CS50 Tideman项目sort_pairs等函数报错求助

问题概述 #

完成CS50的Tideman选举程序后，sort_pairs、lock_pairs和print_winner函数未通过check50测试，以下是代码、测试失败详情及修复方案。

原代码 #

#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; i++)
    {
        for (int j = i + 1; j < candidate_count; j++)
        {
            int p = ranks[i];
            int q = ranks[j];
            preferences[p][q]++;
        }
    }
    return;
}

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

// Sort pairs in decreasing order by strength of victory
void sort_pairs(void)
{
    // TODO
    for (int j = 0; j < pair_count; j++)
    {
        for (int i = 0; i < pair_count - j - 1; i++)
        {
            int p = pairs[i].winner;
            int q = pairs[i].loser;
            int m = pairs[i + 1].winner;
            int n = pairs[i + 1].loser;

            if (preferences[p][q] > preferences[m][n])
            {
                int temp = preferences[p][q];
                preferences[p][q] = preferences[m][n];
                preferences[m][n] = temp;
            }
        }
    }
    return;
}

// Lock pairs into the candidate graph in order, without creating cycles

bool is_cycle(int candidate_a, int candidate_b)
{
    if (candidate_a == candidate_b)
    {
        return true;
    }
    for (int i = 0; i < candidate_count; i++)
    {
        if (locked[candidate_b][i])
        {
            if (is_cycle(i, candidate_a))
            {
                return true;
            }
        }
    }
    return false;
}

void lock_pairs(void)
{
    // TODO
    for (int i = 0; i < pair_count; i++)
    {
        if (!is_cycle(pairs[i].winner, pairs[i].loser))
        {
            locked[pairs[i].winner][pairs[i].loser] = true;
        }
    }
    return;
}

// Print the winner of the election
void print_winner(void)
{
    // TODO
    int fail_count = 0;
    for (int i = 0; i < candidate_count; i++)
    {
        for (int j = 0; j < candidate_count; j++)
        {
            if (!locked[j][i])
            {
                fail_count++;
            }
        }
        if (fail_count == candidate_count)
        {
            printf("%s\n", candidates[i]);
        }
    }
    return;
}

测试失败详情 #

• :( sort_pairs sorts pairs of candidates by margin of victory：sort_pairs未正确排序
• :) lock_pairs locks all pairs when no cycles：无环时lock_pairs正常
• :( lock_pairs skips final pair if it creates cycle：lock_pairs未正确跳过会形成环的最终边
• :( lock_pairs skips middle pair if it creates a cycle：lock_pairs未正确跳过会形成环的中间边
• :) print_winner prints winner of election when one candidate wins over all others：单一胜者时print_winner正常
• :( print_winner prints winner of election when some pairs are tied：存在平局时print_winner未输出胜者

问题修复 #

1. sort_pairs函数错误 #

错误原因：代码错误地交换了preferences数组中的数值，而非pairs数组里的pair结构体元素。排序的目标是调整pairs数组的顺序，而非修改偏好计数。

修复后代码：

void sort_pairs(void)
{
    for (int j = 0; j < pair_count; j++)
    {
        for (int i = 0; i < pair_count - j - 1; i++)
        {
            int strength_current = preferences[pairs[i].winner][pairs[i].loser];
            int strength_next = preferences[pairs[i+1].winner][pairs[i+1].loser];
            
            // 按胜利强度降序排序，若当前强度小于下一个则交换pair
            if (strength_current < strength_next)
            {
                pair temp = pairs[i];
                pairs[i] = pairs[i+1];
                pairs[i+1] = temp;
            }
        }
    }
    return;
}

2. lock_pairs与is_cycle函数错误 #

错误原因：is_cycle的逻辑方向错误。当尝试添加winner -> loser的边时，需要判断从loser能否到达winner（形成环），原代码的参数传递和递归逻辑颠倒了判断方向。

修复后代码：

bool is_cycle(int start, int target)
{
    if (start == target)
    {
        return true;
    }
    for (int i = 0; i < candidate_count; i++)
    {
        if (locked[start][i])
        {
            if (is_cycle(i, target))
            {
                return true;
            }
        }
    }
    return false;
}

void lock_pairs(void)
{
    for (int i = 0; i < pair_count; i++)
    {
        int winner = pairs[i].winner;
        int loser = pairs[i].loser;
        // 检查从loser到winner是否存在路径，不存在则锁定边
        if (!is_cycle(loser, winner))
        {
            locked[winner][loser] = true;
        }
    }
    return;
}

3. print_winner函数错误 #

错误原因：fail_count的初始化位置错误，未在每个候选人的循环内重置，导致计数累加，无法正确判断是否为源节点（无 incoming 边的节点）。

修复后代码：

void print_winner(void)
{
    for (int i = 0; i < candidate_count; i++)
    {
        int fail_count = 0; // 每个候选人单独初始化计数
        for (int j = 0; j < candidate_count; j++)
        {
            if (!locked[j][i])
            {
                fail_count++;
            }
        }
        // 所有j都没有指向i的边，说明i是源节点（胜者）
        if (fail_count == candidate_count)
        {
            printf("%s\n", candidates[i]);
        }
    }
    return;
}

内容的提问来源于stack exchange，提问作者tiant