CS50 Week3: Problem Set, Runoff

하버드 CS50 강의 3주차 Problem Set 과제 Runoff 의 풀이 과정을 다룹니다.
C언어로 이차원 배열을 순회하고, 구조체로 구현된 선거 후보자 데이터를 다루는 문제입니다.

Intro

즉석결선투표제를 구현하는 문제

./runoff Alice Bob Charlie
Number of voters: 5
Rank 1: Alice
Rank 2: Bob
Rank 3: Charlie

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

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

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

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

Alice

Template

이번에도 코드 템플릿이 주어져 있다.
함수를 채워넣기만 하면 된다.

#include <stdio.h>
#include "cs50.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
    return false;
}

// Tabulate votes for non-eliminated candidates
void tabulate(void)
{
    // TODO
    return;
}

// Print the winner of the election, if there is one
bool print_winner(void)
{
    // TODO
    return false;
}

// Return the minimum number of votes any remaining candidate has
int find_min(void)
{
    // TODO
    return 0;
}

// Return true if the election is tied between all candidates, false otherwise
bool is_tie(int min)
{
    // TODO
    return false;
}

// Eliminate the candidate (or candidates) in last place
void eliminate(int min)
{
    // TODO
    return;
}

Code

vote 함수

preferences 배열에 투표자의 선호도를 기록하면 됩니다.

// Record preference if vote is valid
bool vote(int voter, int rank, string name)
{
    for (int i = 0; i < candidate_count; i++)
    {
        if (strcmp(candidates[i].name, name) == 0)
        {
            preferences[voter][rank] = i; // Record the candidate's index in the preferences array
            return true;
        }
    }
    return false;
}

tabulate 함수

i 번째 투표자의 선호도가 가장 높으면서 아직 탈락하지 않은 후보에게 투표를 기록합니다.
while문을 사용했습니다.

// Tabulate votes for non-eliminated candidates
void tabulate(void)
{
    // Loop through each voter
    for (int i = 0; i < voter_count; i++)
    {
        // Loop through each voter's preference(rank)
        int j = 0;
        while (candidates[preferences[i][j]].eliminated) // If candidate is eliminated, next preference is checked
        {
            j++;
        } 
        candidates[preferences[i][j]].votes++;
    }
    return;
}

print_winner 함수

과반 이상의 득표를 받은 후보가 있는지 확인하고 출력.

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

find_min 함수

실수로 대소비교 방향을 잘못잡아서 한 10분을 해맸다.

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

is_tie 함수

모든 후보 동점인지 확인

// 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++)
    {
        if (!candidates[i].eliminated && candidates[i].votes != min)
        {
            return false;
        }
    }
    return true;
}

eliminate 함수

마지막 후보 탈락

// 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)
        {
            candidates[i].eliminated = true;
        }
    }
    return;
}

후기

문제의 난이도가 엄청 높지는 않지만, 이전보다 코드 길이면서 간단한 실수도 찾아내는 게 힘들어짐을 느꼈다.

덕분에 VSC 디버거 사용이 조금 익숙해졌다....