Question

I have made a program which is a small library operated via software. When I add two books and then delete the first book the second book gets the same bookid as the first book because of count-- in the del() function. I cannot rely on printing the count as the bookid. Is there a better option?

#include<stdio.h>
#include<conio.h>
#include<stdlib.h>
static int count;
struct book
{
  int bookid;
  char name[30];
  char author[30];
  float price;
};
struct book b[40];
void add(void);
void del(void);
void sort(void);
void price(void);
void print(void);
void main(void)
{
  char choice;
  while(1)
  {
    clrscr();
    printf("Enter a choice:\n 1.Add a book.\n 2.Delete a book.\n 3.Sort books by price.\n 4.To print all books details.\n 5.To print the names of the books whose price is less than 1000.\n 6.Exit\n");
    choice=getche();//doing by getch() as getche makes the program rough as it is printed
    switch(choice)
    {
      case'1':add();break;
      case'2':del();break;
      case'3':sort();break;
      case'4':print();break;
      case'5':price();break;
      case'6':exit(0);
      default:printf("Enter a valid choice.");break;
    }
  }/*switch ends*/
}
void add(void)
{
  int i;
  char ch[30];
  clrscr();
  for(i=count;i<40;i++)
  {
    printf("Enter books name:\n");
    gets(b[i].name);
    printf("Enter author's name\n");
    gets(b[i].author);
    printf("Enter price:\n");
    gets(ch);
    b[i].price=atoi(ch);
    printf("Dear User,the book has succesfully been added.The book id is %d",i);
    count++;
    break;
  } /* for ends*/
  getch();
}
void print(void)
{
  int i;
  clrscr();
  for(i=0;i<count;i++)
  {
    printf("Bookid=%d,Name=%s,Author=%s,Price=%f\n",b[i].bookid,b[i].name,b[i].author,b[i].price);
  }
  getch();
}

void del(void)
{
  int i,j;
  char ch[10];
  clrscr();
  printf("Enter book id:");
  gets(ch); // how do i put it into the structure as i dont know that which structure it belongs to
  for(i=0;i<count;i++)  //searching
  {
    if(b[i].bookid==atoi(ch))
    {
      for(j=i;j<count;j++)
      {
        b[j]=b[j+1];
      }//for j ends
    }  //if ends
  } /* for of i ends */
  count--;
  getch();
}
//void del(void)
//{

    // int i;
    // char ch[10];
     // clrscr();
 //printf("Enter book id:");
       // gets(ch);
      // for(i=0;i<40;i++)
      // {
     //  b[i]=b[i+1];
    //
   // }
    // count--;
  // printf("Dear user,delete succesful");
//getch();
//}
void sort(void)
{
  int i;
  float temp;
  for(i=0;i<40;i++)
  {
    if(b[i].price>b[i+1].price)
    {
      temp=b[i].price;
      b[i].price=b[i+1].price;
      b[i+1].price=temp;
    }
  }/*for ends*/
  printf("Dear user,the books are sorted by price.\n");

  getch();
}

void price(void)
{
  int i;
  clrscr();
  for(i=0;i<count;i++)
  {
    if(b[i].price<1000)
    {
      printf("%d.%s\n",i+1,b[i].name);
    }
  }
  getch();
}

Answer 1

One way is to have two global counters: one has the number of books stored, the other gives the next (unique) book id, as Tyler McHenry points out. When you delete a book, decrement the number of books, but never decrement the next book id.

I notice that when a book is deleted, you move the rest of the book entries together (“compress” them) so your array is always “dense”. So another solution is to add a “deleted” flag to the book structure. Now you don’t compress the entries when a book is deleted, but when a book is added, the code must search for an empty place in the array. Also the bookid is just the position in the array. (By the way, if you hard-code sizes, like the size of an array, do it once.)

#define MAXSTR (30)
struct book
{
  int bookid;
  int valid; // 1 is valid, 0 is empty or deleted
  char name[MAXSTR];
  char author[MAXSTR];
  float price;
};
#define MAXBOOKS (40)
struct book b[MAXBOOKS];

int findEmpty()
{
  int i;
  for (i=0; i < MAXBOOKS; i++) {
    if (! b[i].valid) return i;
  }
  return -1; // no more space
}

void add(void)
{
  int i = findEmpty();
  char ch[MAXSTR];

  if (i < 0) return; // no room for more books

  clrscr();
  printf("Enter books name:\n");
  fgets(b[i].name, MAXSTR, stdin);
  printf("Enter author's name\n");
  fgets(b[i].author, MAXSTR, stdin);
  printf("Enter price:\n");
  fgets(ch, MAXSTR, stdin);
  b[i].price=atoi(ch);

  /* Assign the empty location found to this book */
  b[i].bookid = i;
  /* mark that it is valid */
  b[i].valid = 1;

  printf("Dear User,the book has succesfully been added.The book id is %d", b[i].bookid);
  getch();
}

del() now just marks the book as invalid. The main part of del() looks like this.

gets(ch);
int idToDelete = atoi(ch);
// find which structure it belongs to
for(i=0;i

Use a selection sort in sort() instead of the one-pass routine (which doesn’t work in all cases) which is there. Printing books should skip any book that is not valid, too.