prg –programming essentials - cvut.cz · •functions, int(), float()and str()convert their...

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PRG– PROGRAMMINGESSENTIALS1

Lecture2– Programflow,Conditionals,Loopshttps://cw.fel.cvut.cz/wiki/courses/be5b33prg/start

MichalReinšteinCzechTechnicalUniversityinPrague,

FacultyofElectricalEngineering,Dept.ofCybernetics,CenterforMachinePerceptionhttp://cmp.felk.cvut.cz/~reinsmic/

reinstein.michal@fel.cvut.cz

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

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PROBLEMSOLVING!2

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

• Problemformulation(input/output)

• Formalism(math?)

• Algorithm(theidea!)

• Implementation(engineering)

• Testing(arewegood?)

3

DATATYPES3

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/variables_expressions_statements.html

• Integers(int) 1,10,124• Strings(str) ”Hello,World!”• Float(float) 1.0,9.999

• StringsinPythoncanbeenclosedineithersinglequotes(')ordoublequotes("),orthreeofeach(''' or """)

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VARIABLES4

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/variables_expressions_statements.html

• Weusevariablestoremember things!• The assignmentstatement givesavaluetoavariable• Donotconfuse= and== !

=isassignment tokensuchthatname_of_variable =value==isoperatortotestequality

• Keypropertyofavariablethatwecanchangeitsvalue• Namingconvention:withfreedomcomesresponsibility!

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VARIABLES5

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/variables_expressions_statements.html

• Thelongerlifethelongername:very_long_name_of_my_var• Themoreimportantthelongername• Meaningfulnamedoesnotaddthemeaningjustbyitself,

thecodemustdothis!• Illegalnamecausesasyntaxerror• Capitals:Variable vsvariable

cannotbeginwithanumber

this$isillegalcharacter

classisreservedkeyword

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KEYWORDS6

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/variables_expressions_statements.html

• Pythonkeywordshavespecial purpose• Alwayschoosenamesmeaningful tohumanreaders• Usecomments(#)and blanklinestoimprovereadability

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BUILT-INFUNCTIONS7

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/variables_expressions_statements.html

• Built-infunctionshavespecial purpose• Studyhttps://docs.python.org/3.4/library/functions.html

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OPERATORS&OPERANDS8

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/variables_expressions_statements.html

• OPERAND OPERATOR OPERAND• Operatorsarespecialtokensthatrepresentcomputationslikeaddition,subtraction,multiplication,division etc

• Thevaluestheoperatorusesarecalledoperands• Whenavariablenameappearsintheplaceofanoperand,itisreplacedwithitsvaluebeforetheoperationisperformed

• Division/ vsfloor division//

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TYPECONVERSION9

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/variables_expressions_statements.html

• Functions,int(),float() andstr() converttheirargumentsintotypesint,float andstr respectively.

• Thetypeconverter float() canturnaninteger,afloat,orasyntacticallylegalstring intoafloat

• Thetypeconverter str() turnsitsargumentintoastring• Onesymbolcanhavedifferentmeaningdependingonthedatatype(s)- try& explore& understand

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ORDEROFOPERATIONS– PEMDAS10

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/variables_expressions_statements.html

• Evaluationdependsonthe rulesofprecedence:1. Parentheses(fororder,readability)2. Exponentiation3. MultiplicationandDivision4. AdditionandSubtraction• Orderleft-to-right evaluationonthesamelevel,withtheexceptionofexponentiation(**)

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OPERATIONSONSTRINGS11

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/variables_expressions_statements.html

• Youcannotperformmathematicaloperationsonstrings,evenifthestringslooklikenumbers

• The + operatorrepresents concatenation,notaddition• The * operatoralsoworksonstrings;itperformsrepetition(oneoftheoperandshastobeastring;theotherhastobeaninteger)

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INPUT12

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/variables_expressions_statements.html

• Built-infunctiontogetinputfromauser:

input(”Messagetotheuser!”)

• Userinputisstoredasstring• Combinewithtypeconversion

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COMPOSITION13

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/variables_expressions_statements.html

• Combinationoftheelementsofaprogram:variables,expressions,statements,andfunctioncalls

• Oneofthemostusefulfeaturesofprogramminglanguages• Takesmallbuildingblocksand compose themintolargerchunks

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MODULUSOPERATOR14

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/variables_expressions_statements.html

• The modulusoperator worksonintegers (integerexpressions)• Definition:modulusistheremainder whenthefirstnumberisdividedbythesecond

• Modulusoperatorisapercentsign%• Syntaxisthesameasforotheroperators• Thesameprecedence asthemultiplication operator

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THEFORLOOP15

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/hello_little_turtles.html

• Thevariable friend atline1isthe loopvariable• Lines2and3arethe loopbody• Theloopbodyisalwaysindented• Theindentation determinesexactlywhatstatements are“inthebodyoftheloop”

• Attheendofeachexecutionofthebodyoftheloop,Pythonreturnstothe for statement,toseeiftherearemoreitemstobehandled,andtoassignthenextoneto theloopvariable

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THEFORLOOP16

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/hello_little_turtles.html

Oneach iteration or pass oftheloop:• Checktoseeiftherearestillmoreitemstobeprocessed• Iftherearenoneleft(the terminatingcondition oftheloop)theloophasfinished

• Ifthereareitemsstilltobeprocessed,theloopvariableisupdated torefertothenextiteminthelist

• Programexecutioncontinuesatthenextstatementaftertheloopbody

• Toexplore:earlybreak,orfor– elseloop

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THEFORLOOP– CONTROLFLOW17

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/hello_little_turtles.html

• Controlflow(controlofthe flowofexecution oftheprogram)

• Asprogramexecutes,theinterpreteralwayskeepstrackofwhichstatementisabouttobeexecuted

• Controlflowuntilnowhasbeenstrictlytoptobottom,onestatementatatime,the for loopchangesthis!

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BOOLEANVALUES&EXPRESSIONS18

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/conditionals.html

• Testconditionsandchangetheprogrambehaviordependingontheoutcomeofthetests

• Boolean valueiseitherTrue orFalse• NamedaftertheBritishmathematician,GeorgeBoole,whofirstformulated Booleanalgebra

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BOOLEANVALUES&EXPRESSIONS19

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/conditionals.html

• Booleanexpression isanexpressionthatevaluatestoproducearesultwhichisaBooleanvalue

• Sixcommon comparisonoperators whichallproducea bool result(differentfromthemathematicalsymbols)

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LOGICALOPERATORS20

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/conditionals.html

• three logicaloperators, and, or,and not,thatallowtobuildmorecomplexexpressionsfromsimpleBooleanexpressions

• semantics(meaning)oftheseoperatorsissimilartonaturallanguageequivalent

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TRUTHTABLES21

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/conditionals.html

Short-circuitevaluation:• OR – iftheexpressionontheleftofthe operatoryieldsTrue,Pythondoesnotevaluatetheexpressionontheright

• AND – iftheexpressionontheleftyields False,Pythondoesnotevaluatetheexpressionontheright.

• Truthtable– listofallthepossibleinputstogivetheresultsforthelogicaloperators

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BOOLEANALGEBRA22

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/conditionals.html

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CONDITIONALEXECUTION23

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/conditionals.html

• ConditionIF– ELSE• Conditionalstatement– theabilitytocheckconditionsandchangethebehavioroftheprogramaccordingly

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CONDITIONALEXECUTION24

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/conditionals.html

• ConditionIFonly• NoELSEstatement• Tocontrolflowonlyforspecificcondition

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CONDITIONALEXECUTION25

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/conditionals.html

• ConditionchainingIF– ELIF– ELSE

• Recommendation:handlealldistinctiveoptionsbyseparatecondition,useelsetohandleallother

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CONDITIONALEXECUTION26

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/conditionals.html

• Nestingconditionsbuildshierarchyofdecisions(decisiontrees)

• Nestingmayreducereadabilityandclarity

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CONDITIONALEXECUTION27

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://book.pythontips.com/en/latest/for_-_else.html

• Earlyreturn/earlybreak• Canbeusedtospeed-upcodeexecution• Specialcondition:FOR– ELSE

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BOOLEANALGEBRA– LOGICOPPOSITES28

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/conditionals.html

• Eachofthesixrelationaloperatorshasalogicalopposite• Recommendation:not operatorsmayreducereadability,uselogicaloppositesinstead

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DEMORGAN‘SLAWS29

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/conditionals.html

• DeMorgan’slaws rulesallowtheexpressionof conjunctions and disjunctions intermsofeachothervia negation

• Example:supposewecanslaythedragononlyifourmagicswordischargedto90%orhigherand wehave100ormoreenergyunitsinourprotectiveshield

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DEMORGAN‘SLAWS30

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/conditionals.html

• Example:supposewecanslaythedragononlyifourmagicswordischargedto90%orhigherand wehave100ormoreenergyunitsinourprotectiveshield

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EXAMPLE31

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

sourcehttp://openbookproject.net/thinkcs/python/english3e/conditionals.html

• Example: completethetable..

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REFERENCES32

03/12/2017 MichalReinštein,CzechTechnicalUniversityinPrague

Thislecturere-usesselectedpartsoftheOPENBOOKPROJECTLearningwithPython3(RLE)

http://openbookproject.net/thinkcs/python/english3e/index.htmlavailableunderGNUFreeDocumentationLicense Version1.3)

• Versiondate:October2012• byPeterWentworth,JeffreyElkner,AllenB.Downey,andChrisMeyers

(basedon2ndeditionbyJeffreyElkner,AllenB.Downey,andChrisMeyers)

• Sourcerepositoryisat https://code.launchpad.net/~thinkcspy-rle-team/thinkcspy/thinkcspy3-rle

• Forofflineuse,downloadazipfileofthehtmlorapdfversionfrom http://www.ict.ru.ac.za/Resources/cspw/thinkcspy3/