C语言指针解引用问题：购票程序指针使用错误排查与修正

需求说明 #

需编写C语言程序，实现以下功能：

• 询问用户可支配金额、欲购买的成人票及儿童票数量
• 程序必须包含main()和PurchaseTickets()两个函数
• 剩余现金变量必须通过指针实现
• 若用户资金不足，程序返回0

以下是提交的代码，存在指针使用相关错误，需排查并修正：

#include <stdio.h>

#define ADULT_TICKET_PRICE 69.00                                                    //define constants
#define CHILD_TICKET_PRICE 35.00

int PurchaseTickets(double *pRemainingCash, int adultTickets, int childTickets);    //declare function PurchaseTickets()

int main(void)
{
    double funds;
    int childTickets;
    int adultTickets;

    printf("How much money do you have to purchase tickets?\n");
    if(scanf("%lf", &funds) != 1)                        //check for errors in user input
    {
        printf("Invalid input, exiting program.\n");
        return 1;                                       //will exit program if invalid characters are entered
    }

    printf("How many child tickets would you like to purchase?\n");
    if(scanf("%d", &childTickets) != 1)                 //check for errors in user input
    {
        printf("Invalid input, exiting program.\n");
        return 1;                                       //will exit program if invalid characters are entered
    }

    printf("How many adult tickets would you like to purchase?\n");
    if(scanf("%d", &adultTickets) != 1)                 //check for errors in user input
    {
        printf("Invalid input, exiting program.\n");
        return 1;                                       //will exit program if invalid characters are entered
    }

    double *pfunds = &funds;

    PurchaseTickets(pfunds, adultTickets, childTickets);

    return 0;
}

/*****************************************************************************************
 *
 *  Function:  PurchaseTickets()
 *
 *  Parameters:
 *
 *      pRemainingCash - points to a variable containing the amt of cash from user
 *      adultTickets - specifies the number of adult tickets the user wants to buy
 *      childTickets - specifies the number of child tickets the user wants to buy
 *
 *  Description: 
 *
 *      The function will determind if the user has enough money to purchase the
 *      specified number of tickets. If they do, the function deducts the proper funds
 *      from their remaining cash and returns the total number of tickets purchased.
 *      if they do not the function returns 0.
 *
*****************************************************************************************/

int PurchaseTickets(double *pRemainingCash, int adultTickets, int childTickets)
{
    double adultCost = (adultTickets * ADULT_TICKET_PRICE);
    double childCost = (childTickets * CHILD_TICKET_PRICE);
    double totalCost = adultCost + childCost;

    int totalTickets = adultTickets + childTickets;

    double remainingCash = *pRemainingCash;
    pRemainingCash = &remainingCash;

    pRemainingCash = remainingCash - totalCost;


    if (*pRemainingCash >= totalCost)
    {
        printf("You have purchased %d tickets, and you have %.2f remaining.",totalTickets, pRemainingCash);
    }

    else
    {
        printf("You do not have enough money to purchase the tickets.");
        return 0;
    }

    return 1;
}

错误分析 #

• 指针指向错误：在PurchaseTickets()中，将pRemainingCash指向局部变量remainingCash，导致后续修改无法同步到main()的funds变量，且局部变量销毁后指针悬空，引发未定义行为。
• 指针赋值逻辑错误：pRemainingCash = remainingCash - totalCost; 试图将double类型的计算结果赋值给指针变量，类型完全不匹配，正确做法是修改指针指向的变量的值（使用*pRemainingCash）。
• 判断条件逻辑错误：if (*pRemainingCash >= totalCost) 错误地用修改后的剩余金额判断，正确逻辑是先检查用户初始可支配金额是否足够支付总票价。
• 输出格式错误：printf中直接使用pRemainingCash（指针变量）会输出内存地址而非金额，需改为*pRemainingCash。
• 返回值不符合要求：函数成功时应返回总票数，但原代码返回1，不符合需求。

修正后的代码 #

#include <stdio.h>

#define ADULT_TICKET_PRICE 69.00
#define CHILD_TICKET_PRICE 35.00

int PurchaseTickets(double *pRemainingCash, int adultTickets, int childTickets);

int main(void)
{
    double funds;
    int childTickets;
    int adultTickets;

    printf("How much money do you have to purchase tickets?\n");
    if(scanf("%lf", &funds) != 1)
    {
        printf("Invalid input, exiting program.\n");
        return 1;
    }

    printf("How many child tickets would you like to purchase?\n");
    if(scanf("%d", &childTickets) != 1)
    {
        printf("Invalid input, exiting program.\n");
        return 1;
    }

    printf("How many adult tickets would you like to purchase?\n");
    if(scanf("%d", &adultTickets) != 1)
    {
        printf("Invalid input, exiting program.\n");
        return 1;
    }

    // 直接传funds的地址，无需额外定义指针变量
    int result = PurchaseTickets(&funds, adultTickets, childTickets);
    
    // 在main中展示最终剩余金额
    printf("\nFinal remaining cash: %.2f\n", funds);

    return result;
}

int PurchaseTickets(double *pRemainingCash, int adultTickets, int childTickets)
{
    double adultCost = adultTickets * ADULT_TICKET_PRICE;
    double childCost = childTickets * CHILD_TICKET_PRICE;
    double totalCost = adultCost + childCost;

    int totalTickets = adultTickets + childTickets;

    // 先判断初始金额是否足够支付总票价
    if (*pRemainingCash >= totalCost)
    {
        // 修改指针指向的变量，更新剩余金额
        *pRemainingCash -= totalCost;
        printf("You have purchased %d tickets, and you have %.2f remaining.", totalTickets, *pRemainingCash);
        // 返回总票数
        return totalTickets;
    }
    else
    {
        printf("You do not have enough money to purchase the tickets.");
        // 资金不足返回0
        return 0;
    }
}

修正说明 #

1. 指针使用修正：不再将指针指向局部变量，直接通过*pRemainingCash修改main()中funds的值，确保剩余金额的修改同步到主函数。
2. 逻辑判断修正：先检查用户初始可支配金额是否足够支付总票价，符合业务逻辑。
3. 输出修正：使用*pRemainingCash输出剩余金额，而非指针地址。
4. 返回值修正：成功购买时返回总票数，资金不足返回0，符合需求。
5. 代码简化：main()中无需额外定义指针变量，直接传递&funds即可。

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