Py4Inf-03-Conditional-PrintPy4Inf-03-Conditional-Print

1、Conditional ExecutionChapter 3Python for Informatics: Exploring IUnless otherwise noted, the content of this course material is licensed under a Creative Commons Attribution 3.0 License.htp:/creativecommons.org/licenses/by/3.0/.Copyright 2010- Charles R. SeveranceConditional StepsOutput:SmalerFinisProgram:x = 5if x 20: print Bigerprint Finisx = 5X 20 ?print Bigerprint FinisYesYesComparison OperatorsBoolean expressions ask a question and produce a Yes or No result which we use to control program

2、flowBoolean expressions using comparison operators evaluate to - True / False - Yes / NoComparison operators lok at variables but do not change the variableshttp:/en.wikipedia.org/wiki/George_BooleRemember: “=” is used for asignment.Python Meaning= Greater than or Equal Greater than!= Not equalComparison Operatorsx = 5if x = 5 : print Equals 5if x 4 :print Greater than 4if x = 5 :print Greater than or Equal 5if x Preferences - Language Menu/Tab SettingsTextWrangler: TextWrangler - Preferences -

3、Editor DefaultsPython cares a *lot* about how far line is indented. If you mix tabs and spaces, you may get “indentation errors” even if everything loks finePlease do this now while you are thinking about it so we can al stay sane.This wil save you much unnecessary pain.x = 5if x 2 :print Biger than 2print Stil bigerprint Done with 2for i in range(5) :print iif i 2 : print Biger than 2print Done with i, ix = 5if x 2 :# commentsprint Biger than 2# dont matterprint Stil biger# but can confuse youp

4、rint Done with 2# if you dont line # them upincrease / maintain after if or fordecrease to indicate end of blockblank lines and comment lines ignoredMental begin/end squaresx = 5if x 2 :print Biger than 2print Stil bigerprint Done with 2for i in range(5) :print iif i 2 : print Biger than 2print Done with i, ix = 5if x 2 :# commentsprint Biger than 2# dont matterprint Stil biger# but can confuse youprint Done with 2# if you dont line # them upx 1print More than onex 1 :print More than oneif x 1pr

5、int More than onex 1 :print More than oneif x 1print More than onex 1 :print More than oneif x 2print BigeryesnoX = 4print Not bigerprint All DoneTwo-way using else :x = 4if x 2 :print Bigerelse :print Smalerprint All donex 2print BigeryesnoX = 4print Smalerprint All DoneTwo-way using else :x = 4if x 2 :print Bigerelse :print Smalerprint All donex 2print BigeryesnoX = 4print Smalerprint All DoneMulti-wayif x = 2 : print Two or moreelse :print Something elseWhich wil never print?The try / except

6、StructureYou surround a dangerous section of code with try and except.If the code in the try works - the except is skippedIf the code in the try fails - it jumps to the except section$ cat notry.py astr = Hello Bobistr = int(astr)print First, istrastr = 123istr = int(astr)print Second, istr$ python notry.py Traceback (most recent cal last):File notry.py, line 2, in istr = int(astr)ValueError: invalid literal for int() with base 10: Hello BobThe program stops hereAllDoneSoftwareInputDevicesCentra

7、lProcessingUnitMainMemoryOutputDevicesSecondaryMemoryGenericComputer$ cat tryexcept.py astr = Hello Bobtry:istr = int(astr)except:istr = -1print First, istrastr = 123try:istr = int(astr)except:istr = -1print Second, istr$ python tryexcept.py First -1Second 123When the first conversion fails - it just drops into the except: clause and the program continues.When the second conversion succeeds - it just skips the except: clause and the program continues.try / exceptastr = Bobastr = Bobtry:print Hel

8、loistr = int(astr)print Thereexcept:istr = -1print Done, istrprint Helloprint Thereistr = int(astr)print Done, istristr = -1Safety netSample try / except$ python trynum.py Enter a number:42Nice work$ python trynum.pyEnter a number:fourtytwoNot a number$rawstr = raw_input(Enter a number:)try: ival = int(rawstr)except: ival = -1if ival 0 : print Nice workelse: print Not a numberExerciseRewrite your pay computation to give the employee 1.5 times the hourly rate for hours worked above 40 hours.Enter

9、 Hours: 45Enter Rate: 10 Pay: 475.0475 = 40 * 10 + 5 * 15ExerciseRewrite your pay program using try and except so that your program handles non-numeric input gracefully.Enter Hours: 20 Enter Rate: nine Error, please enter numeric inputEnter Hours: forty Error, please enter numeric inputSummaryComparison operators = = !=Logical operators: and or notIndentationOne Way DecisionsTwo way Decisions if : and else :Nested DecisionsMultiway decisions using elifTry / Except to compensate for errorsShort circuit evaluations

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