| filehandle = the variable returned by the Readfile WriteFile or OpenFile when the file was opened. The value returned is the offset from the start of the file. ( 0 = Start of the file ) |
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This command returns the current position within a file that is being processed following ReadFile, WriteFile or OpenFile. The returned integer is the offsets in bytes from the start of the file to the current read/write position. Note, this is zero when pointing to the first byte of the file.
By using FilePos and SeekFile the position within the file that is being read or written can be determined and also changed. This allows a file to be read and updated without having to make a new copy of the file or working through the whole file sequentially. This could be useful if you have created a database file and you want to find and update just a few records within it. It is also possible to create an index file that contains pointers to where each record starts in a data file. |
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; Changing part of a file using OpenFile, SeekFile, FilePos
; note FilePos is used in the SearchFile function at the end of this example ; Open/create a file to Write fileout = WriteFile("mydata.dat") ; Write the information to the file WriteInt( fileout, 1 ) WriteInt( fileout, 2 ) WriteInt( fileout, 3 ) WriteInt( fileout, 4 ) WriteInt( fileout, 5 ) ; Close the file CloseFile( fileout ) DisplayFile( "The file as originally written", mydata.dat" ) Position = SearchFile( 4 , "mydata.dat" ) Write "Value 4 was found " Write Position Print " bytes from the start." ; Open the file and change the value 3 to 9999 file = OpenFile("mydata.dat") SeekFile( file, Position ) ; Move to the found location WriteInt( file, 9999 ) ; Replace the original value with 9999 CloseFile( file ) DisplayFile( "The file after being modified", "mydata.dat" ) WaitKey() End ; End of program ; **** Function Definitions follow **** ; Read the file and print it Function DisplayFile( Tittle$, Filename$ ) Print tittle$ file = ReadFile( Filename$ ) While Not Eof( file ) Number = ReadInt( file ) Print Number Wend CloseFile( file ) End Function ; Search a file of integers for the Wanted data value ; Note the need to subtract 4 from the location since having read it ; we are now pointing at the next Integer also Return() was placed ; after closing the file so it is closed properly Function SearchFile( Wanted, Filename$ ) file = ReadFile( Filename$ ) While Not Eof( file ) If ReadInt( file ) = Wanted Then Location = FilePos( file ) - 4 Wend CloseFile( file ) Return( Location ) End Function |