 0 064 32 16 8 4 2 1

26 25 24 23 22 21 20

OtherRelevantBases

•  InComputerScience/Engineering,otherbinary-relatednumericalbasesareusedtoo.

•  OCTAL:Base8 (notethat8is23)– Usesthesymbols:0,1,2,3,4,5,6,7

•  HEXADECIMAL:Base16 (notethat16is24)– Usesthesymbols:0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F

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ConvertingBinarytoOctalandHexadecimal(oranybasethat’sapowerof2)

•  Binaryis 1bit•  Octalis 3bits(23=8)octalisbase8•  Hexadecimalis4bits(24=16)hexisbase16

•  Usethe“groupthebits”technique– Alwaysstartfromtheleastsignificantdigit– Groupevery3bitstogetherforbinàoct– Groupevery4bitstogetherforbinàhex

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ConvertingBinarytoOctalandHexadecimal

•  Taketheexample:10100110…tooctal:

10100110…tohexadecimal:

10100110

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2 4 6

10 6

246inoctal

A6inhexadecimal

0

1

2

3

4

5

6

7

8

9

A

B

C

D

E

F

Octalsymbols

Hex.symbols

Decimalsymbols

While (the quotient is not zero) 1.  Divide the decimal number by the new base 2.  Make the remainder the next digit to the left in the answer 3.  Replace the original decimal number with the quotient 4.  Repeat until your quotient is zero

Algorithmforconvertingnumberinbase10tootherbases

ConvertingDecimaltoOtherBases

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EXAMPLE:Convertthedecimal(base10)number79intohexadecimal(base16)79/16=4R15(15inhexisthesymbol“F”)4/16=0R4

Theansweris:4F

ConvertingDecimalintoBinaryConvert54(base10)intobinaryandhex:•  54/2=27R0•  27/2=13R1•  13/2=6R1•  6/2=3R0•  3/2=1R1•  1/2=0R1

54(decimal)=(binary)=36(hex)

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Sanitycheck:110110=2+4+16+32=54

011011

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WhatisaString?

•  Charactersconnectedtogetherinasequence

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H i M o m !

P i k a c h u

StringsinC/C++

•  Recall:C++isbasedonC•  Originally(inC),stringsweredefinedasan“arrayofcharacters”

–  CalledC-Stringsandare“legacy”datatypesinC++–  Camewiththelibrary<cstring>–  Containslotsofbuilt-infunctionsthatgowithC-Strings

•  InC++,wegotanewlibrary:<string>•  Madeimprovementsovertheold“C-String”

–  LibrarycontainsanothercollectionoffunctionsthatworkwithStrings,butnotC-Strings!

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WhyDoWeCareAboutC-Strings??

•  TheiruseSTILLcomesupinC++– Recall:commandlinearguments…

•  Recallthatcommand-linearguments,specificallyargv[x]aredefinedas: char*[]

•  That’saclassicdefinitionofaC-String

– Soifwewanttousetheseargv[x],we’llhavetotreattheminaC-Stringfashion…

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Cstringsvs.C++strings

•  StringsinC++andStringsinC– C++ismeanttobebackwardscompatiblewithC– Chasonewayofdealingwithstrings,whileC++hasanother

•  C++’suseismucheasierandsaferwithmemoryallocation– Thisiswhatyou’velearnedsofarwith<string>– Let’sbrieflyreviewtheother(older)waywithC-strings…

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What’saC++ProgrammertoDo?!

•  AC-string– Anarrayofcharactersterminatedbythenullcharacter‘\0’–  ThenullcharacterhasanASCIIcodeof0.–  Libraryfordealingwiththesetypes:<cstring>

•  AC++stringobject– Aninstanceofa“class”datatype–useda“blackbox”–  Libraryfordealingwiththesetypes:<string>

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TheC-String•  Anarrayofcharactersthatterminatesinthenullcharacter

–  Thisterminatestheactualstring,butnotthearraynecessarily

•  Example:aC-stringstores“HiMom!”inacharacterarrayofsize10–  Thecharactersoftheword“HiMom!”willbeinpositionswithindices0to6–  Therewillbeanullcharacteratindex7,andthelocationswithindices8to9willcontainsomeunknownvalue.

–  Butwedon’tcareaboutpositions8and9!–  Thenullcharactersays“STOPHERE!”

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s[0] s[1] s[2] s[3] s[4] s[5] s[6] s[7] s[8] s[9]

H i M o m ! \0 ?? ??

C-strings•  TodeclareaC-stringvariable,include<cstring>andusethissyntax:charArray_name[Maximum_C_String_Size+1];–  The“+1”reservestheadditionalcharacterneededby‘\0’

•  WithC-Strings,youcannotdothese: myString=“Hello!” //assignment if(myString==“Jimbo”)… //comparison

–  Insteadusestrncpy()andstrcmp()fromthe<cstring>library

•  ButyouCANuse=and==withC++Strings–  Andsomuchmoreusefulthings!J

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TheStandardC++stringClass

•  Thestringsweknowandlove…•  Thestringclassallowstheprogrammertotreatstringsasabasicdatatype

–  NoneedtodealwiththeimplementationsofC-strings

•  Thestringclassisdefinedinthe<string>library

•  Wewilldiscussmanydifferentmemberfunctionsthatareextremelyusefultouse–  Like.length(),.erase(),.substr(),.find(),etc…

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DeclaringaStringinC++

•  Youhavetoincludethecorrectlibrarymodulewith: #include<string>

•  Declarethem(andinitializethem)with: stringMyString=“”;//Notetheuseofdouble-quotes!

•  Sincestringsaremadeupofcharacters,youcanindexindividualcharactersinstrings(startingatposition0):

If MyString=“Hello!”ThenMyString[0]=‘H’,MyString[1]=‘e’,etc…

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StringBasics

•  Usethe+operatortoconcatenate2stringsstringstr1=“Hello”,str2=“world!”,str3;str3=str1+str2;//str3willbe“Helloworld!”

•  Usethe+=operatortoappendtoastringstr1+=“Z”;//str1willbe“HelloZ”

•  Calloutacharacterinthestringbasedonposition,using[]braces–  RecallarrayindicesinC++startatzero(0)cout<<str1[0];//printsout‘H’cout<<str2[3];//printsout‘l’

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Built-InStringMemberFunctions

•  Searchfunctions– find,rfind,find_first_of,find_first_not_of

•  Descriptorfunctions– length,size

•  Contentchangers– substr,replace,append,insert,erase

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SearchFunctions:find 1

•  Youcansearchforathefirstoccurrenceofastringinastringwiththe.findfunction

stringstr=“Withabanjoonmykneeandbanthebomb-ban!”;intposition=str.find(“ban”);cout<<position; //Willdisplaythenumber7

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[7]

SearchFunctions:find 2

•  Youcanalsosearchforathefirstoccurrenceofastringinastring,startingatpositionn,usingaslightmodto.find()stringstr=“Withabanjoonmykneeandbanthebomb-ban!”;intposition=str.find(“ban”,12);cout<<position; //Willdisplaythenumber28

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?[28]

SearchFunctions:find 3

•  YoucanusethefindfunctiontomakesureasubstringisNOTinthetargetstringusingthe“noposition”value

string::npos isreturnedifnopositionexistsif(MyStr.find("piano")==string::npos) cout<<"Thereisnopianothere!”//Thiswillhappenif“piano”isNOTinthestringMyStr

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SearchFunctions:rfind

•  Youcansearchforathelastoccurrenceofastringinastringwiththe.rfindfunction

stringstr=“Withabanjoonmykneeandbanthebomb-ban!”;intrposition=str.rfind(“ban”);cout<<rposition; //Willdisplaythenumber41

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[41]

SearchFunctions:find_first_ofandfind_first_not_of

•  find_first_of–  Finds1stoccurrenceofanyofthecharactersincludedinthespecifiedstring

•  find_first_not_of–  Finds1stoccurrenceofacharacterthatisnotanyofthecharacters

includedinthespecifiedstring

•  Example:

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Seedemofile:non_numbers.cpp

DescriptorFunctions:lengthandsize

•  Thelengthfunctionreturnsthelengthofthestring•  Thememberfunctionsizeisthesameexactthing…

–  So,ifstringstr1=“MamaMia!”, thenstr1.length()=9 andstr1.size()=9also

Example–whatwillthiscodedo?:

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stringname=“BubbaSmith”;for(inti=name.length();i>0;i--) cout<<name[i-1];

ContentChangers:append

•  Usefunctionappendtoappendonestringtoanotherstringname1=“Max”;stringname2=“Powers”;cout<<name1.append(name2); //Displays“MaxPowers”

•  Doesthesamethingas:name1+name2

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ContentChangers:erase

•  Usefunctionerasetoclearastringtoanemptystring

•  Oneuseis:name1.erase()--Doesthesamethingas:name1=“”

•  Anotheruseis:name1.erase(startposition,howmanycharstoerase)–  Erasesonlypartofthestring–  Example:strings=“Hello!”;cout<<s.erase(2,2);//Displays“Heo!”

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ContentChangers:replaceandinsert•  Usefunctionreplacetoreplacepartofastringwithanother

– PopularUsage:string.replace(startposition,#ofplacesafterstartpositiontoreplace, replacementstring)

•  Usefunctioninserttoinsertasubstringintoastring– PopularUsage:string.insert(startposition,insertionstring)

Example:stringcountry=“BackintheUSSR”; //lengthis16cout<<country.replace(14,2,“A”); //Displayscout<<country.insert(15,“BC”); //Displays

“BackintheUSABC”“BackintheUSA”

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ContentChangers:substr

•  Usefunctionsubstr(shortfor“substring”)toextractandreturnasubstringofthestringobject– PopularUsage:string.substr(startposition,#ofplacesafterstartposition)

Example:stringcity=“SantaBarbara”;cout<<city.substr(3,5)//Displays

“taBa”

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YOURTO-DOs

q PrepareLab4forWednesday!q DoHW8bynextThursday

q VisitProf’sandTAs‘officehoursifyouneedhelp!

q Runamile.Ortwo.

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