Professor Donald J. Patterson - Converting File...
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Converting File Input • As with the input func.on, the readline() method can only return strings • If the file contains numerical data, the strings must be converted to the numerical value using the int() or float() func.on: 10/17/16 value = float(line) Page 15 • The newline character at the end of the line is ignored when the string is converted to a numerical value
Transcript of Professor Donald J. Patterson - Converting File...
Converting File Input • Aswiththeinputfunc.on,thereadline()methodcanonlyreturnstrings
• Ifthefilecontainsnumericaldata,thestringsmustbeconvertedtothenumericalvalueusingtheint()orfloat()func.on:
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value=float(line)
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• Thenewlinecharacterattheendofthelineisignoredwhenthestringisconvertedtoanumericalvalue
Writing To A File • Forexample,wecanwritethestring"Hello,World!"toouroutputfileusingthestatement:
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outfile.write("Hello,World!\n")
outfile.write("Numberofentries:%d\nTotal:%8.2f\n"%(count,total))
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• Unlikeprint()whenwri.ngtexttoanoutputfile,youmustexplicitlywritethenewlinecharactertostartanewline
• YoucanalsowriteformaJedstringstoafilewiththewritemethod:
Example: File Reading/Writing • Supposeyouaregivenatextfilethatcontainsasequenceoffloa.ng-pointvalues,storedonevalueperline
• Youneedtoreadthevaluesandwritethemtoanewoutputfile,alignedinacolumnandfollowedbytheirtotalandaveragevalue
• Iftheinputfilehasthecontents32.054.067.580.25115.0
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Example: File Reading/Writing (2) • Theoutputfilewillcontain
32.0054.0067.5080.25115.00--------Total:348.75Average:69.75
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Example One • Openthefiletotal.py
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Common Error • BackslashesinFileNames
• WhenusingaStringliteralforafilenamewithpathinforma.on,youneedtosupplyeachbackslashtwice:
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infile=open("c:\\homework\\input.txt","r")
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• Asinglebackslashinsideaquotedstringistheescapecharacter,whichmeansthenextcharacterisinterpreteddifferently(forexample,‘\n’foranewlinecharacter)
• Whenausersuppliesafilenameintoaprogram,theusershouldnottypethebackslashtwice
Text Input and Output SECTION 7.2
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Text Input and Output • Inthefollowingsec.ons,youwilllearnhowtoprocesstextwithcomplexcontents,andyouwilllearnhowtocopewithchallengesthato^enoccurwithrealdata
• ReadingWordsExample:
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forlineininputFile:line=line.rsplit()
Mary had a little lamb
Mary had a little lamb
input output
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Processing Text Input • Thereare.meswhenyouwanttoreadinputby:
• Eachword• Eachline• Asinglecharacter
• Pythonprovidesmethodssuch:read(),split()andstrip()forthesetasks
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Processingtextinputisrequiredforalmostalltypesofprogramsthat
interactwiththeuser
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Text Input and Output • Pythoncantreataninputfileasthoughitwereacontainerofstringsinwhicheachlinecomprisesanindividualstring
• Forexample,thefollowingloopreadsalllinesfromafileandprintsthem:
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forlineininfile:print(line)
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• Atthebeginningofeachitera.on,theloopvariablelineisassignedthevalueofastringthatcontainsthenextlineoftextinthefile
• Thereisacri.caldifferencebetweenafileandacontainer:• Onceyoureadthefileyoumustcloseitbeforeyoucaniterateoveritagain
An Example of Reading a File • Wehaveafilethatcontainsacollec.onofwords;oneperline:
spam
and
eggs
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Removing The Newline (1) • Recallthateachinputlineendswithanewline(\n)character• Generally,thenewlinecharactermustberemovedbeforetheinputstringisused
• Whenthefirstlineofthetextfileisread,thestringlinecontains
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Removing The Newline (2) • Toremovethenewlinecharacter,applytherstrip()methodtothestring:
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line=line.rstrip()
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• Thisresultsinthestring:
Character Strip Methods
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Character Strip Examples
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Reading Words • Some.mesyoumayneedtoreadtheindividualwordsfromatextfile
• Forexample,supposeourinputfilecontainstwolinesoftextMaryhadaliJlelamb,whosefleecewaswhiteassnow
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Reading Words (2) • Wewouldliketoprinttotheterminal,onewordperline
MaryhadaliJle...
• Becausethereisnomethodforreadingawordfromafile,youmustfirstreadalineandthensplititintoindividualwords
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line=line.rstrip()wordlist=line.split()
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Reading Words (3) • Thesplitmethodreturnsthelistofsubstringsthatresultsfromspliengthestringateachblankspace
• Forexample,iflinecontainsthestring:
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• Itwillbesplitinto5substringsthatarestoredinalistinthesameorderinwhichtheyoccurinthestring:
Reading Words (4) • No.cethatthelastwordinthelinecontainsacomma
• Ifweonlywanttoprintthewordscontainedinthefilewithoutpunctua.onmarks,wecanstripthosefromthesubstringsusingtherstrip()methodintroducedintheprevioussec.on:
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word=word.rstrip(".,?!")
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Reading Words: Complete Example inputFile=open("lyrics.txt","r")
forlineininputFile:
line=line.rstrip()
wordList=line.split()
forwordinwordList:
word=word.rstrip(".,?!")
print(word)
inputFile.close()
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Example Two • Openthefilelyrics.py
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Additional String Splitting Methods
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Additional String Splitting Examples
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