数组的内存分配[重复]

Question

可能重复：
Using Dynamic Memory allocation for arrays

我最初有这个程序存储价格，数量大小为 10，并意识到我想让程序更具动态性，因为我可能需要在某个给定点存储超过 10 个项目。我很难理解如何重新分配额外的内存，以便我可以存储我需要的任何数量的项目。这是处理此任务的正确方法吗？

主要功能：

double *purchases = (double*)malloc(QUANTITY_SIZE);

外部函数

double startShopping(double *purchases, double *taxAmount, double *subTotal, double *totalPrice)
{
    double itemPrice = 0.00;
    double* storeMoreItems;

    for(int i = 0; i < QUANTITY_SIZE; *subTotal +=purchases[i++])
    {
        while(itemPrice != -1)
        {
            printf("Enter the price of the item :");
            scanf("%lf", &itemPrice); 

            storeMoreItems = (double*)realloc(storeMoreItems, i * sizeof(int));

            if(storeMoreItems != NULL)
            {
                storeMoreItems = purchases;
                purchases[i-1] = itemPrice;
            }

           else
           {
               free(purchases);
           }
       }
  }

  displayCart(purchases);

  *taxAmount = *subTotal * TAX_AMOUNT;

  *totalPrice = *taxAmount + *subTotal;

  printf("\nTotal comes to : $%.2lf\n", *totalPrice);

  return *totalPrice;
}

Answer 1

这是错误的：

        if(storeMoreItems != NULL)
        {
            storeMoreItems = purchases;
            purchases[i-1] = itemPrice;
        }

首先，你覆盖刚才的 realloced 指针，你应该有

purchases = storeMoreItems;

在那里，而不是相反。但这不会影响传入的purchases指针在调用函数中的值。

为此，您需要将purchases 的地址从main 传递到

double startShopping(double **purchases_ptr, double *taxAmount, double *subTotal, double *totalPrice)

并分配

*purchases_ptr = storeMoreItems;

重新分配本身，

storeMoreItems = (double*)realloc(storeMoreItems, i * sizeof(int));

使用错误的类型来计算要分配的大小，这几乎肯定也是非常错误的。

---

在main:

size_t purchase_count = QUANTITY_SIZE;
double *purchases = malloc(purchase_count * sizeof *purchases);
// ...
startShopping(&purchases, &purchase_count, taxAmount, subTotal, totalPrice);
// ...

startShopping 看起来像

double startShopping(double **purchases_ptr, size_t *purchase_count,
                     double *taxAmount, double *subTotal, double *totalPrice)
{
    double itemPrice = 0.00;
    double* purchases = *purchases_ptr;
    size_t i;

    for(i = 0; ; *subTotal += purchases[i++])
    {
        printf("Enter the price of the item :");
        scanf("%lf", &itemPrice);

        // I'm assuming you don't really want to add the -1
        // entered for termination
        if (itemPrice == -1) {
            break;
        }

        if (i == *purchase_count) {
            // array filled, let's get more space
            // double it, or add a fixed amount,
            // but rather not just one element each round
            *purchase_count *= 2;

            // we have the address saved in another variable, so here we can
            // store the pointer returned by realloc in purchases without losing
            // the handle if realloc fails
            purchases = realloc(purchases, *purchase_count * sizeof *purchases);

            if (purchases == NULL) {
                // reallocation failed, now what?

                // throw a tantrum?
                free(*purchases_ptr);
                exit(EXIT_FAILURE);

                // or can something less drastic be done?
            } else {
                // Okay, got the needed space, let's record the address
                *purchases_ptr = purchases;
            }
        }
        purchases[i] = itemPrice;
    }

    // store the number of items actually read in?
    *purchases_count = i;

    // That should probably also get passed the number of items stored
    displayCart(purchases);

    *taxAmount = *subTotal * TAX_AMOUNT;

    *totalPrice = *taxAmount + *subTotal;

    printf("\nTotal comes to : $%.2lf\n", *totalPrice);

    return *totalPrice;
}

Answer 2

double* purchases = (double*)malloc(sizeof(double)*QUANTITY_SIZE);

还有什么：

storeMoreItems = (double*)realloc(storeMoreItems, i * sizeof(double));

您尝试分配i*sizeof(int)，然后将其转换为double*。当double 和int 的大小不同时，您的代码将很难找到错误。

接下来的事情：

当i 等于 0 时，您分配初始大小为 0 字节 (i*sizeof(int)) 的内存，然后尝试使用它。它行不通。尝试以这种方式更改循环：for (int i = 1, i <= QUANTITY_SIZE;... 并保留purchases[i-1]。

Answer 3

查看 realloc。

void * realloc ( void * ptr, size_t size );

重新分配内存块改变ptr指向的内存块的大小。

从这里获取更多详细信息http://www.cplusplus.com/reference/cstdlib/realloc/

Answer 4

您要做的第一件事是确保分配正确的字节数。 Malloc 不知道你想将内存用于双精度所以你需要乘以sizeof(double)：

double *purchases = (double*)malloc(QUANTITY_SIZE * sizeof(double));