CN101
Lecture11-12
ListsandTuples
Topics
• Sequences
• IntroductiontoLists
• ListSlicing
• FindingItemsinListswiththeinOperator
• ListMethodsandUsefulBuilt-inFunctions
• CopyingLists
• Two-DimensionalLists
• Tuples
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Sequences
• Sequence:anobjectthatcontainsmultipleitemsofdata
• Theitemsarestoredinsequenceoneafteranother
• Pythonprovidesdifferenttypesofsequences,including
listsandtuples
• Thedifferencebetweentheseisthatalistismutableanda
tupleisimmutable
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Introduction to Lists
• List:anobjectthatcontainsmultipledataitems
• Element:Aniteminalist
• Format:list = [item1, item2, etc.]
• Canholditemsofdifferenttypes
•printfunctioncanbeusedtodisplayanentirelist
•list()functioncanconvertcertaintypesofobjects
tolists
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Introduction to Lists (cont’d.)
• Hereisastatementthatcreatesalistofintegers:
even_numbers = [2, 4, 6, 8, 10]
• Thefollowingisanotherexample:
names = ['Molly', 'Steven', 'Will', 'Alicia', 'Adriana']
• Alistcanholditemsofdifferenttypes,asshowninthefollowing
example:
info = ['Alicia', 27, 1550.87]
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The Repetition Operator and
Iterating over a List
• Repetitionoperator:makesmultiplecopiesofalistand
joinsthemtogether
• The*symbolisarepetitionoperatorwhenappliedtoa
sequenceandaninteger
• Sequenceisleftoperand,numberisright
• Generalformat:list * n
• Youcaniterateoveralistusingafor loop
• Format:for x in list:
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Indexing
• Index:anumberspecifyingthepositionofanelementin
alist
• Enablesaccesstoindividualelementinlist
• Indexoffirstelementinthelistis0,secondelementis1,and
n’thelementisn-1
• Negativeindexesidentifypositionsrelativetotheendofthe
list
• Theindex-1identifiesthelastelement,-2identifiesthenexttolast
element,etc.
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The len function
• AnIndexErrorexceptionisraisedifaninvalidindex
isused
•lenfunction:returnsthelengthofasequencesuchas
alist
• Example:size = len(my_list)
• Returnsthenumberofelementsinthelist,sotheindexof
lastelementislen(list)-1
• CanbeusedtopreventanIndexError exceptionwhen
iteratingoveralistwithaloop
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Lists Are Mutable
• Mutablesequence:theitemsinthesequencecanbe
changed
• Listsaremutable,andsotheirelementscanbechanged
• Anexpressionsuchas
• list[1] = new_valuecanbeusedtoassigna
newvaluetoalistelement
• MustuseavalidindextopreventraisingofanIndexError
exception
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Concatenating Lists
• Concatenate:jointwothingstogether
• The+operatorcanbeusedtoconcatenatetwolists
– Cannotconcatenatealistwithanotherdatatype,suchasa
number
• The+=augmentedassignmentoperatorcanalsobe
usedtoconcatenatelists
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List Slicing
• Slice:aspanofitemsthataretakenfromasequence
• Listslicingformat:list[start : end]
• Spanisalistcontainingcopiesofelementsfromstartup
to,butnotincluding,end
• Ifstartnotspecified,0isusedforstartindex
• Ifendnotspecified,len(list)isusedforendindex
• Slicingexpressionscanincludeastepvalueandnegative
indexesrelativetoendoflist
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Finding Items in Lists with the in
Operator
• Youcanusetheinoperatortodeterminewhetheran
itemiscontainedinalist
• Generalformat:item in list
• ReturnsTrueiftheitemisinthelist,orFalseifitisnotin
thelist
• Similarlyyoucanusethenot inoperatorto
determinewhetheranitemisnotinalist
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List Methods
•append(item):usedtoadditemstoalist–itemis
appendedtotheendoftheexistinglist
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List Methods (cont’d.)
•index(item):usedtodeterminewhereanitemis
locatedinalist
• Returnstheindexofthefirstelementinthelistcontaining
item
• Raises ValueErrorexceptionifitemnotinthelist
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List Methods (cont’d.)
•insert(index, item):usedtoinsertitemat
positionindexinthelist
•sort():usedtosorttheelementsofthelistin
ascendingorder
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List Methods (cont’d.)
•remove(item):removesthefirstoccurrenceof
iteminthelist
• Raises ValueErrorexceptionifitemnotinthelist
•reverse():reversestheorderoftheelementsinthe
list
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Useful Built-in Functions
•del statement:removesanelementfromaspecific
indexinalist
• Generalformat:del list[i]
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Useful Built-in Functions (cont’d.)
•min and max functions:built-infunctionsthat
returnstheitemthathasthelowestorhighestvaluein
asequence
• Thesequenceispassedasanargument
•Sum functions:built-infunctionsthatreturnsthesum
ofallvaluesinasequence
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List Referencing
• Afterthiscodeexecutes,both
variableslist1andlist2will
referencethesamelistinmemory.
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Copying Lists
• Tomakeacopyofalistyoumustcopyeachelementof
thelist
• Twomethodstodothis:
• Creatinganewemptylistandusingaforlooptoaddacopyofeach
elementfromtheoriginallisttothenewlist
• Creatinganewemptylistandconcatenatingtheoldlisttothenew
emptylist
Asaresult,list1andlist2will
referencetwoseparatebutidenticallists.
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Two-Dimensional Lists
• Two-dimensionallist:alistthatcontainsotherlistsasits
elements
• Alsoknownasnestedlist
• Commontothinkoftwo-dimensionallistsashavingrowsand
columns
• Usefulforworkingwithmultiplesetsofdata
• Toprocessdatainatwo-dimensionallistneedtouse
twoindexes
• Typicallyusenestedloopstoprocess
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Two-Dimensional Lists (cont’d.)
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Tuples
• Tuple:animmutablesequence
• Verysimilartoalist
• Onceitiscreateditcannotbechanged
• Format:tuple_name = (item1, item2)
• Tuplessupportoperationsaslists
• Subscriptindexingforretrievingelements
• Methodssuchasindex
• Builtinfunctionssuchaslen, min, max, sum
• Slicingexpressions
• Thein,+,and*operators
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Tuples (cont’d.)
• Tuplesdonotsupportthemethods:
• append
• remove
• insert
• reverse
• sort
• Tuplesdonotsupportdelstatement
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Tuples (cont’d.)
• Advantagesforusingtuplesoverlists:
• Processingtuplesisfasterthanprocessinglists
• Tuplesaresafe
• SomeoperationsinPythonrequireuseoftuples
•list()function:convertstupletolist
•tuple()function:convertslisttotuple
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Summary
• Thischaptercovered:
• Lists,including:
• Repetitionandconcatenationoperators
• Indexing
• Techniquesforprocessinglists
• Slicingandcopyinglists
• Listmethodsandbuilt-infunctionsforlists
• Two-dimensionallists
• Tuples,including:
• Immutability
• Differencefromandadvantagesoverlists
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