runoff.c

#include <cs50.h>
#include <stdio.h>
#include <string.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
{
    string 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, string 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, string 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;
    }

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

    // Keep querying for votes
    for (int i = 0; i < voter_count; i++)
    {

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

            // 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) {
  int valid = false;
  int candidate_no;
  for (int i = 0; i < candidate_count; i++) {
    if (strcmp(candidates[i].name, name) == 0) {
      candidate_no = i;
      valid = true;
      break;
    }
  }
  if (valid) {
    preferences[voter][rank] = candidate_no;
  }
  return valid;
}

// Tabulate votes for non-eliminated candidates
void tabulate(void) {
  for (int voter_i = 0; voter_i < voter_count; voter_i++) {
    int *voter = preferences[voter_i];
    for (int rank = 0; rank < candidate_count; rank++) {
      int candidate_i = voter[rank];
      candidate curr_choice = candidates[candidate_i];
      if (!curr_choice.eliminated) {
        curr_choice.votes++;
	candidates[candidate_i] = curr_choice;
        break;
      }
    }
  }
  return;
}

// Print the winner of the election, if there is one
bool print_winner(void) {
  for (int candidate_i = 0; candidate_i < candidate_count; candidate_i++) {
    candidate curr_candidate = candidates[candidate_i];
    if (curr_candidate.votes > voter_count / 2) {
      printf("%s\n", curr_candidate.name,
	     curr_candidate.votes);
      return true;
    }
  }
  // TODO
  return false;
}

// Return the minimum number of votes any remaining candidate has
int find_min(void) {
  int lowest_vote;

  for(int i=0; i<candidate_count; i++){
    int first_run = i == 0;
    int eliminated_candidate = candidates[i].eliminated;
    if ((first_run || lowest_vote > candidates[i].votes) && !eliminated_candidate)
      lowest_vote = candidates[i].votes;
  }
  return lowest_vote;
}

// Return true if the election is tied between all candidates, false otherwise
bool is_tie(int min) {
  for(int i=0; i<candidate_count; i++){
    int eliminated_candidate = candidates[i].eliminated;
    if (!eliminated_candidate && candidates[i].votes > min)
      return false;
  }
  return true;
}

// Eliminate the candidate (or candidates) in last place
void eliminate(int min) {
  for(int i=0; i<candidate_count; i++){
    if (candidates[i].votes == min)
      candidates[i].eliminated = true;
  }
}