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NumericalConversionsStringsinC++
CS16:SolvingProblemswithComputersILecture#8
ZiadMatni
Dept.ofComputerScience,UCSB
5/1/18 Matni,CS16,Sp18 2
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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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