CS50 plurality问题：check50检测print_winner函数异常求助

完成CS50的plurality（多数决）编程任务时，自行编写的C语言代码手动测试正常，但在check50自动检测中，print_winner函数无法正确识别Bob、Charlie为单独获胜者，其余测试项均通过，调试器无法查看候选者的票数与名称，需要定位问题。

任务要求 #

• 程序需统计得票最多的候选人，无效选票需提示
• 若多位候选人得票相同，则全部输出

代码实现 #

#include <cs50.h>
#include <stdio.h>
#include <string.h>

// Max number of candidates
#define MAX 9

// Candidates have name and vote count
typedef struct
{
    string name;
    int votes;
}
candidate;

// Array of candidates
candidate candidates[MAX];

// Number of candidates
int candidate_count;

// Function prototypes
bool vote(string name);
void print_winner(void);

int main(int argc, string argv[])
{
    // Check for invalid usage
    if (argc < 2)
    {
        printf("Usage: plurality [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].name = argv[i + 1];
        candidates[i].votes = 0;
    }

    int voter_count = get_int("Number of voters: ");

    // Loop over all voters
    for (int i = 0; i < voter_count;)
    {
        string name = get_string("Vote: ");

        // Check for invalid vote
        if (!vote(name))
        {
            printf("Invalid vote.\n");
        }
        else
        {
            i++;
        }
    }

    // Display winner of election
    print_winner();
}

// Update vote totals given a new vote
bool vote(string name)
{
    for (int i = 0; i < candidate_count; i++)
    {
        if (strcmp(name, candidates[i].name) == 0)
        {
            candidates[i].votes = candidates[i].votes + 1;
            return true;
        }
    }
    return false;
}

// Print the winner (or winners) of the election
void print_winner(void)
{
    char t[100];
    for (int i = 0; i < candidate_count; i++)
    {
        for (int j = 0; j < candidate_count - i - 1; j++)
        {
            if (candidates[j].votes < candidates[j + 1].votes)
            {

                int temp = candidates[j].votes;
                candidates[j].votes = candidates[j + 1].votes;
                candidates[j + 1].votes = temp;

                strcpy(t, candidates[j].name);
                strcpy(candidates[j].name, candidates[j + 1].name);
                strcpy(candidates[j + 1].name, t);
            }
        }
    }
    printf("%s\n", candidates[0].name);
    for(int k = 1; k < candidate_count; k++)
    {
        if (candidates[0].votes == candidates[k].votes)
        {
            printf("%s\n", candidates[k].name);
        }
    }

    return;
}

check50检测结果 #

:) plurality.c exists
:) plurality compiles
:) vote returns true when given name of first candidate
:) vote returns true when given name of middle candidate
:) vote returns true when given name of last candidate
:) vote returns false when given name of invalid candidate
:) vote produces correct counts when all votes are zero
:) vote produces correct counts after some have already voted
:) vote leaves vote counts unchanged when voting for invalid candidate
:) print_winner identifies Alice as winner of election
:( print_winner identifies Bob as winner of election
    print_winner function did not print winner of election
:( print_winner identifies Charlie as winner of election
    print_winner function did not print winner of election
:) print_winner prints multiple winners in case of tie
:) print_winner prints all names when all candidates are tied

补充说明：数组t有100个元素，不会出现段错误，此为非问题点。

问题分析与解决 #

问题根源 #

print_winner函数中排序候选人时，使用strcpy操作candidates[j].name是错误的。因为candidates[j].name指向的是argv传入的只读字符串常量，对其执行strcpy写入操作会触发未定义行为，导致程序运行异常，这就是check50无法识别Bob、Charlie为单独获胜者的原因。

修复方案 #

排序时不需要复制字符串内容，直接交换name指针即可：

// Print the winner (or winners) of the election
void print_winner(void)
{
    for (int i = 0; i < candidate_count; i++)
    {
        for (int j = 0; j < candidate_count - i - 1; j++)
        {
            if (candidates[j].votes < candidates[j + 1].votes)
            {
                // 交换票数
                int temp_votes = candidates[j].votes;
                candidates[j].votes = candidates[j + 1].votes;
                candidates[j + 1].votes = temp_votes;

                // 交换name指针，而非复制字符串
                string temp_name = candidates[j].name;
                candidates[j].name = candidates[j + 1].name;
                candidates[j + 1].name = temp_name;
            }
        }
    }
    printf("%s\n", candidates[0].name);
    for(int k = 1; k < candidate_count; k++)
    {
        if (candidates[0].votes == candidates[k].votes)
        {
            printf("%s\n", candidates[k].name);
        }
    }

    return;
}

额外说明 #

手动测试时可能未触发未定义行为的显性错误，但check50的测试环境会严格检测这类内存操作问题，导致函数输出异常。修改后即可通过所有测试项。

内容的提问来源于stack exchange，提问作者Bulat_Ishet_otvety