Tag: answer to runoff problem

This is my solution to cs50 psets 3 runoff problem. In this problem set, we have to create a program that runs a program of the runoff election.

In the previous post, I brought you my solution to the CS50 psets 3 plurality problem. But that program follows a very simple algorithm to determine the winner of an election. It is simply every voter gets one vote and the candidate with the most votes wins.

But what if two candidates get the same amount of votes? the above program does not have a solution for that. In this runoff, the method gives a solution to that problem. So in this method, each voter gets 3 chances to vote and rank their favorite candidate.

When you read the full description of psets 3 you’ll understand what is runoff method and how it solve the voting problem. I’m not going to explain it here again.

Let’s see what is my solution to the cs50 psets 3 runoff problem. Here is my code,

#include <cs50.h>
#include <stdio.h>
#include <string.h>
#include <math.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, string name)
{
    // TODO
    for (int i = 0; i < candidate_count; i++)
    {
        if (strcmp(name, candidates[i].name) == 0)
        {
            preferences[voter][rank] = i;
            return true;
        }
    }
    return false;
}

// Tabulate votes for non-eliminated candidates
void tabulate(void)
{
    // TODO
    for (int i = 0; i < voter_count; i++)
    {
        for (int j = 0; j < candidate_count; j++)
        {
            if (candidates[preferences[i][j]].eliminated == false)
            {
                candidates[preferences[i][j]].votes += 1;
                break;
            }
        }
    }
    return;
}

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

// Return the minimum number of votes any remaining candidate has
int find_min(void)
{
    // TODO
    int min_votes = voter_count;
    for (int i = 0; i < candidate_count; i++)
    {
        if (candidates[i].eliminated == false && candidates[i].votes < min_votes)
        {
            min_votes = candidates[i].votes;
        }
    }
    return min_votes;
}

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

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

Hope this helps you!