c - 基于C的抽象数据类型列表数组中的memcpy

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我目前正在努力理解结构数组的工作原理。作为引用，这里是完整的代码。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define SIZE 14
/******************************************************************
 * Data Structure Definition                                      *
 ******************************************************************/
typedef struct {
  unsigned int prodID;    /*  product ID, uniquely identifies an element */ 
  char prodDesc[50];      /*  product description*/  
  float prodPrice;        /*  product price*/  
  int prodQty;            /* product count or quantity  */
}product, products[SIZE];                 /* product record */

typedef struct cell {
   products item;             /* array of products */
   int prodCtr;           /* number of products in the list */
}*chocoList;              /*  Definition of the ADT List ver 2 */  

typedef enum {
    TRUE, FALSE
}boolean;

/******************************************************************
 * Function Prototypes                                            *
 ******************************************************************/
void initializeList(chocoList *L);             
void populateSortedList(chocoList L);         
void displayList(chocoList L, char * listName);
void displayProduct(product X);                 
int insertSorted(chocoList L, product X);    
product deleteFirstOccurence(chocoList L, float prodPrice);

int main(void) 
{
    chocoList  L;      /* sorted List  */
    product deleted;     /* container for the deleted record */

    /*---------------------------------------------------------------------------------
    * Problem #1 :: Initialize the choco list. Display the products in the list after* 
    * calling populateSortedList().                                                  *
    * printf("\n\n\nProblem #1:: ");                                                 *
    *--------------------------------------------------------------------------------*/
    printf("\n\n\nProblem #1:: ");
    initializeList(&L); 
    populateSortedList(L);
    displayList(L, "Problem 1");

    /*---------------------------------------------------------------------------------
    * Problem #2 :: Delete 1 product from the list by calling deleteFirstOccurence(). * 
    * Display the returned product record by calling displayProduct().               *
    * printf("\n\n\nProblem #2:: ");                                                 *
    *--------------------------------------------------------------------------------*/
    printf("\n\n\nProblem #2:: ");     
    deleted = deleteFirstOccurence(L, 75.50);
    displayProduct(deleted);

    /*---------------------------------------------------------------------------------
    * CHALLENGE :: Redo Problem #s 1 & 2 using either versions 3 or 4                       *
    * Keep in mind the changes when converting the ADT List Versions                 *
    *--------------------------------------------------------------------------------*/

    getch();
    return 0;
}

/****************************************************************************************
 * This function initializes the list for its first use.                                     *
 ****************************************************************************************/
void initializeList(chocoList *L)
{
    *L = (chocoList)malloc(sizeof(struct cell));
    (*L)->prodCtr = 0;
}

/****************************************************************************************
 * This function populates the list by calling insertSorted().                          *
 ****************************************************************************************/
void populateSortedList(chocoList L)
{
    int x, result = 1;

    product data[] = {  {1501, "Hersheys", 100.50, 10},
                                {1502, "Hersheys Almond", 100.50, 15},
                                {1503, "Hersheys Krackel", 100.50, 15},
                                {1701, "Toblerone", 150.75, 20},
                                {1702, "Toblerone Milk", 150.75, 40},
                                {1703, "Toblerone Honey", 150.75, 10},
                                {1550, "Cadbury", 200.00, 30},
                                {1201, "Kitkat", 97.75, 40},
                                {1450, "Ferrero", 150.50, 50},
                                {1601, "Meiji", 75.50, 60},
                                {1301, "Nestle", 124.50, 70},
                                {1525, "Lindt", 175.50, 80},
                                {1545, "Valor", 100.50, 90},
                                {1455, "Tango", 49.50, 100}
                    };
    for(x = 0; x < 10 && result == 1; x++){
        result = insertSorted(L, data[x]);
    }                 
}

/****************************************************************************************
 * This function display the details of all products in the list.                       *
 ****************************************************************************************/            
void displayList(chocoList L, char * listName) 
{
    int x;
    system("CLS");       /* clears the screen before displaying succeeding lines */

    if(L->prodCtr != 0){
        printf("\nElements of the Product List %s:", listName);
        printf("\n\n%-10s%-15s%10s%10s","ID","Description","Price","Quantity");
        printf("\n%-10s%-15s%10s%10s","--","-----------","-----","--------");

        for(x = 0 ; x < L->prodCtr; x++){
            printf("\n%-10d", L->item[x].prodID);
            printf("%-15s", L->item[x].prodDesc);   
            printf("%10.2f", L->item[x].prodPrice);
            printf("%10d", L->item[x].prodQty);     
        }
    }else{
        printf("\n\tChoco list is currently empty!\n");
    }

    printf("\n\n Press any key to continue . . . ");
    getch();
}

/****************************************************************************************
 * This function inserts a product X in an alphabetically sorted list according to its  *
 * product description. It returns a value of 1 for successful insertion. Otherwise, 0. *
 * Use memcpy() in shifting the elements downward to provide space for the new product. *
 ***************************************************************************************/
int insertSorted(chocoList L, product X)
{
    int ctr;
    if(L->prodCtr<14){
        for(ctr=0; ctr<L->prodCtr && strcmp(L->item[ctr].prodDesc, X.prodDesc)<0;ctr++){    }
        memcpy(L->item + ctr + 1, L->item + ctr, sizeof(product) * (L->prodCtr - ctr));
        L->item[ctr] = X;
        L->prodCtr++;
        ctr = 1;
    }
    else{
        ctr = 0;
    }
    return ctr;
}

/****************************************************************************************
 * This function deletes the first occurence of a product given the prodPrice.It returns* 
 * the product record ones it is found. Otherwise, it returns a dummy record containing *
 * "XXX" for string values and 0 for integer and float values.  Use memcpy() in shifting *
 * the elements upward to avoid having empty indices within the array.                       *
 ***************************************************************************************/
product deleteFirstOccurence(chocoList L, float prodPrice)
{
    int ctr;
    product dummy;
    strcpy(dummy.prodDesc , "XXX");
    dummy.prodID = 0;
    dummy.prodPrice = 0;
    dummy.prodQty = 0;
    for(ctr = 0; ctr<L->prodCtr && L->item[ctr].prodPrice != prodPrice; ctr++){ }
    if(ctr!=L->prodCtr){
        dummy = L->item[ctr];
        memcpy(L->item + ctr, L->item + ctr + 1, sizeof(product) * (L->prodCtr - ctr));
        L->prodCtr--;
    }
    return dummy;
}

/****************************************************************************************
 * This function display the details of 1 product.                                       *
 ***************************************************************************************/
void displayProduct(product X)
{
    //system("CLS");       /* clears the screen before displaying succeeding lines */
    printf("\n\nElements of Product %d:", X.prodID);
    printf("\n\n%-10s%-15s%10s%10s","ID","Description","Price","Quantity");
    printf("\n%-10s%-15s%10s%10s","--","-----------","-----","--------");

    printf("\n%-10d", X.prodID);
    printf("%-15s", X.prodDesc);    
    printf("%10.2f", X.prodPrice);
    printf("%10d", X.prodQty);      

    printf("\n\n Press any key to continue . . . ");
    getch();    
}

我对这一部分特别有几个问题:

int insertSorted(chocoList L, product X)
{
    int ctr;
    if(L->prodCtr<14){
        for(ctr=0; ctr<L->prodCtr && strcmp(L->item[ctr].prodDesc, X.prodDesc)<0;ctr++){    }
        memcpy(L->item + ctr + 1, L->item + ctr, sizeof(product) * (L->prodCtr - ctr));
        L->item[ctr] = X;
        L->prodCtr++;
        ctr = 1;
    }
    else{
        ctr = 0;
    }
    return ctr;
}

首先，有人可以向我解释一下这句话吗？

memcpy(L->item + ctr + 1, L->item + ctr, sizeof(product) * (L->prodCtr - ctr));

第二，使用memcpy是不是效率更高？或memmove在这种情况下？

Answer 1

memcpy 正在尝试移动数组的一部分，以便为新项目腾出空间。从视觉上看，它正在这样做:

哪里

• 绿色是数组中不需要移动的项
• 蓝色是需要移动以为新项目腾出空间的项目
• 白色是数组中未使用的项目
• 黄色是新项目X

为了这个示例，我选择了以下值:

• L->prodCtr 最初为 6
• for 循环将 ctr 设置为 3

这就是 memcpy 的工作原理。第一个参数 (L->item + ctr + 1) 是目标地址，即之后第一个蓝色项目的地址移动。第二个参数L->item + ctr是源地址，即移动之前第一个蓝色项目的地址。第三个论点由两部分组成。 sizeof(product) 是每个项目的大小(以字节为单位)。 (L->prodCtr - ctr) 是需要移动的项目数。将它们相乘即可得到要移动的字节数。

<小时/>

这就是 memcpy 应该做的事情。但是，要回答你的第二个问题，你不能使用memcpy来做到这一点。原因是 memcpy 仅在源和目标不重叠时才起作用。另一方面，即使源和目标重叠，memmove 也能保证正常工作。所以这里没有任何选择，你必须使用memmove。