Procedures, Parameters & Sub-Programs

In any modern programming language, procedures play a vital role in the
construction of any new software. These days, procedures are used instead of
the old constructs of GOTO and GOSUB, which have since become obsolete.

Procedures provide a number of important features for the modern software
engineer:-

Programs are easier to write. Procedures save a large amount of time during
software development as the programmer only needs to code a procedure once, but
can use it a number of times. Procedures are especially useful in recursive
algorithms where the same piece of code has to be executed over and over again.
The use of procedures allows a large and complex program to be broken up into a
number of much smaller parts, each accomplished by a procedure. Procedures also
provide a form of abstraction as all the programmer has to do is know how to
call a procedure and what it does, not how it accomplishes the task.

Programs are easier to read. Procedures help to make programs shorter, and thus
easier to read, by replacing long sequences of statements with one simple
procedure call. By choosing goo procedure names, even the names of the
procedures help to document the program and make it easier to understand.

Programs are easier to modify. When repeated actions are replaced by one
procedure call, it becomes much easier to modify the code at a later stage, and
also correct any errors. By building up the program in a modular fashion via
procedures it becomes much easier to update and replace sections of the program
at a later date, if all the code for the specific section is in a particular
module.

Programs take less time to compile. Replacing a sequence of statements with
once simple procedure call usually reduces the compilation time of the program,
so long as the program contains more than one reference to the procedure!

Object programs require less memory. Procedures reduce the memory consumption
of the program in two ways. Firstly they reduce code duplication as the code
only needs to be stored once, but the procedure can be called many times.
Secondly, procedures allow more efficient storage of data, because storage for a
procedure\'s variables is allocated when the procedure is called and deallocated
when it returns.

We can divide procedures into two groups:-

Function procedures, are procedures which compute a single value and whose calls
appear in expressions

For example, the procedure ABS is a function procedure, when given a number x,
ABS computes the absolute value of x; a call of ABS appears in an expression,
representing the value that ABS computes.

Proper procedures, are procedures whose calls are statements

For example, the procedure INC is a proper procedure. A call of INC is a
statement; executing INC changes the value stored in a variable.

Procedures have only one real disadvantage: Executing a procedure requires
extra time because of the extra work that must be done both when the procedure
is called, and when it returns.

Most of the time however, the advantages of using procedures heavily outweigh
this minor disadvantage.

Most procedures depend on data that varies from one call to the next, and for
this reason, Modula-2 allows a procedure heading to include a list of
identifiers that represent variables or expressions to supply when calling the
procedure. The programmer can use these identifiers, known as formal
parameters, in the body of the procedure in the same fashion as ordinary
variables.

A call of a procedure with parameters must include a list of actual parameters.
The number of actual parameters must be the same as the number of formal
parameters. Correspondence between the actual and formal parameters is by
position, so the first actual parameter corresponds to the first formal
parameter, the second actual parameter corresponds to the second formal
parameter, and so on. The type of each actual parameter must match the type of
the corresponding formal parameter.

Modula-2 provides to kinds of formal parameters:-

Variable parameters. In a procedure heading, if the reserved word VAR precedes
a formal parameter, then it is a variable parameter. Any changes made to a
variable parameter within the body of the procedure also affect the
corresponding actual parameter in the main body of the program.

Value parameters. If the reserved word VAR does not precede a formal parameter
then it is a value parameter. If a formal parameter is a value parameter, the
corresponding actual parameter is protected from change, no matter what changes
are made to the corresponding parameter in the procedure.

To sum up, variable parameters allow information to flow both into and/or out of
a procedure, whereas