Chapter 2 Input, Processing, and Output
Chapter2 Input,Processing,andOutput
DisplayingScreenOutput
TheprintstatementinPythondisplaysoutputonthescreen.Hereisanexample: print 'Hello world' ThepurposeofthisstatementistodisplaythemessageHelloworldonthescreen.Noticethat afterthewordprint,wehavewrittenHello worldinsidesinglequotemarks.Thequote markswillnotbedisplayedwhenthestatementexecutes.Theysimplymarkthebeginningand theendofthetextthatwewishtodisplay. Supposeyourinstructortellsyoutowriteaprogramthatdisplaysyournameandaddresson thecomputerscreen.Program21showsanexampleofsuchaprogram,withtheoutputthatit willproducewhenitruns.(Thelinenumbersthatappearinaprogramlistinginthisbookare notpartoftheprogram.Weusethelinenumbersinourdiscussiontorefertopartsofthe program.)
Program21(output.py)
1 print 'Kate Austen' 2 print '123 Dharma Lane'
ThisprogramisthePython versionofProgram21inyour
3 print 'Asheville, NC 28899'
textbook!
ProgramOutput
Kate Austen
123 Dharma Lane
Asheville, NC 28899
Remember,theselinenumbersareNOTpartoftheprogram!Don'ttypethelinenumbers
whenyouareenteringprogramcode.Alloftheprogramsinthisbookletwillshowline
numbersforreferencepurposesonly.
Itisimportanttounderstandthatthestatementsinthisprogramexecuteintheorderthatthey
appear,fromthetopoftheprogramtothebottom.Whenyourunthisprogram,thefirst
statementwillexecute,followedbythesecondstatement,andfollowedbythethird
statement.
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StringsandStringLiterals
InPythoncode,stringliteralsmustbeenclosedinquotemarks.Asmentionedearlier,thequote markssimplymarkwherethestringdatabeginsandends. InPythonyoucanenclosestringliteralsinasetofsinglequotemarks(')orasetofdouble quotemarks(").ThestringliteralsinProgram21areenclosedinsinglequotemarks,butthe programcouldalsobewrittenasshownhere: print "Kate Austen" print "123 Dharma Lane" print "Asheville, NC 28899" Ifyouwantastringliteraltocontaineitherasinglequoteoranapostropheaspartofthe string,youcanenclosethestringliteralindoublequotemarks.Forexample,Program22prints twostringsthatcontainapostrophes. Program22(apostrophe.py) 1 print "Don't fear!" 2 print "I'm here!" ProgramOutput Don't fear! I'm here! Likewise,youcanusesinglequotemarkstoencloseastringliteralthatcontainsdoublequotes aspartofthestring.Program23showsanexample. Program23(display_quote.py) 1 print 'Your assignment is to read "Hamlet" by tomorrow.' ProgramOutput Your assignment is to read "Hamlet" by tomorrow. Pythonalsoallowsyoutoenclosestringliteralsintriplequotes(either"""or''').Triple quotedstringscancontainbothsinglequotesanddoublequotesaspartofthestring.The followingstatementshowsanexample:
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print """I'm reading "Hamlet" tonight.""" Thisstatementwillprint: I'm reading "Hamlet" tonight. Triplequotescanalsobeusedtosurroundmultilinestrings,whichissomethingthatsingleand doublequotescannotbeusedfor.Hereisanexample: print """One Two Three""" Thisstatementwillprint: One Two Three
Variables
VariablesarenotdeclaredinPython.Instead,youuseanassignmentstatementtocreatea variable.Hereisanexampleofanassignmentstatement: age = 25 Afterthisstatementexecutes,avariablenamedagewillbecreatedanditwillreferencethe value25.ThisconceptisshowninFigure21.Inthefigure,thinkofthevalue25asbeingstored somewhereinthecomputer'smemory.Thearrowthatpointsfromagetothevalue25 indicatesthatthenameagereferencesthevalue. Figure21Theagevariablereferencesthevalue25
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Anassignmentstatementiswritteninthefollowinggeneralformat: variable=expression Theequalsign(=)isknownastheassignmentoperator.Inthegeneralformat,variableisthe nameofavariableandexpressionisavalue,oranypieceofcodethatresultsinavalue.After anassignmentstatementexecutes,thevariablelistedontheleftsideofthe=operatorwill referencethevaluegivenontherightsideofthe=operator. ThecodeinProgram24demonstratesavariable.Line1createsavariablenamedroomand assignsitthevalue503.Theprintstatementsinlines2and3displayamessage.Noticethat line3displaysthevaluethatisreferencedbytheroomvariable. Program24(variable_demo.py) 1 room = 503 2 print 'I am staying in room number' 3 print room ProgramOutput I am staying in room number 503
VariableNamingRules
YoumaychooseyourownvariablenamesinPython,aslongasyoudonotuseanyofthe Pythonkeywords.Thekeywordsmakeupthecoreofthelanguageandeachhasaspecific purpose.Table21showsalistofthePythonkeywords. Additionally,youmustfollowtheseruleswhennamingvariablesinPython:
x Avariablenamecannotcontainspaces. x Thefirstcharactermustbeoneofthelettersathroughz,AthroughZ,oranunderscore
character(_). x AfterthefirstcharacteryoumayusethelettersathroughzorAthroughZ,thedigits0
through9,orunderscores. x Uppercaseandlowercasecharactersaredistinct.Thismeansthevariablename
ItemsOrderedisnotthesameasitemsordered.
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Table21ThePythonkeywords
and
del
from
not
while
as
elif
global or
with
assert else
if
pass
yield
break
except import print
class
exec
in
raise
continue finally is
return
def
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lambda try
DisplayingMultipleItemswiththeprintStatement
IfyoulookbackatProgram24youwillseethatweusedthefollowingtwoprintstatements
inlines3and4:
print 'I am staying in room number'
print room
Weusedtwoprintstatementsbecauseweneededtodisplaytwopiecesofdata.Line3
displaysthestringliteral'I am staying in room number',andline4displaysthe
valuereferencedbytheroomvariable.
Thisprogramcanbesimplified,however,becausePythonallowsustodisplaymultipleitems
withoneprintstatement.Wesimplyhavetoseparatetheitemswithcommasasshownin
Program25.
Program25(variable_demo3.py)
1 room = 503
2 print 'I am staying in room number', room
ProgramOutput
I am staying in room number 503
Theprintstatementinline2displaystwoitems:astringliteralfollowedbythevalue
referencedbytheroomvariable.NoticethatPythonautomaticallyprintedaspacebetween
thesetwoitems.WhenmultipleitemsareprintedthiswayinPython,theywillautomaticallybe
separatedbyaspace.
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