9: Arrays and iteration

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9 Arrays and iteration

• Mark Elsom-Cook 1999

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Contents

• Collections of data
• Accessing arrays
• When to use arrays
• Recursion on arrays
• Iteration on arrays

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Collections of data

• Variable a thing
• (e.g. integer, string)
• Object a group
• Related things
• Possibly different types
• Accessed by name
• Array
• Collection of things, (same type)
• Accessed by index (number)

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Declaring arrays

• Need to name it
• Instance of array object
• allocates memory
• Normally(!) set size
• but not in JavaScript
• var pig new Array()

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Accessing arrays

• Array has an index range
• From 0 to ??
• pig.length
• Dynamic in JavaScript
• Out of range error
• This is square brackets!
• pig0Hello Mark,
  pig27Thursday
• document.writeln(pig27)

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Arrays - a picture

• var pignew Array()
• Array extends to size of assignment
• Access above assignment is out-of-range error
• Access of unassigned is undefined error

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When to use arrays

• Lots of things of the same sort, OR
• ...you dont know how many there are, OR
• they are in some sort of order, OR
• ...you want to do something to all of them

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Three ways to make arrays

• Always use new to allocate memory
• var freeble new Array( )
• no arguments creates an array - Array( )
• one argument creates an array of a size
• Array(15)
• many arguments sets initial values
• Array(Eat, apples, daily, get, big,
  Ears)

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Displaying arrays with index

• Var frood new Array(Before, the, wire)
• document.writeln(1 is frood0 )
• document.writeln(2 is frood1 )
• document.writeln(3 is frood2 )
• document.writeln(4 is frood3 )
• document.writeln(5 is frood4 )
• document.writeln( There are frood.length
  items)

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Recursion on arrays

• Call with an array showArray(frood)
• Write it like this
• function showArray(arr)
• showallArr(arr, arr.length, 0)
• function showAllArr(a, alen, count)
• if (count gt alen) document.writeln(Finished)
  
• else
• document.writeln(count acount ltbrgt)
• showAllArr(a, alen, count1)

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Iteration

• Iteration does the same thing as recursion
• i.e. execute things repeatedly
• More commonly used
• Sometimes faster and more concise
• Various forms
• e.g. for, while, repeat

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The for statement

• for ( start stop change)
• things to do

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A for example

• for (var x1 xlt10x)
• document.writeln(x (xx))

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Iteration on arrays

• function showArray(arr)
• for (var j0 j lt arr.length j)
• document.writeln(j arrj)

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Summary

• Whats an array?
• Why use one?
• How to access them
• What is iteration?
• Using iteration with arrays