Searching a file of records in Delphi data file

Question

Im creating a program that displays a list of 10 books with the title, author and ISBN. The program can display the records and sort them but I am having trouble to searching for him. I do not know where to implement the FindArray identifier into the code and I don't know how to get the program to search for the Book. The records are stored in a .dat file. Any help would be appreciated. Here's the code:

 uses SysUtils;

 type TBook = Record

           Number : integer;
           Title : String[50];
           Author : String[50];
           ISBN : String[13];
         end;

 var Book : TBook;


CurrentFile : File Of TBook;

index : integer;
FindArray : array[1..10] of TBook;

Procedure Add_Book;

var Title, Author, ISBN : String;
index : integer;

begin
 Assign (CurrentFile, 'C:\Books.dat');
 reset (CurrentFile);
 Seek (CurrentFile, filesize(CurrentFile));
 Book.Number := (filepos(CurrentFile)+1);
 writeln ('Type in book title');
 readln (Title);
 For index := 1 to Length(Title) do
 Title[index] := Upcase(Title[index]);
 Book.Title := Title;
 writeln ('Type in author');
 readln (Author);
 For index := 1 to Length(Author) do
 Author[index] := Upcase(Author[index]);
 Book.Author := Author;
 writeln ('Type in ISBN');
 readln (ISBN);
 For index := 1 to Length(ISBN) do
 ISBN[index] := Upcase(ISBN[index]);
 Book.ISBN := ISBN;
 write (CurrentFile, Book);
 Close (CurrentFile);
 end;

 Procedure Show_All;
 begin
   Assign (CurrentFile, 'C:\Books.dat');
   reset (CurrentFile);
   while filepos(CurrentFile) <> filesize(CurrentFile) do
   begin
     read (CurrentFile, Book);
     writeln ('File: ', Book.Number);
     writeln ('Title: ', Book.Title);
     writeln ('Author: ', Book.Author);
     writeln ('ISBN: ', Book.ISBN);
     writeln;
  end;
 writeln;
 write ('END OF FILE!');
 readln;
 Close (CurrentFile);
 end;

Procedure Delete_All;
begin
  Assign (CurrentFile, 'C:\Books.dat');
  reset (CurrentFile);
  seek (CurrentFile,0);
  Truncate (CurrentFile);
  Close (CurrentFile);
end;

Procedure Menu;

var option :integer;

begin
  writeln ('Press appropriate number');
  writeln;
  writeln ('1. Search book');
  writeln ('2. Quit');
  readln (option);
  CASE option of
   1: show_all;
   2: delete_all
   else;menu
   end; {End Case}
end;

begin
  Assign (CurrentFile, 'C:\Books.dat');
  reset (CurrentFile);
  index := 0;
  while not eof(CurrentFile) do
  begin
    index := index+1;
    read (CurrentFile, Book);
  end;
  index := Book.Number;
  repeat
  menu
   until eof(CurrentFile);
   close (CurrentFile)
  end.

This is my code so far. What do I have to do to make it search for the book title or the author?

Answer 1

If I had a list of records that all used the same basic structure, and I wanted to be able to search through them and find records matching a certain filter pattern, the first thing I would do is put it into a SQL database instead of a file of [record type]. Then I could simply run a query: select * from BOOKS where TITLE = :title and let the DBMS handle the details.

If you can't do that, there are two basic ways to run your own filtering: by linear search, and by key lookup.

They both start the same way: read the entire file into memory, into an array of TBook. What you do next depends on your strategy.

Linear search is very simple:

function FindBookIndex(const searchTitle: string): integer;
begin
   for i := 0 to high(bookArray) do
      if bookArray[i].Title = searchTitle then
         exit(i);
   result := -1; //not found
end;

Indexed search takes a bit more work up-front, but is a lot faster to actually run the search. You set up a TDictionary mapping strings (titles) to integers (array indices). Then to run a lookup, call TryGetValue on the dictionary, and return -1 if there's no result. Note that this will only work on exact matches; if you want to do a partial match (books where the title contains the word "Wind," for example,) you either need a linear search or a much more sophisitcated indexing scheme.

But again, really the simplest way to do this is to put it in a database.