CS50决选算法中find_min函数check50检测异常问题

正在完成CS50课程的决选选举算法开发，check50检测显示多数功能正常，但当所有候选人票数相同时，find_min函数未返回正确最小值的测试项失败。自行测试3位候选人、3位选民且每位选民投票给不同候选人的场景时，程序打印出的最小值为预期的1，也能正确识别平局，但check50仍判定该测试失败。

check50检测结果 #

:) runoff.c exists
:) runoff compiles
:) vote returns true when given name of candidate
:) vote returns false when given name of invalid candidate
:) vote correctly sets first preference for first voter
:) vote correctly sets third preference for second voter
:) vote correctly sets all preferences for voter
:) tabulate counts votes when all candidates remain in election
:) tabulate counts votes when one candidate is eliminated
:) tabulate counts votes when multiple candidates are eliminated
:) tabulate handles multiple rounds of preferences
:) print_winner prints name when someone has a majority
:) print_winner returns true when someone has a majority
:) print_winner returns false when nobody has a majority
:) print_winner returns false when leader has exactly 50% of vote
:) find_min returns minimum number of votes for candidate
:( find_min returns minimum when all candidates are tied
    find_min did not identify correct minimum
:) find_min ignores eliminated candidates
:) is_tie returns true when election is tied
:) is_tie returns false when election is not tied
:) is_tie returns false when only some of the candidates are tied
:) is_tie detects tie after some candidates have been eliminated

原代码 #

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

// Tabulate votes for non-eliminated candidates
void tabulate(void)
{
    int cont;
    for (int i = 0; i < voter_count; i++)
    {
        cont = 0;
        while (candidates[preferences[i][cont]].eliminated)
        {
            cont++;
        }
        candidates[preferences[i][cont]].votes++;
    }
    return;
}

// 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\n", candidates[i].name);
            return true;
        }
    }
    return false;
}

// Return the minimum number of votes any remaining candidate has
int find_min(void)
{
    int min = candidate_count + 1;
    int max = 0;
    for (int i = 0; i < candidate_count; i++)
    {
        if (candidates[i].eliminated)
        {
            printf("%s eliminado\n", candidates[i].name);
        }
        if (!candidates[i].eliminated)
        {
            if (candidates[i].votes < min)
            {
                min = candidates[i].votes;
            }
        }
    }
    printf("%i", min);
    return min;
}

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

// 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;
        }
    }
    return;
}

问题分析与修复 #

问题根源 #

check50测试失败的核心原因是find_min函数中添加的调试打印语句干扰了测试的输出判断：

• 函数内的printf("%s eliminado\n", candidates[i].name);和printf("%i", min);会向标准输出打印额外内容，而check50是通过捕获程序的标准输出来验证功能正确性的，这些额外输出会导致测试逻辑误判。
• 虽然本地测试时能看到正确的最小值，但check50会将这些额外输出视为程序的异常输出，从而判定测试失败。

修复步骤 #

1. 删除调试打印语句：移除find_min函数中的所有printf语句，只保留核心逻辑。
2. 优化初始值（可选但更严谨）：将min的初始值从candidate_count + 1改为第一个未被淘汰候选人的票数，避免极端情况下初始值不合理的问题。

修复后的find_min函数：

// Return the minimum number of votes any remaining candidate has
int find_min(void)
{
    // 初始化min为第一个未被淘汰候选人的票数
    int min = -1;
    for (int i = 0; i < candidate_count; i++)
    {
        if (!candidates[i].eliminated)
        {
            min = candidates[i].votes;
            break;
        }
    }
    // 遍历所有未被淘汰的候选人，找到最小票数
    for (int i = 0; i < candidate_count; i++)
    {
        if (!candidates[i].eliminated && candidates[i].votes < min)
        {
            min = candidates[i].votes;
        }
    }
    return min;
}

验证 #

修复后重新运行check50，该测试项即可通过。本地测试时若需要调试，可临时添加打印语句，但提交前务必删除，避免干扰测试。

内容的提问来源于stack exchange，提问作者m4theus