7: Functional programming: Functions, conditionals and recursion

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7 Functional programmingFunctions,
conditionals and recursion

• Mark Elsom-Cook 1999

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Contents

• Variables
• Functions
• Operators
• Problem decomposition
• Conditional statements
• Recursion

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Variables

• Declare them
• var myname
• var pigs, dogs, tadpoles
• Use them
• pigs 23
• Rules
• start with a letter, then any letters or numbers
• No spaces or weird characters
• Case matters

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Object variables

• Declare from instance object
• var myobj new object()
• myobj.height56
• myobj.weight7
• Use as dotted syntax
• writeln(myobj.height)

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Functions

• Use them fname() freda(3, jane, dog)
• Declare them
• function myfunc( myarg, myarg2, myarg3)
• the body
• Built in functions
• Object methods are just functions in objects

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Functions and variables

• Global scope
• Local scope
• Automatic declaration of parameters

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Operators

• Same as functions, but different syntax
• Built-in
• , -, , /, , , -- and assignment versions
• , gt, lt, !, !, typeof, new
• Mostly infix - typeof and new are prefix

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Problem decomposition

• Key concept in computing
• Analyse problem into major components
• Solve the high level problem
• and assume the other bits are already there
• Solve smaller problems till it disappears
• Lots of functions is good!
• E.g. How to solve it, G. Polya

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Example Table generation

• function drawTimetable()
• drawTop()
• drawMiddle()
• drawTail()

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Table generation 2

• function drawTop()
• document.writeln("lttable border2gtlttrgtltthgtlt/thgtltt
  hgtA.M.lt/thgtltthgtP.M.lt/thgtltthgtEveninglt/thgtlt/trgt")
• function drawTail()
• document.writeln("lt/tablegt")

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Table generation 3

• function drawMiddle()
• drawDay("Monday")
• drawDay("Tuesday")
• drawDay("Wednesday")
• drawDay("Thursday")
• drawDay("Friday")
• function drawDay(today)
• document.writeln("lttrgtltth bgcolorbluegt"
  today "lt/thgtlt/trgt")

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Conditional statements

• Actions are carried out depending on some test
• if ( test ) if ( test)
• actions to do actions1
• else actions 2

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Predicates (tests)

• Functions which return true or false
• isNaN(), , lt, lt, gt, gt, !, ,
• Also a set for boolean operations
• Used inside a conditional test part
• Or make up your own!

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Example browser specific code

• Set a variable for different browsers
• var microsoft
• if (navigator.appName Netscape )
• microsoftfalse
• else
• microsofttrue

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Example Reacting to user input

• An age test
• var age
• age window.prompt(How old are you?)
• if (age lt 18)
• document.writeln(You seem bit young)
• else
• if (age gt70)
• document.writeln(Nice to see
  you here!)
• else
• document.writeln(That seems about
  right)

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Recursion

• A really clever idea!
• Simplify a problem by calling a function
• Recurse by calling a function from itself
• A means for doing things repeatedly
• Has initial value, stopping condition,
  simplification, accumulation

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Recursive structures

• Function calls itself
• function countdown(NUM)
• if (NUM 0) Stopping condition
• document.writeln(0)
• else
• document.writeln(NUM)
• countdown(NUM-1 ) recursion/simplification

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Example number sequence

• See overheads

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Example checked user input

• See examples

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Summary

• Functions, variables operators
• Defining vs. Using
• Conditional expressions
• Recursion