Download Session 4

Introductiontoprogramming
usingPython
Session4
MatthieuChoplin
[email protected]
http://moodle.city.ac.uk/
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Objectives
Tocomebackonthenotionofobjectandtype.
Tointroducetothetype"List"anditsmethods.
Tousethelen,min/max,sum,andrandom.shufflefunctionsforalist.
Todevelopandinvokefunctionswithlistargumentsandreturnvalue.
Toaccesslistelementsusingindexedvariables.
Toobtainasublistusingtheslicingoperator[start:end].
Touse+,*,andin/notinoperatorsonlists.
Totraverseelementsinalistusingafor-eachloop.
Tocreatelistsusinglistcomprehension.
Tosplitastringtoalistusingthestr’ssplitmethod.
Tocopycontentsfromonelisttoanother.
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Whatisthedifferencebetweenanobject
andatype?
Atypeoraclassiswhatisgoingtocreateanobject
Typeandobjectseensofar:
Types
Objects
Constructor
Integer
1,3,4,5,999,-3,-4
int()
Float
1.333,-0.5,0.001
float()
String
"Foo",'bar',""
str()
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Anobjecthasmethods
Youcanfindthemethodofanobjectwiththefunctiondir(),
whichreturnstheattributesofanobject.
>>>dir("abc")
['__add__','__class__','__contains__','__delattr__','__dir__','__doc__','__eq__','__format_
NB:thedundermethods(withdoubleunderscore),are
"specialmethods"inpythonthatcanbeoverridden.Wewill
comebackonthatlater.
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Differencebetweenmethodsofobjectsand
builtinfunctions
Themethodsofanobjectcanonlybecalledonanobject.
>>>"speaklouder".upper()
'SPEAKLOUDER'
Abuiltinfunctiondoesnotneedanobjecttobecalled.
>>>len("numberofcharacter")
19
NB:len()givethenumberofelementinasequence
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ThetypeList
Creatinglistusingthelistconstructor
list1=list()#Createanemptylist
list2=list([2,3,4])#Createalistwithelements2,3,4
list3=list(["red","green","blue"])#Createalistofstrings
list4=list(range(3,6))#Createalistwithelements3,4,5
list5=list("abcd")#Createalistwithcharactersa,b,c
Thatistheequivalentof:
list1=[]#Sameaslist()
list2=[2,3,4]#Sameaslist([2,3,4])
list3=["red","green"]#Sameaslist(["red","green"])
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TheListmethods
Youcanfindthedifferentmethodsofalistthankstothe
functiondir()
>>>dir([])
['__add__','__class__','__contains__','__delattr__','__delitem__','__dir__','__doc__','__eq
Wearegoingtolookat:'append','clear','copy','count',
'extend','index','insert','pop','remove','reverse','sort'
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Howtoseewhatamethodcando
Lookatthebuiltinhelp:
>>>help([].append)
Helponbuilt-infunctionappend:
append(...)methodofbuiltins.listinstance
L.append(object)->None--appendobjecttoend
Experimentintheinterpreter:
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main.py
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Remix
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my_list = ["foo", "bar", 124, []]
print(my_list)
my_list.append("abc")
print(my_list)
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Summaryofthelistmethods
append(x:object):None
Addanitemxtotheendofthelist.
insert(index:int,x:
object):None
Insertanitemxatagivenindex.Notethatthefirstelementin
thelisthasindex0.
remove(x:object):None
Removethefirstoccurrenceoftheitemxfromthelist.
index(x:object):int
Returntheindexoftheitemxinthelist.
count(x:object):int
Returnthenumberoftimesitemxappearsinthelist.
sort():None
Sorttheitemsinthelist.
reverse():None
Reversetheitemsinthelist.
extend(L:list):None
AppendalltheitemsinlistLtothelist.
pop([i]):object
Removetheitematthegivenpositionandreturnit.The
squarebracketdenotesthatparameterisoptional.Ifnoindex
isspecified,list.pop()removesandreturnsthelastiteminthe
list.
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Exercise1
Writeaprogramthatreadsintegersfromtheuserandstorestheminalist(use
input()andappend()).Yourprogramshouldcontinuereadingvaluesuntiltheuser
enters'q'(thesentinelvalue).Thenitshoulddisplayallofthevaluesenteredbythe
userinorderfromsmallesttolargest,withonevalueappearingoneachline.Use
eitherthesortmethodorthesortedbuiltinfunctiontosortthelist.
 Solution
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Builtinfunctionforlistorsequences
>>>list1=[2,3,4,1,32]
>>>len(list1)
5
>>>max(list1)
32
>>>min(list1)
1
>>>sum(list1)
42
>>>importrandom
>>>random.shuffle(list1)#Shuffletheitemsinthelist
>>>list1
[4,1,2,32,3]
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Iteratingonalist
Thelistisasequenceonwhichyoucaniterate.
Withfor:
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main.py
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Remix
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for element in ["foo", 11, "bar"]:
print(element)
Withwhile:
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main.py
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Remix
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my_list = ["foo", 11, "bar"]
i=0
while i < len(my_list):
print(my_list[i])
i+=1
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Reminderaboutfunctions
Wedefinethefunctionlikethis:
defmain():
print('Thefunction',main.__name__,'hasbeencalled')
Andwecallthefunctionslikethis:
main()
NB:noticethebrackets:whenwedefineandwhenwecall!
Trytousefunctionsinthenextexercises.
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Exercise2:ChineseZodiacsignwithlist
Simplifytheexercisewesawattheendofsession2byusinga
listofstringstoringalltheanimalsname,insteadofmultipleif
andelifstatement.
 Solution
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PassingListstoFunctions
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main.py
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Remix
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def printList(lst):
for element in lst:
print(element)
# Invoke the function
lst = [3, 1, 2, 6, 4, 2]
printList(lst)
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ReturningaListfromaFunction
Example:afunctionthatreturnsareversedlist
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main.py
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Remix
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def reverse(lst):
result = []
for element in lst:
result.insert(0, element)
return result
list1 = [1, 2, 3, 4, 5, 6]
list2 = reverse(list1)
print(list2)
Thefunctionreverseactuallyexistsfordoingthesamething
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Exercise3:
Completethisprogramtogettheminimumnumberofthelist
anditsindex
importrandom
random_list=[random.choice(list(range(1,100)))for_inrange(10)]
defget_min(random_list):
#tocomplete
pass
get_min(random_list)
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Solutionwithoutusingbuiltinfunctionsnon
listmethods
 Solution
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Solutionusingbuiltinfunctionsandlist
methods
 Solution
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Reminder
Thestringisasequence
Theitemsofasequencecanbeaccessedthroughindexes
Items
(characters)
a
b
r
a
c
a
d
a
b
r
a
Indexes
0
1
2
3
4
5
6
7
8
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Getthefirstelementofthesequence:
my_string_variable="abracadabra"
first_elem=my_string_variable[0]
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ManipulateelementofaListwithindexes
YoucanalsoaccesselementofalistwithindexesBUTyou
canalsomodifythem:
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main.py
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Remix
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my_list = ["foo", 11, "bar"]
print(my_list)
my_list[1] = "eleven"
print(my_list)
contrarytothestringtype.
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main.py
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Remix
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my_string = "abracadabra"
my_string[0] = "O"
print(my_string)
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Differencebetweenmutableandimmutable
objects
Youcannotmodifyanimmutableobjectsuchasastring.
Youcanmodifyamutableobjectsuchasalist.
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The+,*,[:],andinOperators(1/2)
+isforconcatenatinglist
*isforrepeatingalist
[:]isthesliceoperator,forextractingasublistfromalist
>>>list1=[2,3]
>>>list2=[1,9]
>>>list3=list1+list2
>>>list3
[2,3,1,9]
>>>list3=2*list1
>>>list3
[2,3,2,3]
>>>list4=list3[2:4]
>>>list4
[2,3]
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The+,*,[:],andinOperators(2/2)
Getthelastelementofalistwithanegativeindex
Checkifanelementisinalistwiththeinoperator
>>>list1=[2,3,5,2,33,21]
>>>list1[-1]
21
>>>list1[-3]
2
>>>list1=[2,3,5,2,33,21]
>>>2inlist1
True
>>>list1=[2,3,5,2,33,21]
>>>2.5inlist1
False
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Listcomprehensions
Listcomprehensionsprovideaconcisewaytocreatelists
Transformingalistwithoperationoneachelement
Filteringalist,keepingonlyelementsthatsatisfya
condition
>>>list1=[xforxinrange(0,5)]
>>>list1
[0,1,2,3,4]
>>>list2=[0.5*xforxinlist1]
>>>list2
[0.0,0.5,1.0,1.5,2.0]
>>>list3=[xforxinlist2ifx<1.5]
>>>list3
[0.0,0.5,1.0]
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SplittingaStringtoaList
Youcanconvertastringtoalistwiththesplitfunctionon
string.
>>>items="WelcometotheUK".split()
>>>print(items)
['Welcome','to','the','UK']
>>>items="34#13#78#45".split("#")
>>>print(items)
['34','13','78','45']
Youcanconvertbackalisttoastringwiththejoinfunction
onstring

>>>print(items)
['Welcome','to','the','UK']
>>>print("".join(items))
'WelcometotheUK'
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Exercise4-Eliminateduplicates
Writeafunctionthatreturnsanewlistbyeliminatingthe
duplicatevaluesinthelist.Usethefollowingfunctionheader:
defeliminateDuplicates(lst):
Writeatestprogramthatreadsinalistofintegers,invokes
thefunction,anddisplaystheresult.Hereisthesamplerunof
theprogram:
Entertennumbers:1232163452
Thedistinctnumbersare:123645
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Solution
 Solution
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Exercise5=Anagrams
Writeafunctionthatcheckswhethertwowordsare
anagrams.Twowordsareanagramsiftheycontainthesame
letters.Forexample,silentandlistenareanagrams.The
headerofthefunctionis:
defisAnagram(s1,s2):
(Hint:Obtaintwolistsforthetwostrings.Sortthelistsand
checkiftwolistsareidentical.)
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Writeatestprogramthatpromptstheusertoentertwo
stringsand,iftheyareanagrams,displaysisananagram;
otherwise,itdisplaysisnotananagram.
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Solution
 Solution
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CopyingLists
Often,inaprogram,youneedtoduplicatealistorapartofa
list.Insuchcasesyoucouldattempttousetheassignment
statement(=):
list1=[1,2,3]
list2=list1
Butyouarenotcopyingthelisthere!Youarecopyingits
reference.
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Whatishappeninginmemory
Python3.3
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list1=[1,2,3]
list2=list1
list2.append('four')
print(list1)
print(list2)
linethathasjustexecuted
nextlinetoexecute
Printoutput(draglowerrightcornertoresize)
[1,2,3,'four']
[1,2,3,'four']
Frames
Globalframe
list1
list2
Objects
list
0
1
1
2
2
3
3
"four"
<Back Programterminated Forward>
VisualizedusingOnlinePythonTutorbyPhilipGuo
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Copyingalistthecorrectway
>>>list2=[xforxinlist1]
>>>list2=list1[:]
>>>list2=list(list1)
>>>list2=list(list1)
>>>importcopy
>>>list2=copy.copy(list1)
>>>list2=copy.deepcopy(list1)#willcopytheobjectaswell
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Whatishappeninginmemoryforareal
copy
Python3.3
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list1=[1,2,3]
list2=list1[:]
list1.append('four')
print(list1)
print(list2)
linethathasjustexecuted
nextlinetoexecute
<Back Programterminated Forward>
VisualizedusingOnlinePythonTutorbyPhilipGuo
Printoutput(draglowerrightcornertoresize)
[1,2,3,'four']
[1,2,3]
Frames
Globalframe
list1
list2
Objects
list
0
1
1
2
2
3
3
"four"
list
0
1
1
2
2
3
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PassByValue
Thereareimportantdifferencesbetweenpassingimmutable
ormutableobjectsasargumentstoafunction.
Stringandnumericvalues(integerandfloat)areimmutable,
theydonotgetchanged
Listsaremutable,theycanbechanged
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Example
Python3.3
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defm(number,list_of_numbers):
number=1001
list_of_numbers[0]=5555
defmain():
x=1
y=[1,2,3]
m(x,y)
print("xis",str(x))
print("y[0]is",str(y[0]))
main()
Printoutput(draglowerrightcornertoresize)
xis1
y[0]is5555
Frames
Globalframe
m
main
Objects
function
m(number,list_of_numbers)
function
main()
linethathasjustexecuted
nextlinetoexecute
<Back Programterminated Forward>
VisualizedusingOnlinePythonTutorbyPhilipGuo
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Exercise6:Hangman
Writeahangmangamethatrandomlygeneratesawordandpromptstheusertoguessoneletteratatime,as
showninthesamplerun.
Eachletterinthewordisdisplayedasanasterisk.Whentheusermakesacorrectguess,theactualletteris
thendisplayed.Whentheuserfinishesaword,displaythenumberofmissesandasktheuserwhetherto
continueplaying.Createalisttostorethewords,asfollows:
words=["write","that","program",...]
(Guess)Enteraletterinword*******>p
(Guess)Enteraletterinwordp******>r
(Guess)Enteraletterinwordpr**r**>p
pisalreadyintheword
(Guess)Enteraletterinwordpr**r**>o
(Guess)Enteraletterinwordpro*r**>g
(Guess)Enteraletterinwordprogr**>n
nisnotintheword
(Guess)Enteraletterinwordprogr**>m
(Guess)Enteraletterinwordprogr*m>a
Thewordisprogram.Youmissed1time
Doyouwanttoguessanotherword?Enteryorn>
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
Solution
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