File Access/Creation

[Walt Decker]

A file can be thought of as a one dimensional array located on a mass storage device. Various portions of the data can be accessed via a simple algorithm:

Row * Width + Column (provided that the result is not beyond the end of the file)

where: Row = a down position
Width = either an arbitrary length or a known length
Column = a position on Row

For example, portions of data stored using an 8 X 8 array can be extracted into a smaller array by:

Wide = 8
Row = 3
Column = 4
Col = Column

FOR I = 1 TO 2
  R1 = Row * Wide
  FOR J = 1 TO 3
    Rec = R1 + Col
    GET #1, Rec, A(I, J)
    INCR Col
  NEXT J
  INCR Row
  Col = Column
NEXT I

My question:

How would one use a small two dimensional array to create a large random access file that emulates the above process? I've tried several different methods but with each I either get way too many records, too few records, overlaps, or gaps. The reason I'm trying to do this is so that I can access portions of up to four "areas" in the file and modify the records at the same time.

Visualization:

.......... ,,,,,,,,,,
.......... ,,,,,,,,,,
.......... ,,,,,,,,,,
.......... ,,,,,,,,,,
;;;;;;;;;; :::::::::
;;;;;;;;;; :::::::::
;;;;;;;;;; :::::::::
;;;;;;;;;; :::::::::

------------------------

.......... ,,,,,,,,,,
.......... ,,,,,,,,,,
.....----- -----,,,,,,
.....----- -----,,,,,,
;;;;;----- -----:::::
;;;;;----- -----:::::
;;;;;;;;;; :::::::::
;;;;;;;;;; :::::::::

[Michael Mattias]

Memory Mapped Files instead of RANDOM disk file access 5-8-04

Demo uses a one-dimension array. You can use two or more.

[Walt Decker]

Michael,

That's interesting, but I don't see how it relates to the problem. Perhaps the following will illustrate the problem better than I can explain it. The code is PBCC 4x.

#COMPILE EXE
DEFLNG A - Z

TYPE SampStruct
  X   AS SINGLE
  Y   AS SINGLE
  Elv AS SINGLE
END TYPE

TYPE ImpStruct
  LandImp(10)     AS WORD
  LandImpRec(10)  AS DWORD
END TYPE

TYPE WorldStruct
  Landtype    AS INTEGER
  RawIdx      AS DWORD
  BmpIdx      AS DWORD
  ImpStruct
END TYPE

$WORLD_RAW = "RAW_WORLD.RAW"

FUNCTION PBMAIN () AS LONG

STATIC tWorld() AS WorldStruct

DIM tWorld(1 TO 2, 1 TO 2)

Tile_High = 80
Tile_Wide = 80

CALL Base_World(Tile_High, Tile_Wide, 2, 2)

END FUNCTION

'------------------------------------------------------------------
'------------------------------------------------------------------

SUB Base_World(BYVAL Tile_High, BYVAL Tile_Wide, BYVAL Nright, BYVAL Ndown)

'==============================================================
' Ndown = # of bitmaps in virtical direction
' Nright = # of bitmaps in horizontal direction
' Tile_High = height of bitmap
' Tile_Wide = width of bitmap
'==============================================================

LOCAL Map() AS SINGLE
LOCAL Rad   AS SINGLE

Tile_High = Tile_High - 1
Tile_Wide = Tile_Wide - 1
Rad = (4 * ATN(1)) / 180

DIM Map(Tile_High, Tile_Wide)

NumRecs = ((Tile_High + 1) * NDown) * ((Tile_Wide + 1) * NRight)

'-----------------------------------------
' center of bitmap for circle calculation
'-----------------------------------------

Cx = Tile_Wide \ 2
Cy = Tile_High \ 2

N = 0

'---------------------------------------------------------------
'         WRITE RANDOM ACCESS FILE IN LINEAR FASHION
'---------------------------------------------------------------

IF DIR$($WORLD_RAW) <> "" THEN KILL $WORLD_RAW

OPEN $WORLD_RAW FOR RANDOM AS #1 LEN = SIZEOF(Map(0, 0)) BASE = 0
FOR I = 1 TO Ndown
  FOR J = 1 TO Nright

'-----------------------------------------------------------
'   draw circle in memory
'-----------------------------------------------------------

    FOR K = 0 TO 360
      L = Cx + (Cx - 5) * SIN(K * Rad)
      M = Cy - (Cy - 5) * COS(K * Rad)
      IF N = 0 THEN
        Map(M, L) = %WHITE
      ELSEIF N = 1 THEN
        Map(M, L) = %GREEN
      ELSEIF N = 2 THEN
        Map(M, L) = %RED
      ELSEIF N = 3 THEN
        Map(M, L) = %CYAN
      END IF
    NEXT K

'-------------------------------------------------------
'   write map to file
'-------------------------------------------------------

    FOR K = 0 TO Tile_High
      FOR L = 0 TO Tile_Wide
        PUT #1, , Map(K, L)
      NEXT L
    NEXT K
    INCR N
    RESET Map()
  NEXT J
NEXT I
N = SEEK (#1) - 1

PRINT "Number of records written = "; N
PRINT "Calculated records = "; NumRecs
PRINT "Press a key"

CLOSE #1

WAITKEY$


GRAPHIC WINDOW "test", 0, 0, (Tile_Wide + 1) * NRight, (Tile_High + 1) * NDown TO WinHndl
GRAPHIC BITMAP NEW Tile_Wide + 1, Tile_High + 1 TO BmpHndl
GRAPHIC ATTACH BmpHndl, 0

X = 0
Y = 0
X1 = 0
Y1 = 0

OPEN $WORLD_RAW FOR RANDOM AS #1 LEN = SIZEOF(Map(0, 0)) BASE = 0

'-------------------------------------------------------------------------
'     extract map in same order it was written and display it
'-------------------------------------------------------------------------

FOR I = 1 TO Ndown
  FOR J = 1 TO Nright
    FOR K = 0 TO Tile_High
      FOR L = 0 TO Tile_Wide
        GET #1, , Rad
        GRAPHIC SET PIXEL (X1, Y1), Rad
        INCR X1
      NEXT L
      INCR Y1
      X1 = 0
    NEXT K
    GRAPHIC ATTACH WinHndl, 0
    GRAPHIC COPY BmpHndl, 0 TO (X, Y)
    GRAPHIC ATTACH BmpHndl, 0
    X = X + Tile_Wide + 1
    X1 = 0
    Y1 = 0
  NEXT J
  Y = Y + Tile_High + 1
  X = 0
NEXT I

PRINT "circles done"
PRINT "Press a key"

WAITKEY$

'--------------------------------------------------------
'   extract center portion of map and display it
'--------------------------------------------------------

Nwide = (Tile_Wide + 1) * NRight
Nhigh = (Tile_High + 1) * NDown

Cx = Nwide \ 2
Cy = Nhigh \ 2

Y = Cy - (Tile_High + 1) \ 2
X = Cx - (Tile_Wide + 1) \ 2

Row = 0
Col = X
X1 = 0
Y1 = 0

GRAPHIC ATTACH BmpHndl, 0

FOR I = 0 TO Tile_High
  Row = Y * NWide
  FOR J = 0 TO Tile_Wide
    Rec = Row + Col
    GET #1, Rec, Rad
    GRAPHIC SET PIXEL (X1, Y1), Rad
    INCR X1
    INCR Col
  NEXT J
  INCR Y
  INCR Y1
  X1 = 0
  Col = X
NEXT I

GRAPHIC ATTACH WinHndl, 0
GRAPHIC COPY BmpHndl, 0 TO (0, 0)


CLOSE #1
PRINT "all done"
PRINT "Press a key"
WAITKEY$
GRAPHIC ATTACH BmpHndl, 0
GRAPHIC BITMAP END
GRAPHIC ATTACH WinHndl, 0
GRAPHIC WINDOW END

KILL $WORLD_RAW

END SUB

The last portion of the code should extract the lower right 1/4 of the white circle, the lower left 1/4 of the green circle, the upper right 1/4 of the red circle, and the upper left 1/4 of the cyan circle. However, because the data is written in a strictly linear fashion the code fails to extract the correct data. In addition, some of the data is missing and the rest is distorted. So, the problem is not reading and writing to a random access file but writing to it in such a fashion that beginning at any location and reading from it will produce the correct data.

[Michael Mattias]

I deal with this kind of thing (sort of) in an application where I am reading a print image file.... each "page" is known to contain 66 rows of 82 columns

I could I suppose use something like....

 GET$ hfile, 82 * 60 , somestring
 REDIM page (65, 81)  AT STRPTR(somestring) 
 ' get a character
 Z =  Page (Row, column)

The only thing I really have going for me here is that I know the dimensions (66 x 82) and therefore exactly how much space in the file is being taken by each page.

So.... if you had that info you could do the same thing.

BTW, I have no idea if I have the subscripts in the correct order. Nor do I particularly care, since this is an example only.

I can offer this much... I know a BMP file has a header so you'd have to offset your array ... ASSUMING BMP pixel data are actually stored in some kind of "row, column" format...

  GET$ hFile, LOF(hFile), bmpString$   ' get whole file into string var
  REDIM rowsandColumns (nRow-1, ncol-1) AT STRPTR (bmpstring$) [b]+ SIZEOF(bmpfileheader)[/b]

(For one who admittedly does not know squat about graphics.... don't you do stuff like get "quadrants" by MASKING the BMP when you display it?)

MCM

[Walt Decker]

Thanks, for the help Michael. I finally found the problem; I was just making it too complicated.

Actually the project has to do with graphics only in a roundabout way. It's a real world mapping project that incorporates land use, hazard, topographic, resource, ownership, and a variety of other data. The data has to be arranged in such a way that the user can bring up a map and click on a location, enter coordinates, an owner's name, a resource, or any of a variety of things and find out what's there or where it is.

Again, thanks.