File Access and File Structure

Fortran supports two methods of file access:

• Sequential

• Direct

Fortran supports three kinds of file structure:

• Formatted

• Unformatted

• Binary

Sequential-access and direct-access files can have any of the three file structures.

The following kinds of files are possible:

• Formatted Sequential

• Formatted Direct

• Unformatted Sequential

• Unformatted Direct

• Binary Sequential

• Binary Direct

Choosing a File Access and File Structure

Each kind of file has advantages and the best choice depends on the application you are developing:

• Formatted Files

  You create a formatted file by opening it with the FORM='FORMATTED' option, or by omitting the FORM parameter when creating a sequential file. The records of a formatted file are stored as ASCII characters; numbers that would otherwise be stored in binary form are converted to ASCII format. Each record ends with the ASCII carriage return (CR) and line feed (LF) characters.

  If you need to view a data file's contents, use a formatted file. You can load a formatted file into a text editor and read its contents directly, that is, the numbers would look like numbers and the strings like character strings, whereas an unformatted or binary file looks like a set of hexadecimal characters.

• Unformatted Files

  You create an unformatted file by opening it with the FORM='UNFORMATTED' option, or by omitting the FORM parameter when creating a direct-access file. An unformatted file is a series of records composed of physical blocks. Each record contains a sequence of values stored in a representation that is close to that used in program memory. Little conversion is required during input/output.

  The lack of formatting makes these files quicker to access and more compact than files that store the same information in a formatted form. However, if the files contain numbers, you will not be able to read them with a text editor.

• Binary Files

  You create a binary file by specifying FORM='BINARY'. Binary files are similar to unformatted files, except binary files have no internal record format associated with them.

• Sequential-Access Files

  Data in sequential files must be accessed in order, one record after the other (unless you change your position in the file with the REWIND or BACKSPACE statements). Some methods of I/O are possible only with sequential files, including nonadvancing I/O, list-directed I/O, and namelist I/O. Internal files also must be sequential files. You must use sequential access for files associated with sequential devices.

  A sequential device is a physical storage device that does not allow explicit motion (other than reading or writing). The keyboard, screen, and printer are all sequential devices.

• Direct-Access Files

  Data in direct-access files can be read or written to in any order. Records are numbered sequentially, starting with record number 1. All records have the length specified by the RECL option in the OPEN statement. Data in direct files is accessed by specifying the record you want within the file. If you need random access I/O, use direct-access files. A common example of a random-access application is a database.