Algorithms and Data Structures

N.Wirth. Algorithms and Data Structures. Oberon version 26
A sequence is generated by appending elements at its end after having placed a rider on the file.
Assuming the declaration
VAR r: Rider
we position the rider r on the file f by the statement
Set(r, f, pos)
where pos = 0 designates the beginning of the file (sequence). A typical pattern for generating the
sequence is:
WHILE more DO compute next element x; Write(r, x) END
A sequence is inspected by first positioning a rider as shown above, and then proceeding from element to
element. A typical pattern for reading a sequence is:
Read(r, x);
WHILE ~r.eof DO process element x; Read(r, x) END
Evidently, a certain position is always associated with every rider. It is denoted by r.pos. Furthermore,
we postulate that a rider contain a predicate (flag) r.eof indicating whether a preceding read operation had
reached the sequence's end. We can now postulate and describe informally the following set of primitive
operators:
1a. New(f, name) defines f to be the empty sequence.
1b. Old(f, name) defines f to be the sequence persistently stored with given name.
2. Set(r, f, pos) associate rider r with sequence f, and place it at position pos.
3. Write(r, x) place element with value x in the sequence designated by rider r, and advance.
4. Read(r, x) assign to x the value of the element designated by rider r, and advance.
5. Close(f) registers the written file f in the persistent store (flush buffers).
Note. Writing an element in a sequence is often a complex operation. However, mostly, files are created
by appending elements at the end.
Translator's note. The program examples in the book use two more commands:
6. WriteInt(r, n) place the integer n in the sequence designated by rider r, and advance.
7. ReadInt(r, n) assign to n the integer value designated by rider r, and advance.
In order to convey a more precise understanding of the sequencing operators, the following example of
an implementation is provided. It shows how they might be expressed if sequences were represented by
arrays. This example of an implementation intentionally builds upon concepts introduced and discussed
earlier, and it does not involve either buffering or sequential stores which, as mentioned above, make the
sequence concept truly necessary and attractive. Nevertheless, this example exhibits all the essential
characteristics of the primitive sequence operators, independently of how the sequences are represented in
store.
The operators are presented in terms of conventional procedures. This collection of definitions of types,
variables, and procedure headings (signatures) is called a definition. We assume that we are to deal with
sequences of characters, i.e. text files whose elements are of type CHAR. The declarations of File and Rider
are good examples of an application of record structures because, in addition to the field denoting the array
which represents the data, further fields are required to denote the current length and position, i.e. the state
of the rider.