Chapter 7

Functions and Procedures

So far, all the programs in this tutorial have been contained inside a single block. This has been fine since all of the programs in this tutorial, so far, have been very short. Larger programs should be divided into smaller blocks called SUBPROGRAMS. Subprograms are implemented in Pascal as FUNCTIONS and PROCEDURES.

PROCEDURES

Program Intro_To_Procedures (input, output);

var

  i : integer;


{The following is a procedure}
{it looks like a small program}


Procedure Sample_Procedure;
Begin
  Writeln('Greetings from');
  Writeln('the Sample_Procedure');
  Writeln('now back to your );
  Writeln('regularly scheduled program');
End;
  

{now begins the main block}
{ of the program.}

Begin


  Sample_Procedure; {call the subprogram}


  Writeln('The End');

End.

Procedures go between the variable section and the main program. A procedure must have a procedure heading. The procedure heading is similar to the program heading. It starts with the keyword "PROCEDURE." It must be followed by the name of the procedure. The name must be a valid identifier. In the example above, the procedure name is followed by a semicolon. This means that no variables must be passed to the procedure.

Some procedures require that variables be passed to them. These variables are called arguments. Take the following program for example. It includes a procedure that multiplies the first argument by ten and the second argument by twenty.

Program Proc_with_args(input, output);

var 
  i : integer;


Procedure Multiply(x : integer; y : integer);
var
  z : integer
Begin
  z := 10 * x;  {multiply the argument by 10}

  Writeln('10 multiplied by ', x, ' is ', z);

  z := 20 * y;

  Writeln('20 multiplied by ', y, ' is ', z);

end;


Begin
  
  For i := 1 to 10 do Begin
    Multiply(i, i);  {call the procedure}
  End;

End.

In this example the procedure heading is slightly modified. Instead of being followed by a semicolon, the procedure name is followed by an argument list. The argument list tells the compiler how many arguments the procedure takes and their datatypes.

GLOBAL AND LOCAL VARIABLES

Notice, in the example above that there is a VAR section after the procedure heading. This VAR section allows LOCAL VARIABLES to be declared. Local Variables are variables that exist only inside a function or procedure. They can only be accessed from within the function or procedure. In previous examples, all programs had GLOBAL VARIABLES. Global variables are declared in the VAR section below the program heading and can be accessed by the main program, all procedures, and functions. Local variables should be used whenever possible. Since they can only be accessed from within the function or procedure that they are in, there is less of a risk that they will be accidentaly modified.

FUNCTIONS

Functions are similar to procedures. Functions in Pascal are also similar to functions in algebra. They take a value, perform various calculations, and then return a new value. Take the following for instance.

Program Intro_to_functions (input, output);

Var
  i : integer;
  n : integer;


Function  Test_Function( x : integer):integer;
Begin
  {This function performs the }
  { mathematical equivalent of}
 {  f(x) = 2x + 1 }
  
  Writeln('performing calculation');
  Test_Function := 2 * x + 1;

End;


Begin

  for i := 1 to 10 do begin
    n := test_function(i);
    Writeln(n);
  end;


End.

In this example the argument list is followed by a colon and a datatype. This tells the compiler what type of data the function returns. In this case the function returns an integer. To return a value the return value is assigned to the name of the function.