OperationsonStrings,ListsIntroductiontoFunctions

CS8:IntroductiontoComputerScience,Spring2019Lecture#4

ZiadMatni,Ph.D.

Dept.ofComputerScience,UCSB

Administrative•  Hw02–dueTuesdayinclass•  Lab01– dueonSundaybymidnight(11:59pm)onGradescope!

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LectureOutline•  OperationsonStrings

•  IntrotoLists&Tuple

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Yellow Band = Class Demonstration! J

Strings•  Thesearealloktouse:

S='hello!'S="hello!"S="Isaid\"hello\"!"S='Isaid"hello"!'S="Isaid'hello'!"

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Notethealternateuseof“and‘

AddingaNewlineCharacter•  Ifyouwanttoprintastringwitha“newline”characterin

it…–  i.e.equivalenttohittingthe“Return”key

print("Hello!\nMynameisZed")Thiswillprintout:Hello!MynameisZed

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Recall:Indexing•  Everycharacterinastringhasanindexassociatedwithit

•  InPython,indexingalwaysstartsat0.–  Sothe1stcharacterinthestringischaracter#0–  Indexingiscalledoutwithsquarebrackets[n]

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I ' m h e r e !

0 1 2 3 4 5 6 7 8

IndicesandSlices•  Tosliceastringintoasmallerstring,use[i:j]– Wherei=startingindex,j=endingindex(NOTincluded)–  Example:"Gaucho"[2:4]is"uc"

•  Combinationsarepossible!–  Example,whatdoesthisspellout?(("o"+"Gaucho"[2:5]+"")*3)+"!"

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NegativeIndicesinStrings•  If s="hello"•  Then s[-1]="o" s[-2]="l",etc…

•  Intheexampleabove, s[-2:]="lo" etc…

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Exercise1•  Whatisthevalueofsafterthefollowingcoderuns?

s='abc'

s='d'*3+ss=s+e*2

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A.‘abcd3e2’B.‘abcdddabc’C.‘dddabcee’D.‘abcdddabce2’E.Error

Exercise2•  Whatisthevalueofsafterthefollowingcoderuns?

s='abc'

s='d'*3+ss=s+'e'*2

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A.‘abcd3e2’B.‘abcdddabc’C.‘dddabcee’D.‘abcdddabce2’E.Error

SomeOperationsonStrings•  GivenastringS,forexample,"Tunneling":

len(S) Lengthofstring e.g.9 S.upper() Makestringallupper-case e.g.“TUNNELING” S.lower() Makestringalllower-case e.g.“tunneling” S.find('n') Findthe1stoccurrenceof e.g.2 S.find('z') ifnotfound… e.g.-1

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Thesearecalled

methods

MoreStringMethodsAssume:name=‘Bubba’

•  name.count(‘b’)is 2 ßcountshowmanytimes‘b’occurs

•  name.count(‘ubb’)is 1 ßcountshowmanytimes‘ubb’occurs

•  name.center(9)is ‘Bubba’ ßcentersw/spacesoneachside

•  name.ljust(9)is ‘Bubba’ ßleftjustifiesnamein9spaces

•  name.rjust(9)is ‘Bubba’ ßrightjustifiesnamein9spaces

•  name.replace(‘bb’,‘dd’)is‘Budda’ ßReplacesonesub-stringforanother

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More(Fun)ctionswecanusewithStrings!

•  Booleanoperatorsinandnotinaregreatwaystocheckifasub-stringisfoundinsidealongerstring

Examples:•  “fun”in“functions” =•  “fun”in“Functions” =•  “Fan”notin“Functions” =

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True

True

False

ExampleAssumestrings=“hownowbrowncowmeow!”Whatis:•  s.find(‘m’) =•  s.find(‘r’) =•  s.find(‘ow’) =•  s.find(‘s’) =•  s.replace(‘meow’,‘moo?’)=•  ‘nc’ins =

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h o w n o w b r o w n c o w m e o w !0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

“ ”

1891-1

“hownowbrowncowmoo?!”True

note:onespacebeforemeownote:spacebetweennandc

Lists•  Alistisacollectionofmultiplevalues

–  Similartohowastrisacollectionofcharacters

•  Note:InPython,listscanbeofheterogenous–  Ofdifferenttypes(i.e.intsorstringsoretc…)

•  Listscanalsohaveduplicatevalues•  Listsaremutable:elementsofalistcanbemodified4/12/19 Matni,CS8,Sp19 15

ExampleofListsNameList=[“Abby”,“Bruce”,“Chris”]Student=[“JillJillson”,19,3.7,“F”]

NameListandStudentarevariablesoftypelist

•  YoucancalluplistelementsbyindexingthelistExample:NameList[0]=“Abby”4/12/19 Matni,CS8,Sp19 16

SomeOperationsonLists•  GivenalistL,forexample,[1,2,-5,9,0,1]:

len(L) Lengthoflist e.g.6 max(L) Maxvalueinalist e.g.9 min(L) Minvalueinalist e.g.-5 sum(L) Sumofallvaluesinalist e.g.8

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Mostlyusedonlistsof

numbers

Tuples•  Tuplesareavariabletypethat’sverysimilartolists

–  Excepttheyareimmutable!

–  Thatis,oncethey’reset,theycannotchange

•  Exampleofatuple:

collection=(1,2,“bucklemyshoe”,3,4)

•  Youcancalluplistelementsbyindexingthelist

Example:collection[1]=2

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Functions

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FUNCTION

Input

Output

ProceduralAbstraction:TheFunction

•  A“blackbox”–apieceofcodethatcantakeinputsandgivesmesomeexpectedoutput

•  Afunction,forexample,isakindofproceduralabstraction 25àSquareRootFunctionà5

•  What’shappeninginsidethefunction?•  Doesn’talwaysmatter!...Aslongasitworks!!

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ProgrammedFunction•  Does“something”toinput(s)andsendsbackoutput(s)

–  Alwayshasparenthesesto“carry”theinputs–  Theseinputsarecalledthefunctionarguments

•  Example:thesqrt()function(findthesquareroot)–  Withaninputof25,Iexpectanoutputof5–  Thatis,sqrt(25)willgiveme(RETURNStome)5

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Function

Input

Output

MoreAboutFunctions

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•  Definition:“Selfcontained”modulesofcodethataccomplishaspecifictask.

•  Thefunctionoften(althoughnotalways)“returns”anoutput(result)•  The“returned”outputislinkedtothefunctionname(examplescoming…)

•  Sometimesthefunctiondoesnotreturnanything…

•  Functionscanbe“calledfrom”themainblockofaprogram–  Orfrominsideotherfunctions!

Function

Input

Output

MoreAboutFunctions

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•  Afunctioncanbeusedoverandoveragain.

•  Example:Considerafunctioncalled“distance”thatreturnsthevalueofthedistancebetweenapointw/coordinates(a,b)andtheCartesianorigin(0,0)

Wecan“reuse”thisfunctionwithdifferentvaluesforaandb!distance(2,4) distance(92,-41) distance(distance(1,1),4)

distance(a,b)=squarerootof(a2+b2)

DefiningYourOwnFunction•  TodefineafunctioninPython,thenecessarysyntaxis: def functionName (parameters): # a block of statements appear here

# all of them must be indented (with tabs)

–  def –amandatorykeywordthatdefinesafunction–  functionName –anylegalPythonidentifier(e.g.myLittleFunction)–  ( ):–mandatorysetofparenthesesandcolon–  parameters –objectnames(canbenone,1param.,ormultipleparams.)

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ExampleDefinition#Myfirstfunction!Yay!defdbl(x):

"""Thisfunctionreturnsdoubleitsinputx"""print(“I’mdoublingthenumberto:”,2*x)return2*x #Ineedto“return”theresult

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Let’s try it out!

FUNCTIONRULES!#Myfirstfunction!Yay!defdbl(x):

"""Thisfunctionreturnsdoubleitsinputx"""print(“I’mdoublingthenumberto:”,2*x)return2*x #Ineedto“return”theresult

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Function header x is the input parameter (also called argument)

docstring: a comment that becomes part of Python's built-in help system! With each function be sure to include one that: a)  describes overall what the function does, and b)  explains what the inputs mean/are

Function body

Indentation: VERY IMPORTANT Achieved with a tab character or just spaces All the lines in the function body are indented from the function header, and all to the same degree

MoreExampleDefinitions#Thisfunctioncalculatesthedistancebetween(a,b)and(0,0)defdistance(a,b):

x=a**2 #Notethetabindent!!!y=b**2 #Recall**means“tothepowerof”z=(x+y)**0.5returnz #Ineedto“return”theresult

!!!Alternatively,Icanalsodothis!!!

defdistance(a,b):return((a**2)+(b**2))**0.5

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Let’s try it out!

FlowofExecutionofaFunction•  DEFININGvs.CALLINGafunction•  Callingishowyougetto“run”it

fromanotherplaceinthecode

•  UseitsnameandargumentsASDEFINED

•  Example:tocallthedblfunctionforaninputof21,you’dhavetocallitlikethis:dbl(21)

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defdbl(x):y=2*xreturny

……a=dbl(21)print(a)……

FunctionDefinition

FunctionCall

comesbeforethecall

WhatifThereareMultipleParameters??

•  Whenyoucallafunction,thevaluesyouputinparenthesishavetobeintheorderinwhichtheyarelistedinthedefinition!

•  Example:defsubtract(m,n):returnm–n

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Whenyoucallthisfunctiontodoasubtractionof5–99,then:mhastobe5andnhastobe99

So,it’scalledas:subtract(5,99)i.e.notsubtract(99,5)

WhatAbout…NOParameters?!•  Sure,youcandothat!

•  Example:deffortyTwo():return42

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Allthisfunctiondoesisreturnthenumber42towhoevercalledit!Whichwayshouldwecallit?

fortyTwofortyTwo()

Wow.FunctionsareCool.CanTheyCALLEACHOTHER????

Yes!!!!!!!!!!!!!!!Becarefulthatyougettheordercorrect…!

defhalve(x):"""returnshalfitsinput,x"""

returndiv(x,2)defdiv(y,x):"""returnsy/x"""

returny/xWhathappenswhenIsay:>>>halve(85)

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Let’s try it out!

A.  Iget42B.  Iget42.5C.  0D.  0.02352(i.e.,2dividedby85)

YOURTO-DOsq  FinishreadingChapter2q  StartreadingChapter3q  FinishupHW2(dueTuesday)q  FinishupLab1(dueSunday)

q  Rememberofficehours/openlabs!J

q  Eatyourgreens…

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