ELECTION NIGHTCITS1001

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Example development• Understanding the problem• Understanding the solution• Structuring the solution• Implementing the solution• (Testing the solution)

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Preferential voting• An election has

• M candidates (we will use five candidates ABCDE in examples) • N formal votes• One winner, who needs N/2+1 votes to triumph

• Each voter returns a list of preferences for all candidates• e.g. CAEDB • But not CAE or CAEDA or CAXEDB or …

• http://www.aec.gov.au/Voting/counting/hor_count.htm

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The count• Discard informal votes • Allocate each formal vote to its first preference candidate

• Then in each round of the count• If the leading candidate has enough votes,

• Declare them the winner

• Otherwise, • Identify the candidate X with the fewest votes • Redistribute each of X’s votes to its highest-ranked surviving candidate• Eliminate X

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An example • 100 votes: 12 informal, and 88 formal

• 88/2+1 votes = 45 required to win

• After the initial distribution • A 11, B 14, C 29, D 25, E 9

• No winner yet, so E is eliminated in Round 1 • e.g. vote EABDC would be transferred to A • A 17, B 16, C 29, D 26

• No winner yet, so B is eliminated in Round 2• e.g. vote BEDAC would be transferred to D • A 22, C 31, D 35

• No winner yet, so A is eliminated in Round 3 • C 47, D 41

• C has enough votes to be declared the winner

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What types of entities do we have?• Votes

• Each vote is an expression of a voter’s preferences

• Candidates • Each candidate collects votes until they either win or are eliminated

• Elections • Each election represents one constituency

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The Vote class• What is the state of a Vote?

• The list of preferences for surviving candidates

• What inputs does the constructor need? • The voter’s original list of preferences

• What accessor methods do Votes provide?• Who the Vote is currently for:

public char getFirstPreference(String losers)

• What other methods do Votes provide? • A test for whether the Vote is formal:

public boolean isFormal(String candidates)

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The Candidate class• What is the state of a Candidate?

• Their name, and their current pile of Votes

• What inputs does the constructor need? • Their name

• What accessor methods do Candidates provide? • getName(), getVotes(), getCount()

• What other methods do Candidates provide? • A way of adding votes to the pile:

public void addVotes(ArrayList<Vote> vs, String losers)• A test for whether the candidate has won:

public boolean isWinner(int noOfVotes)

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The Election class• What is the state of an Election?

• A pile of Votes for each surviving Candidate

• What inputs does the constructor need? • Where to get the voting papers and the candidates’ names from

• What accessor methods do Elections provide? • None – “top-level” class • Although there could be a higher class with a collection of Elections

• What other methods do Elections provide? • The constructor could do the initial distribution • A method to perform one round of the count • Probably several private methods to structure the code

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Implementation – Class Diagram

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Implementation questions – Vote• What type should the list of preferences be?

• If the candidates’ names are just one char each, a simple String will do

• Otherwise a String[] or an ArrayList<String>, depending on…

• Should we delete the names of eliminated candidates, or just ignore them? • We could ignore them when looking for the next preference • We could delete them as needed • We could delete all mentions of c at the time they are

eliminated

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Implementation questions – Candidate

• What type should the pile of Votes be?• For most candidates, the pile will grow during the count • So an ArrayList<Vote> is best

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Implementation questions – Election

• What type should the pile of Candidates be?• Again, an ArrayList<Candidate> is best

• Should we delete eliminated Candidates, or just ignore them?

• We can use multiple constructors for testing:

public Election(String candidates, String votes){…}

public Election(String votes){this(“candidates.txt”, votes);}

public Election(){this(“votes.txt”);}