在 C 语言中取消引用指针时遇到困难

我查看了您的代码，从我所看到的所需解决方案来看，该程序目的的上下文确实不需要指针。首先，由于“PurchaseTickets”函数并没有真正返回任何所需的信息，因此使用void返回条目对其进行设置更有意义。其次，由于函数所需的输入变量可以全部作为值传递，因此实际上不需要发送指针引用。

为了尽量减少对代码的最小更改，可以在函数中注释掉以下语句以及重构“printf”语句。

    //double remainingCash = *pRemainingCash;           // Not needed
    //pRemainingCash = &remainingCash;

    //pRemainingCash = remainingCash - totalCost;


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

这在终端上产生了以下测试输出。

craig@Vera:~/C_Programs/Console/TicketPurchase/bin/Release$ ./TicketPurchase 
How much money do you have to purchase tickets?
800
How many child tickets would you like to purchase?
4
How many adult tickets would you like to purchase?
4
You have purchased 8 tickets, and you have 384.00 remaining.

注意代码的观察结果并简化事情，以下是程序的重构版本。

#include <stdio.h>

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

void 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? ");
    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? ");
    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? ");
    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
    }

    PurchaseTickets(funds, 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.
 *
*****************************************************************************************/

void 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;

    pRemainingCash -= totalCost;

    if (pRemainingCash >= 0.00)
    {
        printf("You have purchased %d tickets, and you have %.2f remaining.\n",totalTickets, pRemainingCash);
    }

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

    return;
}

请注意以下改进：

• “PurchaseTickets”函数的返回值已更改为“void”，因为似乎没有使用整数返回值。
• 所有变量都作为值传递，而不是使用任何变量指针。
• 删除了不需要的变量，以简化计算和比较。

通过这些改进，以下是对程序和购票功能的一系列额外测试。

craig@Vera:~/C_Programs/Console/TicketPurchase/bin/Release$ ./TicketPurchase 
How much money do you have to purchase tickets? 300.00
How many child tickets would you like to purchase? 4
How many adult tickets would you like to purchase? 4
You do not have enough money to purchase the tickets.
craig@Vera:~/C_Programs/Console/TicketPurchase/bin/Release$ ./TicketPurchase 
How much money do you have to purchase tickets? 600
How many child tickets would you like to purchase? 4
How many adult tickets would you like to purchase? 4
You have purchased 8 tickets, and you have 184.00 remaining.

总而言之，在函数中传递变量地址代替变量值是有正当理由的，但人们应该始终了解函数的上下文，以确定是否有必要这样做。寻找有关函数定义的其他教程，了解指针何时有益可能是明智的。

这是一团糟。在不重复@NoDakker的评论的情况下，下面是代码的缩写版本，显示了所需的操作。请注意，一些变量已被重命名（并删除了注释）以提高清晰度。

// heading stuff omitted for brevity
int main(void)
{
    double funds;
    int childTickets;
    int adultTickets;

    /* Get funds */ // omitted for brevity

    /* Get # of Child tickets */

    /* Get # of Adult tickets */

    int totTckts = PurchaseTickets( &funds, adultTickets, childTickets );

    // Notice that the function performs its task.
    // Reporting those results belongs in the calling function
    if( totTckts )
        printf( "%d tickets, and you have %.2f remaining.", totTckts, funds );
    else
        printf( "Insufficient funds to purchase those tickets." );

    return 0;
}

int PurchaseTickets( double *pFunds, int nAdult, int nChild )
{
    double Cost
        = (nAdult * ADULT_TICKET_PRICE)
        + (nChild * CHILD_TICKET_PRICE);

    if( *pFunds < Cost ) // Insufficient available funds?
        return 0; // no show

    *pFunds -= Cost; // subtract cost from "double funds;" variable in main()

    return nAdult + nChild; // return # of purchased tickets
}

在函数中存在一些问题。PurchaseTickets

double remainingCash = *pRemainingCash;
pRemainingCash = &remainingCash;

此代码创建值的副本，指向本地副本，然后将其指向本地副本。此代码是正确的，但似乎不是正确的做法。pRemainingCash

pRemainingCash = remainingCash - totalCost;

这是由于语法而遇到错误的地方。它试图将浮点值分配给指针，而不是指针指向的内容。为此，您需要*pRemainingCash = remainingCash - totalCost;

这将解决错误，但不能解决问题，因为它正在修改本地副本而不是传入的值。

另一个问题是，在减去成本之前，您应该检查他们是否买得起门票。我希望如果成本超过可用资金，您将保持可用资金不变。

下面是一个清理后的版本：

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;

    if (*pRemainingCash >= totalCost)
    {
        // Subtract cost here
        *pRemainingCash -= totalCost;
        // Note *pRemainingCash in the next line. 
        // You need to dereference to obtain the value pointed at.
        printf("You have purchased %d tickets, and you have %.2f remaining.",
            totalTickets, *pRemainingCash);

        // Return success
        return totalTickets;
    }
    // This code won't be reached in the success case because of the return.
    // Removing the else avoids the possibility of the compiler warning that
    // all paths do not return a value.
    printf("You do not have enough money to purchase the tickets.");
    return 0;
}

中的另一个小建议。main

而不是仅仅调用函数double *pfunds = &funds;PurchaseTickets(&funds, adultTickets, childTickets);

你所拥有的是有效的，但我认为最好不要为此创建一个额外的变量。如果之后有更多的代码，可能会混淆使用哪一个。