| 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 allows the position in a file to be changed. This allows random access to data within files and can be used with files opened by ReadFile, WriteFile and OpenFile. Note, the offset is the number of bytes from the start of the file, where the first byte is at offset 0. It is important to take account of the size of the data elements in your file.
For instance Integers are 4 bytes long so the first integer in the file is at offset 0 and the second at offset 4. If you write Custom Data types out then you must work out haw many bytes each takes so that you can move about the file correctly. Seeking beyond the end of a file does not generate an error but the data is not read or written to the file, and may course unknown side effects. 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. To calculate an offset you need to know how long each data element is; Offset = Wanted_Element * size_of_element - size_of_element For example a file of integers which are 4 bytes long is calculated by: The 7th integer is at offset 7 * 4 - 4 i.e. 24 Note, extreme care needs to be exercised when updating files that contain strings since these are not fixed in length. |
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; Changing part of a file using OpenFile, SeekFile, FilePos
; Open/create a file to Write fileout = WriteFile("mydata.dat") ; Write the information to the file WriteInt( fileout, 100 ) WriteInt( fileout, 200 ) WriteInt( fileout, 300 ) WriteInt( fileout, 400 ) WriteInt( fileout, 500 ) ; Close the file CloseFile( fileout ) DisplayFile( "The file as originally written", mydata.dat" ) Print "Data read in random order" ; Open the file to read just the 4th and 2nd elements from file = OpenFile("mydata.dat") ; read and print the 4th integer ie 4*4-4 = 12 byte from the start of the file SeekFile( file, 12 ) ; Move to the found location Number = ReadInt( file ) Print Number ; read and print the 2th integer ie 2*4-4 = 4 bytes from the start of the file SeekFile( file, 4 ) ; Move to the found location Number = ReadInt( file ) Print Number CloseFile( file ) 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 |