WEBVTT

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In this demo I'm going to show you how a parser object

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does its job of turning whatever the player types in

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at the programs user interface in to a command object

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As you saw in a previous demo

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a command object represents a command 

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with one or two command words

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if the first of those words is a question mark

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that means that the command is not understood

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or doesn't make sense in the context of the program

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where the command class is being used.

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So lets see what happens when we

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create a parser object

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I'm going to call the constructor

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and I'm prompted for an array of command words

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an array of valid command words.

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So its not the parsers responsibility

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to know what are the valid commands in the game

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the parser has to be told what the valid commands are.

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I can give the parser object any list of commands I like

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and it will use that list in deciding what is and

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what isn't a valid command.

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So lets give it some commands

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how about run

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jump

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and

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stop

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that will do fine for now

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and the objects created in the object bench.

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So lets try parsing some input 

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I'm going to call the method

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getCommand

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that opens the terminal window and opens a prompt

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so I can type some input, so lets try run

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fast that should be a valid command according

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to the command list that

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I provided for the parser

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and that returns a command object

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if I inspect that command object

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it has two words, a command word and a second word

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which are run and fast.

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What happens if there are too many words in the input

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lets try that lets getCommand again

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again I have a command prompt so I can try jump

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very

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high that should be valid

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that its a valid command word

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the only problem is there is too many words in the input.

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So lets try that

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again we get a command object, inspect that

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and it only has two words the third word

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of the input has been ignored

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and a two word command has been created.

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what if I enter a one word command

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lets try getCommand again

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and this time I'm just going to type stop

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again its a valid command word

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but I'm only typing one word

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return there and

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re-inspect the command that has returned

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the command has the command word stop

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and the second word is null.

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Finally lets type something that isn't a valid command

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call getCommand again this time i'll type

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literally some nonsense at the command line

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and that does return a command object

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if I inspect that object

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you can see the command word is a question mark

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so thats an unknown or invalid command.

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So a parser object if you tell it what 

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the valid command words are

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can take pretty much any input thats typed in

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and turn that into a command object

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which represents either a valid command

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if the input matches one of the valid command words

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or an invalid command.

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So you've seen in these demos that the command class

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doesn't actually have any knowledge of what

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are the valid command words for this game

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and the parser class doesn't have that knowledge either

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so who does know

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well thats the players responsibility

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lets look at the player class

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the player class has a field called

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commands which is an array 

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containing the valid command words

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for this game go, quit and help

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when a player takes a turn using the takeTurn method

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that array of commands is given

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to the parser object that is created there

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that parser object is then used by the player

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to get a command

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and the player can then process that command object

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to do whatever action needs to be done in that turn.