Random File I/O
Random I/O requires a bit more care and coding than sequential I/O, but is more flexible because you can move back and forth within a disk file. These are the key differences:
•To get data from random disk files to BASIC-80 variables and vice-versa, you must understand how BASIC-80 uses 110 buffer space.
•Random 110 operations (GET and PUT) always read or write all 128 bytes of a floppy disk sector. You must specify how this 128-byte string is divided into fields and assigned to string variables.
•Because all data in random files is treated as string data, you must convert strings that represent numbers into numeric values if you want to do any calculations with them.· Likewise, if you want to write a numeric value to a ran- dom file, you must first convert it to a string. Special functions are provided to do this.
•You must use special assignment statements to put a string variable into an 110 buffer. This is due to the fact that the fields you define in an I/O buffer are of fixed-length; the special assignment statements accommodate variables that are longer or shorter than the specified field.
•Unless you simply want to read or write records in sequence, you .must specify which record you want to read or write.
1/0 Buffers
BASIC-80 reserves space for six I/O buffers, each 128 bytes long. One buffer is assigned to each open file (when an OPEN statement is executed). When you read a record from a random file using GET, the 128-byte string is not assigned directly to a variable. It is placed in the 110 buffer assigned to that file, and it is up to you to define what portions of the string are to be assigned to what variable or variables.
To emphasize: the usual variable and line delimiters (", CR) have no special mean- ing. All 128 bytes from the specified sector are placed in the 110 buffer. If fewer than 128 bytes were written to the sector, the remaining character positions in the buffer are set to ASCII blanks (hexadecimal 20) .
Defining a Random 1/0 Field-FIELD
When a sector has been read into the 110 buffer, it exists as a 128-byte string. You must define how the string is to be assigned to a variable or variables. The FIELD statement specifies to BASIC-80 what character positions in the buffer correspond to string variables (initially they must be string variables, because the sector is treated as a 128-byte string).
The FIELD statement tells BASIC-80, for a given file, what character positions, starting with the first, are to be associated with a variable. For example, assume you have opened a random file, assigned it the file number 3 (the OPEN command works just like in sequential 110), and read the first sector from the file. You know that the first 20 characters of each sector contain a name. To assign these 20 bytes to the string variable N$, enter the following FIELD statement:
FIELD #3, 20 AS N$
