DesignStrategies2:Usingatemplate

CS5010ProgramDesignParadigmsLesson2.1

©MitchellWand,2012-2014ThisworkislicensedunderaCreative Commons Attribution-NonCommercial 4.0 International License. 1

Generalization

OverConstants

OverExpressions

OverContexts

OverDataRepresentations

OverMethodImplementations

MixedData

DataRepresentations

Basics

RecursiveData

FunctionalData

Objects&Classes

Stateful Objects

DesignStrategies

Combinesimplerfunctions

Useatemplate

Callamoregeneralfunction

CommunicateviaState

Module02

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DivideintoCases

Generalization

OverConstants

OverExpressions

OverContexts

OverDataRepresentations

OverMethodImplementations

MixedData

DataRepresentations

Basics

RecursiveData

FunctionalData

Objects&Classes

Stateful Objects

DesignStrategies

Combinesimplerfunctions

Useatemplate

Callamoregeneralfunction

CommunicateviaState

Lesson2.1

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DivideintoCases

Introduction

• Inthislesson,wewillshowhowtotakeapartnon-scalardatausingtheobservertemplateforthatkindofdata.

• Thisisthestrategyyouwilluseforthevastmajorityofyourfunctions.

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Let'sseewhereweare

TheFunctionDesignRecipe

1.DataDesign

2.ContractandPurposeStatement

3. ExamplesandTests

4.DesignStrategy

5.FunctionDefinition

6.Program Review

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TheSixPrinciplesofthiscourse

1.ProgrammingisaPeopleDiscipline

2.RepresentInformationasData;InterpretDataasInformation

3. Programsshouldconsistoffunctionsandmethodsthatconsumeandproducevalues

4.DesignFunctions Systematically

5.DesignSystemsIteratively

6.Passvalueswhenyoucan,sharestateonlywhenyoumust.

DesignStrategies

1.Combine simplerfunctions

2. Usetemplatefor<datadef>on<vble>

3.Divideintocaseson<condition>

4.UseHOF<mapfn>on<vble>

5.Callamoregeneralfunction

Useadestructortemplate

• Usedwhentheproblemcanbesolvedbyexaminingapieceofnon-scalardata.

• Slogan:

Theshapeofthedatadeterminestheshapeof

theprogram.6

Whatdoesitmeanto“examine”apieceofdata?

• Ifthedataiscompounddata,thismeansextractingitsfields.

• Ifthedataisitemizationdata,thismeansdeterminingwhichvariantthedatais.

• Ifthedataismixeddata,thismeansdeterminingwhichvariantthedatais,andthenextractingitsfields,ifany.

• Everydatadefinitionincludesatemplatethatshowshowthisexaminationprocessistobeorganized.

• Writingafunctionusingatemplateisaccomplishedbyfillingintheblanksinthetemplate.– Definitionof"fillingintheblank"tocomeinSlide11.

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FromTemplatetoFunctionDefinition

RecipeforUsingaTemplate1.Make acopyofthetemplateanduncommentit2.Fill inthefunctionnameandaddmoreargumentsifneeded3.Thestrategyis“Usetemplatefor<datadef>on<vble>,”where<datadef>isthekindofdatayouaretakingapart,and<vble>isthevariablewhosevalueyouarelookingat.4.Fillintheblanksinthetemplatebycombiningtheargumentsandthevaluesofthefieldsusingsimplerfunctions.

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Example:book-receipts;; book-receipts : Book NonNegInt -> NonNegInt;; GIVEN: a Book and the number of copies sold;; RETURNS: the total receipts from the sales of the ;; given book. Ignores the number of copies on hand.;; EXAMPLE:;; (book-receipts ;; (make-book "Felleisen" "HtdP2" 13 2795) 100);; = 279500

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Todothis,we’llneedtolookinsidetheBooktoseeitsprice,sowe’llusethe

Booktemplate

1.Makeacopyofthetemplateanduncommentit

(define (book-fn b)(...(book-author b)(book-title b)(book-on-hand b)(book-price b)))

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2.Fillinthefunctionnameandaddmoreargumentsifneeded

(define (book-receipts b sales)(...(book-author b)(book-title b)(book-on-hand b)(book-price b)))

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3.Writedownthestrategy

;; STRATEGY: Use template for Book on b.(define (book-receipts b sales)(...(book-author b)(book-title b)(book-on-hand b)(book-price b)))

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4.Fillintheblanksinthetemplate

;; STRATEGY: Use template for Book on b.(define (book-receipts b sales)(* (book-price b) sales))

Thingswedidn’tuse:(book-author b)(book-title b)(book-on-hand b)

That’sOK!

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Wesaid:“4.Fillintheblanksinthetemplatebycombiningtheargumentsandthevaluesofthefieldsusingsimplerfunctions.”

Example:nextstateoftrafficlight;; DATA DEFINITION:;; a TrafficLightState (TLState) is one of;; -- "red";; -- "yellow" ;; -- "green";; INTERPRETATION: self-evident

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ContractandPurposeStatement;; next-state : TLState -> TLState;; GIVEN: a TLState;; RETURNS: the TLState that follows the given TLState;; EXAMPLES:;; (next-state "red") = "green";; (next-state "yellow") = "red";; (next-state "green") = "yellow"

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1.Makeacopyofthetemplateanduncommentit

(define (tls-fn state)(cond[(string=? state "red") ...][(string=? state "yellow") ...][(string=? state "green") ...]))

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2.Fillinthefunctionnameandaddmoreargumentsifneeded

(define (next-state state)(cond[(string=? state "red") ...][(string=? state "yellow") ...][(string=? state "green") ...]))

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3.Fillinthestrategy;; STRATEGY: Use template for TLState on state

(define (next-state state)(cond[(string=? state "red") ...][(string=? state "yellow") ...][(string=? state "green") ...]))

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4.Fillintheblanks;; STRATEGY: Use template for TLState on state

(define (next-state state)(cond[(string=? state "red") ...][(string=? state "yellow") ...][(string=? state "green") ...]))

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Whatistheanswerfor“red”?

4.Fillintheblanks;; STRATEGY: Use template for TLState on state

(define (next-state state)(cond[(string=? state "red") "green"][(string=? state "yellow") ...][(string=? state "green") ...]))

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Whatistheanswerfor“red”?

Answer(fromexamples):“green”

4.Fillintheblanks;; STRATEGY: Use template for TLState on state

(define (next-state state)(cond[(string=? state "red") "green"][(string=? state "yellow") "red"][(string=? state "green") ...]))

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Whatistheanswerfor“yellow”?

Answer(fromexamples):“red”

4.Fillintheblanks;; STRATEGY: Use template for TLState on state

(define (next-state state)(cond[(string=? state "red") "green"][(string=? state "yellow") "red"][(string=? state "green") "yellow"]))

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Whatistheanswerfor“green”?

Answer(fromexamples):“yellow”

Workingwithotherkindsofdata

• We'veseenhowtousetemplatesforcompounddataanditemizationdata

• Mixeddataworksthesameway.• Copythetemplate,uncommentit,andfillinthemissingpieces.That'sit!

• Ifyou'vethoughthardenoughaboutyourfunction,fillingintheblanksiseasy.

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Whatcanyouputintheblanks?

• Wesaid:Fillintheblanksinthetemplatebycombiningtheargumentsandthevaluesofthefieldsusingsimplerfunctions.

• Thismeans:– Youdon'thavetouseallofthefields– Youcanuseafieldtwice– Youdon'thavetousethefields"inorder"

• Butithastobesimple,asinLesson1.7

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NextSteps

• Study02-1-book-receipts.rktand02-2-traffic-light.rktintheExamplesfolder.– Besuretofinishtheprevious-state examplein02-2-traffic-light.rkt

• Ifyouhavequestionsorcommentsaboutthislesson,postthemonthediscussionboard.

• DotheGuidedPractices• Goontothenextlesson.

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