更新函数中的结构数组并将其作为参数传递给其他函数

Question

我正在学习用 C 编程，但在一些事情上仍然很挣扎，尤其是指针。

我被要求制作一个模拟库管理器的程序。它将具有添加书籍的功能和检查特定书籍信息的选项（检查 isbn 以查找该特定书籍）。

我的代码是：

#include <stdio.h>
#include <string.h>

#define MENU_MESSAGE "\n1 Add book\n2 Check book info\n3 Exit"
int books_added = 0; //counts books added to the library (mainly for usage as index)

struct Bookinfo
{
    char nIsbn[10];
    char szTitle[60];
    char szType[15];
    char szPublisher[25];
    int nPages;
    float nPrice;
    int nYear;
    int nStatus;
    char szName[40];
};
struct Bookinfo Library_manager[100]; // up to 100 books

//---------------------------------------------------------------

void menu(struct Bookinfo Library_manager[])
{
    int nUser; // user's choice for the menu
    printf(MENU_MESSAGE);
    printf("\n\nSelect a number from the menu: ");
    scanf("%d", &nUser);
    menu_direct(nUser, Library_manager);
}
//---------------------------------------------------------------

int check_ISBN(char *szUser_isbn, int *nUser,struct Bookinfo Library_manager[])
{
    int i; // counter

    if(books_added==0) // no need to check for first book
        return 0;
    else
    {
        for(i=0;i<=(books_added-1);i++) //iterates over library to check isbn
        {
            printf("\n\nentered ISBN: %s", szUser_isbn); //for testing
            printf("\nISBN of book in library: \n%s", Library_manager[i].nIsbn); //for testing
            if(szUser_isbn==Library_manager[i].nIsbn)// already in library
            {
                if(*nUser==2)
                    return i; // returns index of book with given isbn
                else
                    return 1;
            }
        }
        return 0; //not found in library
    }
}
//---------------------------------------------------------------

void add_book(char *szUser_isbn, struct Bookinfo Library_manager[])
{
    strcpy(Library_manager[books_added].nIsbn, szUser_isbn);
    printf("\nEnter the book's title: ");
    scanf("%s", &Library_manager[books_added].szTitle);
    getchar(); // takes \n character
    printf("\nEnter the book's type: ");
    scanf("%s", &Library_manager[books_added].szType);
    getchar(); // takes \n character
    printf("\nEnter the book's publisher: ");
    scanf("%s", &Library_manager[books_added].szPublisher);
    getchar(); // takes \n character
    printf("\nEnter the book's number of pages: ");
    scanf("%d", &Library_manager[books_added].nPages);
    getchar(); // takes \n character
    printf("\nEnter the book's price: ");
    scanf("%f", &Library_manager[books_added].nPrice);
    getchar(); // takes \n character
    printf("\nEnter the book's year of publication: ");
    scanf("%d", &Library_manager[books_added].nYear);
    getchar(); // takes \n character
    Library_manager[books_added].nStatus=0; // book is available
    strcpy(Library_manager[books_added].szName, "None"); // not owned by anyone

    printf("\nISBN of book in library: %s", Library_manager[books_added].nIsbn); //for testing

    books_added+=1;
    printf("\nBook added succesfully to the library.\n\n");
    menu(Library_manager);
}
//---------------------------------------------------------------

void display_book_details(int nUser)
{
    char szUser_isbn[10]; // book's isbn
    int nCheck_result; // 0 if book not in library, or index at wich it was found

    do
    {
        printf("\nEnter a valid ISBN: ");
        scanf("%s", szUser_isbn); // get book's isbn for checking
        getchar(); // gets \n character
        nCheck_result=check_ISBN(szUser_isbn,nUser, Library_manager);

        if (nCheck_result==0)
        {
            printf("\nThe ISBN was not found.");
            continue;
        }
        else
        {
            printf("\n-----------------------------------------------\n");
            printf("Book's Title: %s\t", Library_manager[nCheck_result].szTitle);
            printf("\nBook's ISBN: %s", szUser_isbn);
            printf("\nBook's Type: %s", Library_manager[nCheck_result].szType);
            printf("\nBook's Publisher: %s", Library_manager[nCheck_result].szPublisher);
            printf("\nBook's number of pages: %d", Library_manager[nCheck_result].nPages);
            printf("\nBook's Price: %f", Library_manager[nCheck_result].nPrice);
            printf("\nBook's publication year: %d", Library_manager[nCheck_result].nYear);

            if(Library_manager[nCheck_result].nStatus==0)
            {
                printf("\nBook's Status: Available");
            }
            else if (Library_manager[nCheck_result].nStatus==1)
            {
                printf("\nBook's Status: Checked-out");
            }

            printf("\nBook's current holder: %s", Library_manager[nCheck_result].szName);
        }
    }while(nCheck_result==0);

    printf("\n-----------------------------------------------\n");
    printf("\nPress any key to continue to menu...");
    getch();
    menu(Library_manager);
}
//---------------------------------------------------------------

int menu_direct(int nUser,struct Bookinfo Library_manager[]) // directs user's input
{
    if(nUser==1)
    {
        char szUser_isbn[10]; // new book's isbn
        int nCheck_result; // 0 if book not in library, 1 if found in library

        printf("Enter the book's ISBN: ");

        do
        {
            scanf("%s", szUser_isbn); // get book's isbn for checking
            getchar(); // takes \n character
            nCheck_result=check_ISBN(szUser_isbn,nUser, Library_manager);

            if (nCheck_result==0)
                add_book(szUser_isbn, &nUser);
            else
                printf("\nThe ISBN entered is already in library. Enter another ISBN: ");
        }while(nCheck_result==1);
    }
    else if (nUser==2)
    {
        if(books_added==0)
        {
            printf("\nThere are no books in the library yet\n");
            menu(Library_manager);
        }
        else
            display_book_details(nUser);
    }
    else if (nUser==3)
        return 0;
}
//---------------------------------------------------------------

int main(void)
{
    printf("-------------- Welcome --------------\n");
    menu(Library_manager);
    printf("Bye...");

    return 0;
}

如果我运行它，它确实可以编译而不会显示任何错误，但是当我进入检查一本书信息的部分时（在我输入至少一本书之后）我无法从我之前的书中获取 ISBN从数组中添加，以便我可以将其与此代码 if(szUser_isbn==Library_manager[i].nIsbn) 进行比较。

我认为这与指针有关。我已经查看了大量示例，但没有一个适合我的情况。

提前致谢。

Answer 1

如果您使用-Wall -Wextra 编译，那么您已经发现了99% 的问题。还有其他逻辑错误，例如比较strings 需要strcmp（或类似），而== 将不起作用。 check_ISBN 的返回每次第一本书都失败，因为数组索引是基于 C 的zero（即0 - n-1，而不是1 - n）。

我还更改了每个scanf 语句中的format string 以自动使用尾随newline。 （例如scanf("%d%*c", &nUser);）这可以防止需要getchar()，但这是一个临时修复，因为输入nothing会将newline传递给下一个输入。您可能希望使其更加健壮。

我在您的代码中留下了逻辑错误（已注释掉），并提出了其他建议来整理代码。它们包含在下面：

#include <stdio.h>
#include <string.h>

#define MENU_MESSAGE "\n1 Add book\n2 Check book info\n3 Exit"

int books_added = 0; //counts books added to the library (mainly for usage as index)

struct Bookinfo
{
    char nIsbn[10];
    char szTitle[60];
    char szType[15];
    char szPublisher[25];
    int nPages;
    float nPrice;
    int nYear;
    int nStatus;
    char szName[40];
};
struct Bookinfo Library_manager[100]; // up to 100 books

//---------------------------------------------------------------

int menu_direct(int nUser,struct Bookinfo Library_manager[]);

//---------------------------------------------------------------

void menu(struct Bookinfo Library_manager[])
{
    int nUser; // user's choice for the menu
    printf(MENU_MESSAGE);
    printf("\n\nSelect a number from the menu: ");
    scanf("%d%*c", &nUser);
    menu_direct(nUser, Library_manager);
}

//---------------------------------------------------------------

int check_ISBN(char *szUser_isbn, int *nUser,struct Bookinfo Library_manager[])
{
    int i; // counter

    if(books_added==0) // no need to check for first book
        return 0;
    else
    {
        for(i=0;i<=(books_added-1);i++) //iterates over library to check isbn
        {
            printf("\n\nentered ISBN: '%s'", szUser_isbn); //for testing
            printf("\nISBN of book in library: \n%s", Library_manager[i].nIsbn); //for testing
            // if(szUser_isbn==Library_manager[i].nIsbn)// already in library
            if ( strcmp (szUser_isbn, Library_manager[i].nIsbn) == 0)// already in library
            {
                if(*nUser==2)
                    return i; // returns index of book with given isbn
                else
                    return 1;
            }
        }
        // return 0; //not found in library
        return -1; //not found in library (0 is first index)
    }
}
//---------------------------------------------------------------

void add_book(char *szUser_isbn, struct Bookinfo Library_manager[])
{
    strcpy(Library_manager[books_added].nIsbn, szUser_isbn);
    printf("  Enter the book's title    : ");
    scanf("%[^\n]%*c", Library_manager[books_added].szTitle);
    // getchar(); // takes \n character
    printf("  Enter the book's type     : ");
    scanf("%[^\n]%*c", Library_manager[books_added].szType);
    // getchar(); // takes \n character
    printf("  Enter the book's publisher: ");
    scanf("%[^\n]%*c", Library_manager[books_added].szPublisher);
    // getchar(); // takes \n character
    printf("  Enter the number of pages : ");
    scanf("%d%*c", &Library_manager[books_added].nPages);
    // getchar(); // takes \n character
    printf("  Enter the book's price    : ");
    scanf("%f%*c", &Library_manager[books_added].nPrice);
    // getchar(); // takes \n character
    printf("  Enter year of publication : ");
    scanf("%d%*c", &Library_manager[books_added].nYear);
    // getchar(); // takes \n character
    Library_manager[books_added].nStatus=0; // book is available
    strcpy(Library_manager[books_added].szName, "None"); // not owned by anyone

    printf("\nISBN of book in library: %s", Library_manager[books_added].nIsbn); //for testing

    books_added+=1;
    printf("\nBook added succesfully to the library.\n\n");
    menu(Library_manager);
}
//---------------------------------------------------------------

void display_book_details(int nUser)
{
    char szUser_isbn[10]; // book's isbn
    int nCheck_result; // 0 if book not in library, or index at wich it was found

    do
    {
        printf("\nEnter a valid ISBN (-1 to exit): ");
        scanf("%[^\n]%*c", szUser_isbn); // get book's isbn for checking
        //getchar(); // gets \n character
        nCheck_result=check_ISBN (szUser_isbn, &nUser, Library_manager);

        if (nCheck_result==-1)
        {
            printf("\nThe ISBN was not found.");
            continue;
        }
        else
        {
            printf("\n-----------------------------------------------\n");
            printf("  Book's Title           : %s\n", Library_manager[nCheck_result].szTitle);
            printf("  Book's ISBN            : %s\n", szUser_isbn);
            printf("  Book's Type            : %s\n", Library_manager[nCheck_result].szType);
            printf("  Book's Publisher       : %s\n", Library_manager[nCheck_result].szPublisher);
            printf("  Book's number of pages : %d\n", Library_manager[nCheck_result].nPages);
            printf("  Book's Price           : $%.2f\n", Library_manager[nCheck_result].nPrice);
            printf("  Book's publication year: %d\n", Library_manager[nCheck_result].nYear);

            if(Library_manager[nCheck_result].nStatus==0)
            {
                printf("\nBook's Status: Available");
            }
            else if (Library_manager[nCheck_result].nStatus==1)
            {
                printf("\nBook's Status: Checked-out");
            }

            printf("\nBook's current holder: %s", Library_manager[nCheck_result].szName);
        }
    } while (nCheck_result != -1);

    printf("\n-----------------------------------------------\n");
    printf("\nPress any key to continue to menu...");
//     getch();
    menu(Library_manager);
}
//---------------------------------------------------------------

int menu_direct(int nUser,struct Bookinfo Library_manager[]) // directs user's input
{
    if(nUser==1)
    {
        char szUser_isbn[10]; // new book's isbn
        int nCheck_result; // 0 if book not in library, 1 if found in library

        printf("\n  Enter the book's ISBN     : ");

        do
        {
            scanf("%[^\n]%*c", szUser_isbn); // get book's isbn for checking
            // getchar(); // takes \n character
            nCheck_result=check_ISBN(szUser_isbn, &nUser, Library_manager);

            if (nCheck_result==0)
                add_book(szUser_isbn, Library_manager);
            else
                printf("\nThe ISBN entered is already in library. Enter another ISBN: ");
        } while(nCheck_result==1);
    }
    else if (nUser==2)
    {
        if(books_added==0)
        {
            printf("\nThere are no books in the library yet\n");
            menu(Library_manager);
        }
        else
            display_book_details(nUser);
    }
    else if (nUser==3)
        return 0;

    return 0;
}
//---------------------------------------------------------------

int main(void)
{
    printf("-------------- Welcome --------------\n");
    menu(Library_manager);
    printf("Bye...\n\n");

    return 0;
}

输出：

$ ./bin/bookmgr
-------------- Welcome --------------

1 Add book
2 Check book info
3 Exit

Select a number from the menu: 1

  Enter the book's ISBN     : 123
  Enter the book's title    : Huck Finn
  Enter the book's type     : fiction
  Enter the book's publisher: Random House
  Enter the number of pages : 432
  Enter the book's price    : $19.99
  Enter year of publication : 2001

ISBN of book in library: 123
Book added succesfully to the library.


1 Add book
2 Check book info
3 Exit

Select a number from the menu: 2

Enter a valid ISBN (-1 to exit): 123


entered ISBN: '123'
ISBN of book in library:
123
-----------------------------------------------
  Book's Title           : Huck Finn
  Book's ISBN            : 123
  Book's Type            : fiction
  Book's Publisher       : Random House
  Book's number of pages : 432
  Book's Price           : 19.990000.2
  Book's publication year: 2001

Book's Status: Available
Book's current holder: None
Enter a valid ISBN (-1 to exit): -1


entered ISBN: '-1'
ISBN of book in library:
123
The ISBN was not found.
-----------------------------------------------

Press any key to continue to menu...
1 Add book
2 Check book info
3 Exit

Select a number from the menu: 3
Bye...

Answer 2

您的函数check_ISBN() 无法区分在索引 0 找到的书和未找到的书。此外，您不是在比较 ISBN，而是在比较它们的指针。 books_added 的初始测试是不必要的 - for 循环会为您完成。我建议函数返回-1如果没有找到书，那么调用者可以检查函数结果为>= 0

int check_ISBN(char *szUser_isbn, int *nUser,struct Bookinfo Library_manager[])
{
    int i;
    for(i=0; i<books_added; i++) //iterates over library to check isbn
        if(strcmp (szUser_isbn, Library_manager[i].nIsbn) == 0) // already in library
            return i; // returns index of book with given isbn
    return -1; //not found in library
}