Random Mode Files Record length

[Charles Perry]

Below is a single record write to a random access file with the consol screen output below.
Why is the read back only 18 bytes when the write string is 20 bytes
from the LEN = 20. Are these 2 missing bytes the string length?

#COMPILE EXE
'#DIM ALL
FUNCTION PBMAIN () AS LONG
OPEN "f:\pb35\new.dat" FOR RANDOM AS #1 LEN =20
A$ = "12345678911234567892"
St% = LEN(A$)
PRINT St%, A$
PUT #1,1,A$
CLOSE #1
OPEN "F:\pb35\new.dat" FOR RANDOM AS #2 LEN=20
GET #2,1,B$
Stb% = LEN( B$)
PRINT Stb%,B$
INPUT C$
END FUNCTION
20 12345678911234567892
18 123456789112345678

[Michael Mattias]

Because when you write a dynamic string variable to a file opened FOR RANDOM, you're not really writing just the string unless you do something special.

Look up the options for the PUT and GET statements again.

[Guy Dombrowski]

Put that code before the OPEN

DIM a AS STRING * 20
DIM b as STRING * 20

Then it will work

[Michael Mattias]

 DIM a AS STRING * 20
 DIM b as STRING * 20

Then it will work

If that's all you change, no it won't. Your records will be space-filled to 20 bytes, which is not what the original code is trying to do, which is to store dynamic (dynamic = variable-length) string data.

You, too, might want to mosey on back over to the help file pages for the PUT and GET statements and check out some of the options.

[John Petty]

From the manual "When writing a dynamic string to a random access file, PUT writes a 2-byte descriptor containing the string's length, before the actual string data. This descriptor reduces the available space in a record by two bytes. "

[Guy Dombrowski]

If all you want is write is variable length string, it would be better to switch to sequential mode.
To use random mode you have to know in advance the size of each field.
Still better would be the Binary mode.

[Michael Mattias]

>it would be better to switch to sequential mode.
That may be true in this application, but we don't don't know enough to say that with confidence.

>To use random mode you have to know in advance the size of each field

No, you don't. The length word handles that for you. You just have to make sure your record length is at least SIZEOF(LONG) + max dynamic string length. (It also wastes disk, but that may be a price worth paying for the benefits of random access).

MCM

[Guy Dombrowski]

I have fixed length random customer files but the section for the name and address is made of dynamic string separated with a chr$(13).
So although the maximum per line is 40 characters, the average is a lot less than that. Use PARSE to read back the data.

[Nick Luick]

Good method Guy that I implemented years ago in DOS days when space was more critical. I found that its rare to have all allocated string lengths maxed out.

[Mike Doty]

#COMPILE EXE
#DIM ALL
FUNCTION PBMAIN () AS LONG
 
  LOCAL a AS STRING
  LOCAL st AS INTEGER
  LOCAL b AS STRING
  LOCAL stb AS INTEGER
  LOCAL c AS STRING
 
  OPEN "new.dat" FOR RANDOM AS #1 LEN =20
  A$ = "12345678911234567892"
  St% = LEN(A$)
  PRINT St%, A$
  PUT #1,1, ABS A$  'added ABS
  CLOSE #1
 
  OPEN "new.dat" FOR RANDOM AS #2 LEN=20
  b = SPACE$(20)    'required
  GET #2,1, ABS B$  'added ABS
  Stb% = LEN( B$)
  PRINT Stb%,B$
  INPUT C$
 
END FUNCTION

IF you want multi-user or cgi THEN
  [URL]http://www.powerbasic.com/support/pbforums/showthread.php?t=24345[/URL]
END IF