Py4Inf-03-ConditionalPy4Inf-03-Conditional

Conditional ExecutionChapter 3Python for Informatics: Exploring Informationwww.py4inf.comUnless otherwise noted, the content of this course material is licensed under a Creative Commons Attribution 3.0 License.http:/creativecommons.org/licenses/by/3.0/.Copyright 2010- Charles R. SeveranceConditional StepsOutput:SmallerFinisProgram:x = 5if x 20: print 'Bigger'print 'Finis'x = 5X 20 ?print 'Bigger'print 'Finis'YesYesComparison OperatorsBoolean expressions ask a question and produce a Yes or No result which we use to control program flowBoolean expressions using comparison operators evaluate to - True / False - Yes / NoComparison operators look at variables but do not change the variableshttp:/en.wikipedia.org/wiki/George_BooleRemember: “=” is used for assignment.Python Meaning= Greater than or Equal> Greater than!= Not equalComparison Operatorsx = 5if x = 5 : print 'Equals 5'if x > 4 :print 'Greater than 4'if x >= 5 :print 'Greater than or Equal 5'if x Preferences -> Language Menu/Tab SettingsTextWrangler: TextWrangler -> Preferences -> 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 looks finePlease do this now while you are thinking about it so we can all stay sane.This will save you much unnecessary pain.x = 5if x > 2 :print 'Bigger than 2'print 'Still bigger'print 'Done with 2'for i in range(5) :print iif i > 2 : print 'Bigger than 2'print 'Done with i', ix = 5if x > 2 :# commentsprint 'Bigger than 2'# dont matterprint 'Still bigger'# but can confuse youprint '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 'Bigger than 2'print 'Still bigger'print 'Done with 2'for i in range(5) :print iif i > 2 : print 'Bigger than 2'print 'Done with i', ix = 5if x > 2 :# commentsprint 'Bigger than 2'# dont matterprint 'Still bigger'# but can confuse youprint 'Done with 2'# if you dont line # them upx > 1print 'More than one'x 1 :print 'More than one'if x 1print 'More than one'x 1 :print 'More than one'if x 1print 'More than one'x 1 :print 'More than one'if x 2print 'Bigger'yesnoX = 4print 'Not bigger'print 'All Done'Two-way using else :x = 4if x > 2 :print 'Bigger'else :print 'Smaller'print 'All done'x > 2print 'Bigger'yesnoX = 4print 'Smaller'print 'All Done'Two-way using else :x = 4if x > 2 :print 'Bigger'else :print 'Smaller'print 'All done'x > 2print 'Bigger'yesnoX = 4print 'Smaller'print 'All Done'Multi-wayif x = 2 : print 'Two or more'else :print 'Something else'Which will never print?The try / except 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 Bob'istr = int(astr)print 'First', istrastr = '123'istr = int(astr)print 'Second', istr$ python notry.py Traceback (most recent call last):File "notry.py", line 2, in istr = int(astr)ValueError: invalid literal for int() with base 10: 'Hello Bob'The program stops hereAllDone SoftwareInputDevicesCentralProcessingUnitMainMemoryOutputDevicesSecondaryMemoryGenericComputer$ cat tryexcept.py astr = 'Hello Bob'try:istr = int(astr)except:istr = -1print 'First', istrastr = '123'try: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 = 'Bob'astr = 'Bob'try:print 'Hello'istr = int(astr)print 'There'except:istr = -1print 'Done', istrprint 'Hello'print 'There'istr = 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 work'else: print 'Not a number'ExerciseRewrite your pay computation to give the employee 1.5 times the hourly rate for hours worked above 40 hours.Enter 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