2533CIT Iteration, Arrays, Static Properties and Math

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2533CIT Iteration, Arrays, Static Properties
and Math

• Coridyn Fitzgerald-Hood (2007)

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Arrays

• Arrays are a simple data structure used to group
  related information.
• Instead managing multiple variables we can now
  just manage 1.

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• This is longwinded
• And unnecessarily repetitious.

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• This is a little better
• At least we only have to pass one parameter to
  the function.

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• We can still do better.
• Array shorthand notation
• Use the square brackets
• item1, item2, , itemN

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• var appleBoxArray
• We now have a number of objects stored in their
  own discrete location within the array.

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Accessing Array Items

• Once we have items in the array we need to get
  them back out
• We access the items in an array using the square
  brackets and a numbered index.

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• Array indices (or indexes) start counting from 0
  (zero)

Item 0
Item 1
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• And continue to length - 1.

Item 4 (length - 1)
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• If we run off the end of the array we receive the
  value undefined
• appleBox5

0
1
2
3
4
5
?
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• Code example

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Array Properties

• Arrays are complex data types so they have
  properties and functions defined on them
• Array.length The number of items in the array.

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• Array.push(objInst)
• Add a new item to the end of the array.

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• Array.pop()Object
• Remove an item from the end of the list (and
  return it).

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• Array.unshift(objInst)
• Add a new item to the start of the array.

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• Array.shift()Object
• Remove an item from the start of the array (and
  return it).

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• Array.splice(startIndex, deleteCountNumber,
  valueObject)
• Adds or removes items from the array.

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• Delete n items
• Insert item

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Array Overview

• Arrays hold multiple values in an indexed
  variable.
• Valid indices are between 0 and length 1
• We can add and remove items after creating the
  array.

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Array Exercises
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• Using the Array functions from the previous
  slides how could we mimic the following
  Collection data structures?
• Stack Last on, first off
• Queue First on, first off

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Iteration

• iteration - it-uh-rey-shuh n
• noun
• 1.the act of repeating a repetition.

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Iteration

• We use loops for performing repetitive tasks.
• There are three types of loops
• While,
• Dowhile,
• For.

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Terminology

• The continuation test performed in a loop is also
  known as the guard.
• i.e. it guards the contents of the loop.

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While Loop

• The while loop is a test-first loop
• The guard is evaluated before entering the loop.
• The loop is executed zero or more times.

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Dowhile Loop

• The dowhile loop is a test-last loop
• The guard is evaluated after the previous
  iteration.
• This loop is executed one or more times.

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For Loop

• The for loop is semantically equivalent to a
  while loop
• Adds three statements to the head of the loop,
• Considered clearer and less error-prone than
  while loops,
• Executed zero or more times.

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For Loop continued

• Initialisation
• Setup loop variables prior to execution,
• Comma-separated values,
• Code executed once only.
• Guard
• Continuation test.
• Tested before every iteration.
• Post
• Executed after each iteration.

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For Loop continued
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While Loop Example
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Dowhile Example
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Storing Results

• We often need to store the results of repetitive
  actions
• Individual variables are obviously unwieldy,
• Arrays are perfect for this

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• Now that we have a collection of MovieClips we
  can perform batch processing.

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Iterators

• There is a special fourth type of loop called a
  Property Iterator or forin loop.
• Can be applied to Arrays and Object property
  lists.

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• The forin loop accesses the property names of an
  object instance, which we use to access the
  object values.
• The iterator (the i value) is a string.
• We must use array notation when accessing the
  properties.

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Array Iteration Example
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Static properties
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• Static properties are defined as properties that
  belong to the class definition not an instance
  of the class.
• Static properties are shared amongst all instance
  of the class
• a change made in one instance is reflected in all
  others.

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Definition of static property
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Changing a static property
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Examples of Static Properties

• Static properties are often used for values that
  dont change, or properties that need to be
  easily accessible from outside a class.
• The stage exposes some static properties
• Stage.width
• Stage.height
• Note we did not instantiate the Stage class.

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Static Functions

• We can also define static functions on a class.

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Factory class example
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Limitations of Static Functions

• Static functions cannot access instance
  variables.

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The Math Class
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• Actionscript provides us with a Math class that
  defines a number of useful (mathematically
  related) properties and functions.
• Well only be looking at a few )

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• Math.min(nNumber, mNumber)Number
• Returns the smaller of the two numbers
• Math.max(nNumber, mNumber)Number
• Returns the larger of the two numbers

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• The max and min functions are useful for clamping
  a range of values,
• That is, ensuring values remain within specified
  values.
• Can you think of any useful examples?

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Long and ugly
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Simplified
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Further simplified
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• Semantically
• Return a value no larger than Stage.width.
• Semantically
• Return a value no less than 0.

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• Note The order of the nested min and max
  functions doesnt matter, the comparison values
  do.

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• Math.floor(nNumber)Number
• Return the closest integer less than or equal to
  n.
• Math.ceil(nNumber)Number
• Return the closest integer greater than or equal
  to n.

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• Math.floor(1.2) // Returns 1
• Math.floor(5.0) // Returns 5
• Math.ceil(2.03) // Returns 3
• Math.ceil(-3.3) // Returns -3

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• Math.random()Number
• Return a random value where 0 lt n lt 1

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• Returns a random value between 4 and 11 inclusive.