我正在学习使用C编程，但在某些事情上仍然很困难，尤其是指针。
我被要求制作一个模拟图书馆经理的程序。它将具有添加书籍的功能以及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，它确实编译而没有显示任何错误但是当我到达检查信息的一部分时（在我输入至少一本书之后）我无法从我之前的书中获得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。
我认为它与指针有关。我已经看了很多例子，但没有一个适合我的情况。
提前致谢。
答案 0 :(得分：0)
您的函数check_ISBN()并不区分在索引0处找到的书和未找到的书。你也不是在比较ISBN，而是指出它们的指针。 books_added的初始测试是不必要的 - for循环为您执行此操作。如果找不到书，我建议函数返回-1，然后调用者可以将函数结果检查为>= 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
}
答案 1 :(得分：0)
如果您使用-Wall -Wextra进行编译，那么您已找到99%个问题。还存在其他逻辑错误，例如strings需要strcmp（或类似内容）的比较，而==将无效。每次第一本书的check_ISBN返回失败，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format string语句中的scanf，以自动使用尾随newline。 （例如scanf("%d%*c", &nUser);）这可以防止需要getchar()但是暂时修复，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强大。
我在代码中留下了逻辑错误（已注释掉），并提出了额外的建议来整理代码。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;
}
<强>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...