CS50第二周Plurality问题代码修复：解决2个check50检测错误

原始代码 #

#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; i++)
    {
        string name = get_string("Vote: ");

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

    // Display winner of election
    print_winner();
}

// Update vote totals given a new vote
bool vote(string name)
{
    for (int i = 0; i < 9; i++)
    {
        // If candidate name matches the entered string by voter, then increase vote by 1
        if(strcmp(candidates[i].name,name) == 0)
        {
            candidates[i].votes += 1;
            return true;
        }
    }
    return false;
}

// Print the winner (or winners) of the election
void print_winner(void)
{
    // Check which candidate has the most votes
    int highest = candidates[0].votes;
    string highest_candidate = candidates[0].name;

    // For loop to find out most number of votes
    for (int i = 0; i < 9; i++)
    {
        if (candidates[i+1].votes > candidates[i].votes)
        {
            highest = candidates[i+1].votes;
        }
    }

    // For loop to print candidates with the most number of votes
    for (int i = 0; i < 9; i++)
    {
        if (candidates[i].votes == highest)
        {
            printf("%s\n", candidates[i].name);
        }
    }
    return;
}

check50测试结果 #

:) 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 // Error 1
    vote function did not return false
:) 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 // Error 2
    vote function modified vote totals incorrectly
:) print_winner identifies Alice as winner of election
:) print_winner identifies Bob as winner of election
:) print_winner identifies Charlie as winner of election
:) print_winner prints multiple winners in case of tie
:) print_winner prints all names when all candidates are tied

问题原因 #

两个错误的根源都是数组越界访问未初始化元素：

1. vote函数循环条件写死为i < 9，但实际候选人数量是candidate_count（可能小于9）。投票给无效候选人时，循环会遍历到未初始化的candidates[i].name（野指针），调用strcmp触发段错误，导致程序直接崩溃，无法返回false，同时可能意外修改未初始化的votes值，引发错误2。
2. print_winner函数同样写死遍历9次，不仅访问无效元素，还存在i+1超出数组最大索引（8）的越界风险。

修复方案 #

1. 修复vote函数 #

将循环条件改为遍历实际存在的候选人：

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

2. 修复print_winner函数 #

修正遍历范围与最高票数查找逻辑：

void print_winner(void)
{
    // 初始化最高票数为第一个候选人的票数
    int highest = candidates[0].votes;

    // 遍历所有候选人，找到最高票数
    for (int i = 1; i < candidate_count; i++)
    {
        if (candidates[i].votes > highest)
        {
            highest = candidates[i].votes;
        }
    }

    // 打印所有得最高票的候选人
    for (int i = 0; i < candidate_count; i++)
    {
        if (candidates[i].votes == highest)
        {
            printf("%s\n", candidates[i].name);
        }
    }
    return;
}

修复说明 #

• 修复后vote函数仅访问已初始化的候选人元素，无效投票时不会触发段错误，能正确返回false，也不会修改无效元素的票数，解决错误1和错误2。
• print_winner函数逻辑更严谨，避免越界访问，同时确保正确找到所有最高票候选人。

内容的提问来源于stack exchange，提问作者King Brain