PASCAL: Functions

FUNCTIONS - A SPECIAL TYPE OF PROCEDURE WHICH RETURNS A VALUE
Procedures accept data or variables when they are executed. Functions also accept data, but have the ability to return a value to the procedure or program which requests it. Functions are used to perform mathematical tasks like factorial calculations.

A function

begins with the keyword function
is similar in structure to a procedure
somewhere inside the code associated with the function, a value is assigned to the function name
a function is used on the righthand side of an expression
can only return a simple data type

The actual heading of a function differs slightly than that of a procedure. Its format is,

      function  Function_name (variable declarations) : return_data_type;

After the parenthesis which declare those variables accepted by the function, the return data type (preceeded by a colon) is declared.


	function  ADD_TWO ( value1, value2 : integer ) : integer;
	begin
	     ADD_TWO := value1 + value2
	end;

The following line demonstrates how to call the function,


	result := ADD_TWO( 10, 20 );

thus, when ADD_TWO is executed, it equates to the value assigned to its name (in this case 30), which is then assigned to result.

SELF TEST 24
Determine the output of the following program


	program  function_time (input, output);
	const    maxsize = 80;
	type     line = packed array[1..maxsize] of char;

	function COUNTLETTERS ( words : line) : integer; {returns an integer}
	var      loop_count : integer;                   {local variable}
	begin
	      loop_count := 1;
	      while (words[loop_count] <> '.') and (loop_count <= maxsize) do
		      loop_count := loop_count + 1;
	      COUNTLETTERS := loop_count - 1
	end;

	var   oneline : line;
	      letters : integer;
	begin
	      writeln('Please enter in a sentence terminated with a .');
	      readln( oneline );
	      letters := COUNTLETTERS( oneline );
	      writeln('There are ',letters,' letters in that sentence.')
	end.

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