runoff.c

#include <stdio.h>
#include <string.h>
#include <stdbool.h>

// Max voters and candidates
#define MAX_VOTERS 100
#define MAX_CANDIDATES 9

// preferences[i][j] is jth preference for voter i
int preferences[MAX_VOTERS][MAX_CANDIDATES];

// Candidates have name, vote count, eliminated status
typedef struct
{
    char *name;
    int votes;
    bool eliminated;
} candidate;

// Array of candidates
candidate candidates[MAX_CANDIDATES];

// Numbers of voters and candidates
int voter_count;
int candidate_count;

// Function prototypes
bool vote(int voter, int rank, char *name);
void tabulate(void);
bool print_winner(void);
int find_min(void);
bool is_tie(int min);
void eliminate(int min);

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

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

    printf("Number of voters: ");
    scanf("%d", &voter_count);
    if (voter_count > MAX_VOTERS)
    {
        printf("Maximum number of voters is %i\n", MAX_VOTERS);
        return 3;
    }

    char name[100];
    // Keep querying for votes
    for (int i = 0; i < voter_count; i++)
    {
        // Query for each rank
        for (int j = 0; j < candidate_count; j++)
        {
            printf("Rank %d: ", j + 1);
            scanf("%s", name);

            // Record vote, unless it's invalid
            if (!vote(i, j, name))
            {
                printf("Invalid vote.\n");
                return 4;
            }
        }

        printf("\n");
    }

    // Keep holding runoffs until winner exists
    while (true)
    {
        // Calculate votes given remaining candidates
        tabulate();

        // Check if election has been won
        bool won = print_winner();
        if (won)
        {
            break;
        }

        // Eliminate last-place candidates
        int min = find_min();
        bool tie = is_tie(min);

        // If tie, everyone wins
        if (tie)
        {
            for (int i = 0; i < candidate_count; i++)
            {
                if (!candidates[i].eliminated)
                {
                    printf("%s\n", candidates[i].name);
                }
            }
            break;
        }

        // Eliminate anyone with minimum number of votes
        eliminate(min);

        // Reset vote counts back to zero
        for (int i = 0; i < candidate_count; i++)
        {
            candidates[i].votes = 0;
        }
    }
    return 0;
}

// Record preference if vote is valid
bool vote(int voter, int rank, char *name)
{
    for (int k = 0; k < candidate_count; k++)
    {
        // If the I/P matches the name of a valid candidate then we save the index (k) of
        // candidate as the [rank]th preference of the voter [voter].
        if (strcmp(candidates[k].name, name) == 0)
        {
            preferences[voter][rank] = k;
            return true;
        }
    }
    return false;
}

// Tabulate votes for non-eliminated candidates
void tabulate(void)
{
    for (int l = 0; l < voter_count; l++)
    {
        for (int m = 0; m < candidate_count; m++)
        {
            if (!candidates[preferences[l][m]].eliminated)
            {
                //
                candidates[preferences[l][m]].votes++;
                break;
            }
        }
    }
    return;
}

// Print the winner of the election, if there is one
bool print_winner(void)
{
    // The number of votes required to win the election would be {voter_count/2} + 1
    // But as we are dealing with integers we can allow majority_votes to be GIF of
    // voter_count/2
    int majority_votes = voter_count / 2;

    for (int n = 0; n < candidate_count; n++)
    {
        if (majority_votes < candidates[n].votes)
        {
            printf("%s\n", candidates[n].name);
            return true;
        }
    }
    return false;
}

// Return the minimum number of votes any remaining candidate has
int find_min(void)
{
    /* Set the min_votes variable to the highest possible value i.e. voter_count
    So that we can set the votes of next non-eliminated voter as the minimum
    and compare that value to the other non-eliminated voters */
    int minimum_votes = voter_count;
    for (int p = 0; p < candidate_count; p++)
    {
        // Check if candidate is both non-eliminated and has lower votes than current
        // minimum_votes count
        if (!candidates[p].eliminated && candidates[p].votes < minimum_votes)
        {
            minimum_votes = candidates[p].votes;
        }
    }
    return minimum_votes;
}

// Return true if the election is tied between all candidates, false otherwise
bool is_tie(int min)
{
    int not_eliminated = 0;
    int not_eliminated_with_min_votes = 0;
    for (int q = 0; q < candidate_count; q++)
    {
        // Count candidates i.e. not eliminated
        if (!candidates[q].eliminated)
        {
            not_eliminated++;
        }

        // Count candidates i.e. not eliminated AND have minimum number of votes
        if (!candidates[q].eliminated && candidates[q].votes == min)
        {
            not_eliminated_with_min_votes++;
        }
    }

    // Check if Tie occurs or not
    if (not_eliminated == not_eliminated_with_min_votes)
    {
        return true;
    }

    else
    {
        return false;
    }
}

// Eliminate the candidate (or candidates) in last place
void eliminate(int min)
{
    for (int r = 0; r < candidate_count; r++)
    {
        // Eliminate candidate(s) with minimum number of votes if they are not already eliminated
        if (!candidates[r].eliminated && candidates[r].votes == min)
        {
            candidates[r].eliminated = true;
        }
    }
    return;
}