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pset 3

plurality

Implement a program that runs a plurality election, per the below.

Terminal window
$ ./plurality Alice Bob Charlie
Number of voters: 4
Vote: Alice
Vote: Bob
Vote: Charlie
Vote: Alice
Alice
plurality.c
#include <cs50.h>
#include <stdio.h>
#include <string.h>


// Max number of candidates
#define MAX 9


// Candidates have name and vote count
typedef struct
{
    string name;
    int votes;
}
candidate;


// Array of candidates
candidate candidates[MAX];


// Number of candidates
int candidate_count;


// Function prototypes
bool vote(string name);
void print_winner(void);


int main(int argc, string argv[])
{
    // Check for invalid usage
    if (argc < 2)
    {
        printf("Usage: plurality [candidate ...]\n");
        return 1;
    }


    // Populate array of candidates
    candidate_count = argc - 1;
    if (candidate_count > MAX)
    {
        printf("Maximum number of candidates is %i\n", MAX);
        return 2;
    }
    for (int i = 0; i < candidate_count; i++)
    {
        candidates[i].name = argv[i + 1];
        candidates[i].votes = 0;
    }


    int voter_count = get_int("Number of voters: ");


    // Loop over all voters
    for (int i = 0; i < voter_count; i++)
    {
        string name = get_string("Vote: ");


        // Check for invalid vote
        if (!vote(name))
        {
            printf("Invalid vote.\n");
        }
    }


    // Display winner of election
    print_winner();
}


// Update vote totals given a new vote
bool vote(string name)
{
    // TODO
    for (int i = 0; i < candidate_count; i++)
    {
        if (strcmp(candidates[i].name, name) == 0)
        {
            candidates[i].votes++;
            return true;
        }
    }
    return false;
}


// Print the winner (or winners) of the election
void print_winner(void)
{
    // TODO
    int max = 0;
    for (int i = 0; i < candidate_count; i++)
    {
        if (max < candidates[i].votes)
        {
            max = candidates[i].votes;
        }
    }
    for (int i = 0; i < candidate_count; i++)
    {
        if (max == candidates[i].votes)
        {
            printf("%s\n", candidates[i].name);
        }
    }
    return;
}

tideman

Implement a program that runs a Tideman election, per the below.

Terminal window
./tideman Alice Bob Charlie
Number of voters: 5
Rank 1: Alice
Rank 2: Charlie
Rank 3: Bob


Rank 1: Alice
Rank 2: Charlie
Rank 3: Bob


Rank 1: Bob
Rank 2: Charlie
Rank 3: Alice


Rank 1: Bob
Rank 2: Charlie
Rank 3: Alice


Rank 1: Charlie
Rank 2: Alice
Rank 3: Bob


Charlie
tideman.c
#include <cs50.h>
#include <stdio.h>
#include <string.h>


// Max number of candidates
#define MAX 9


// preferences[i][j] is number of voters who prefer i over j
int preferences[MAX][MAX];


// locked[i][j] means i is locked in over j
bool locked[MAX][MAX];


// Each pair has a winner, loser
typedef struct
{
    int winner;
    int loser;
}
pair;


// Array of candidates
string candidates[MAX];
pair pairs[MAX * (MAX - 1) / 2];


int pair_count;
int candidate_count;


// Function prototypes
bool vote(int rank, string name, int ranks[]);
void record_preferences(int ranks[]);
void add_pairs(void);
void sort_pairs(void);
void lock_pairs(void);
void print_winner(void);


int main(int argc, string argv[])
{
    // Check for invalid usage
    if (argc < 2)
    {
        printf("Usage: tideman [candidate ...]\n");
        return 1;
    }


    // Populate array of candidates
    candidate_count = argc - 1;
    if (candidate_count > MAX)
    {
        printf("Maximum number of candidates is %i\n", MAX);
        return 2;
    }
    for (int i = 0; i < candidate_count; i++)
    {
        candidates[i] = argv[i + 1];
    }


    // Clear graph of locked in pairs
    for (int i = 0; i < candidate_count; i++)
    {
        for (int j = 0; j < candidate_count; j++)
        {
            locked[i][j] = false;
        }
    }


    pair_count = 0;
    int voter_count = get_int("Number of voters: ");


    // Query for votes
    for (int i = 0; i < voter_count; i++)
    {
        // ranks[i] is voter's ith preference
        int ranks[candidate_count];


        // Query for each rank
        for (int j = 0; j < candidate_count; j++)
        {
            string name = get_string("Rank %i: ", j + 1);


            if (!vote(j, name, ranks))
            {
                printf("Invalid vote.\n");
                return 3;
            }
        }


        record_preferences(ranks);


        printf("\n");
    }


    add_pairs();
    sort_pairs();
    lock_pairs();
    print_winner();
    return 0;
}


// Update ranks given a new vote
bool vote(int rank, string name, int ranks[])
{
    // TODO
    for (int i = 0; i < candidate_count; i++)
    {
        if (strcmp(name, candidates[i]) == 0)
        {
            ranks[rank] = i;
            return true;
        }
    }
    return false;
}


// Update preferences given one voter's ranks
void record_preferences(int ranks[])
{
    // TODO
    for (int i = 0; i < candidate_count - 1; i++)
    {
        for (int j = i + 1; j < candidate_count; j++)
        {
            preferences[ranks[i]][ranks[j]]++;
        }
    }
    return;
}


// Record pairs of candidates where one is preferred over the other
void add_pairs(void)
{
    // TODO
    int n = 0;
    for (int i = 0; i < candidate_count - 1; i++)
    {
        for (int j = i + 1; j < candidate_count; j++)
        {
            if (preferences[i][j] > preferences[j][i])
            {
                pairs[n].winner = i;
                pairs[n].loser = j;
                n++;
            }
            else if (preferences[j][i] > preferences[i][j])
            {
                pairs[n].winner = j;
                pairs[n].loser = i;
                n++;
            }
        }
    }
    pair_count = n;


    return;
}


// Sort pairs in decreasing order by strength of victory
void sort_pairs(void)
{
    // TODO
    int k;
    int max;
    pair temp;
    for (int i = 0; i < pair_count; i++)
    {
        max = -1;
        for (int j = i; j < pair_count; j++)
        {
            if (max < preferences[pairs[j].winner][pairs[j].loser])
            {
                max = preferences[pairs[j].winner][pairs[j].loser];
                k = j;
            }
        }
        temp = pairs[k];
        pairs[k] = pairs[i];
        pairs[i] = temp;
    }
    return;
}


// Lock pairs into the candidate graph in order, without creating cycles
void lock_pairs(void)
{
    // TODO
    bool cycle;
    locked[pairs[0].winner][pairs[0].loser] = true;
    for (int i = 1; i < pair_count; i++)
    {
        cycle = false;
        if (pairs[i].winner == pairs[i - 1].loser)
        {
            for (int j = 0; j < i; j++)
            {
                if (pairs[j].winner == pairs[i].loser)
                {
                    cycle = true;
                }
            }
        }
        else if (pairs[i].loser == pairs[i - 1].winner)
        {
            for (int j = 0; j < i; j++)
            {
                if (pairs[j].loser == pairs[i].winner)
                {
                    cycle = true;
                }
            }
        }
        if (cycle == false)
        {
            locked[pairs[i].winner][pairs[i].loser] = true;
        }
    }
    return;
}


// Print the winner of the election
void print_winner(void)
{
    // TODO
    bool winner[candidate_count];
    for (int i = 0; i < candidate_count; i++)
    {
        winner[i] = true;
    }


    for (int i = 0; i < candidate_count; i++)
    {
        for (int j = 0; j < candidate_count; j++)
        {
            if (locked[i][j] == true)
            {
                winner[j] = false;
            }
        }
    }


    for (int i = 0; i < candidate_count; i++)
    {
        if (winner[i] == true)
        {
            printf("%s\n", candidates[i]);
        }
    }
    return;
}


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