The geographical position of books in the library can't be completely fixed. For example, after the books are sorted, a book "A" is placed in the third row of the second bookshelf. When the collection of books increases, the previous category has entered a lot, or the books in this category have entered a lot, so it needs to be reversed. The book "A" may be moved to the fifth row of the second shelf (because the books in the library are either sorted by category or subject, it is impossible to get into a book.

First of all, the books in the library are compiled according to the needs of the library, and the books are arranged on the shelves according to this call number; The retrieval system is actually to retrieve the call number of a book, and then retrieve the book from the stack according to the call number.

If you still don't understand, find a textbook of library science.