Algol 68 programs can be written in one or more parts Here is a valid Algol 68 program:
PROGRAM firstprogram CONTEXT VOID USE standard BEGIN print(20) END FINISH
Only the three lines starting with BEGIN
and ending
with END
are strictly part of the Algol 68 program. The
first, second and last lines are specific to the
a68toc compiler. The first line gives the
identification of the program as firstprogram and the fact
that this file contains a program. The CONTEXT VOID
phrase specifies that the program stands on its own instead of being
embedded in other parts. The phrase is a vestige of the modular
compilation system originally provided by the compiler at the heart of
a68toc
.
A full explanation of the print
phrase will be found in chapter 9 (Transput).
For now, it is enough to know that it causes the value in the
parentheses to be displayed on the screen.2.3 The
standard prelude must be USEd if you want
to use print. You can use any identifier
for the operating system file in which to store the Algol 68
source code of the program. Although it does
not have to be the same as the identifier of the module, it is
advisable to make it so.
Both the print
phrase and the
denotation are units. Chapter 10
will explain units in detail. Phrases are separated by the go-on
symbol (a semicolon ;). Because there is
only one phrase in firstprogram
, no go-on symbols are
required. Here is another valid program:
PROGRAM prog CONTEXT VOID USE standard BEGIN INT special integer = 48930767; print(20) ; print(special integer) END FINISH
The semicolon between the two print
phrases is not a
terminator: it is a separator. It means “throw away any value
yielded by the previous phrase and go on with the succeeding
phrase”. That is why it is called the go-on
symbol. Notice that there is no semicolon after the
third phrase.
Algol 68 programs are written in free format. This means that the meaning of your program is independent of the layout of the source program. However, it is sensible to lay out the code so as to show the structure of the program. For example, you could write the first program like this:
PROGRAM firstprogram CONTEXT VOID USE standard(print(20))FINISH
which is just as valid, but not as comprehensible. Notice that
BEGIN
and END
can be replaced by (
and )
respectively. How you lay out your program is up to you, but writing
it as shown in the examples in this book will help you write
comprehensible programs.
PROGRAM test BEGIN print("A")) END FINISHAns
Sian Mountbatten 2012-01-19