programming-for-dummies

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Storing Varying Size Data in Untyped Files
Random-access files make it easy to store and retrieve data. Because you
retrieve only the data you want, random-access files don’t waste time reading
the entire file.
Storing Varying Size Data in Untyped Files
Random-access files are great for storing chunks of data of equal size.
However, what if you want to store data that may vary in size? You could
define a record as the largest size data you need to store, but that means you
wind up wasting space. As a more efficient alternative, you can store data in
untyped files. Untyped files still organize data in records, but each record can
vary in size, as shown in Figure 8-3.
Figure 8-3:
Untyped
files contain
records that
can vary
in size.
Records can be any size in an untyped file.
Writing data
To store data in an untyped file, you must first name and create an untyped
file. In the Delphi programming language, you can create an untyped file by
declaring a variable as a File type, such as
var
myFile : File;
After you create a variable name, you need to assign that variable name to
an actual filename, such as
AssignFile(myFile, ‘MyData.dat’);
In some languages, such as BASIC, you assign a number to an actual filename.
In Delphi and other languages, you assign a name to an actual file. The
purpose of assigning a number or a name is so that you can refer to a file
without typing the complete filename.
Before you can add any data to an untyped file, use the ReWrite command,
which defines the filename to use and the number of blocks to add to the
file. (Each block of data typically corresponds to one byte of data.) So if you
want to define a 5-byte block of data, you’d use the following:
ReWrite(myFile, 5);