Useing index files

 BBS: Inland Empire Archive
Date: 04-17-93 (20:55)             Number: 302
From: MARTY DUPLISSEY              Refer#: NONE
  To: ALL                           Recvd: NO  
Subj: Useing index files             Conf: (2) Quik_Bas

Blabbering as if I knew what I was talking about!

There has been some discussion about using sorted indexes.
Here is a function I
use to do this. You pass it a Key$ as a string and IDXNUM% as the
File handle you opened the index file with. It returns the Record Number to
look in the main data file to. I am using this with a large (4meg) data file
and it works extremely fast on my 386 40. It will return
the first match the first time you run it and the next one
the next time you run it etc. It will return 0 if no match
found.



DECLARE FUNCTION GetRecord& (IdxNum%, Key$)

TYPE indextype
 Key AS STRING * 15
 Record AS LONG
END TYPE '19



FUNCTION GetRecord& (IdxNum%, Key$) STATIC

DIM index AS indextype
A% = 0

'IF SECOND+ PASS THROUGH JUST Get the next one

IF Found% = 1 THEN
    GET #IdxNum%, Middle&, index
      IF LEFT$(index.Key, LEN(Key$)) = Key$ THEN
 GetRecord& = index.Record
 Middle& = Middle& + 1
 EXIT FUNCTION
      ELSE
 GetRecord& = 0
 Found% = 0
 EXIT FUNCTION
      END IF
END IF

'On more than 1st pass the folowing code is not run
'FIRST PASS THROUGH LETS INITILIZE EVERYTHING

  Lower& = 1
  IndexLen% = LEN(index)
  Upper& = LOF(IdxNum%) / IndexLen%
  TimesToLook% = 1

'Find out how many times to look before giving up

  Counter& = Upper& / 2
  DO UNTIL Counter& <= 1
      Counter& = Counter& \ 2
      TimesToLook% = TimesToLook% + 1
  LOOP

' Lets find it

NoFind% = 0
DO WHILE Found% = 0
    Middle& = (Lower& + Upper&) / 2
    GET #IdxNum%, Middle&, index
      SELECT CASE Key$

'Found it lets see if its the first occurance and if not step up
'Till it is

       CASE LEFT$(index.Key, LEN(Key$))
     DO UNTIL A% = 1
        IF LEFT$(index.Key, LEN(Key$)) = Key$ THEN
    Middle& = Middle& - 1
    GET #IdxNum%, Middle&, index
        ELSE

'Reset pointer to the last match

    A% = 1
    GET #IdxNum%, Middle& + 1, index
        END IF
     LOOP
  Found% = 1
  Middle& = Middle& + 1

'Exit found loop couse this is the last one

  EXIT DO
      CASE IS < LEFT$(index.Key, LEN(Key$))
  Upper& = Middle& - 1
  Found% = 0
  NoFind% = NoFind% + 1
  IF NoFind% = TimesToLook% THEN EXIT DO
      CASE ELSE
  Lower& = Middle& + 1
  Found% = 0
  NoFind% = NoFind% + 1
  IF NoFind% = TimesToLook% THEN EXIT DO
      END SELECT
LOOP

   IF Found% = 0 THEN
      GetRecord& = 0
   ELSE
      GetRecord& = index.Record&

'Reset middle for the next pass

      Middle& = Middle& + 1
   END IF


END FUNCTION

MARTY

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