PASCAL: Procedures

SIMPLE PROCEDURES
Procedures are used to perform tasks such as displaying menu choices to a user. The procedure (module) consists of a set of program statements, grouped by the begin and end keywords. Each procedure is given a name, similar to the title that is given to the main module.

Any variables used by the procedure are declared before the keyword begin.


	PROCEDURE  DISPLAY_MENU;
	begin
	     writeln('<14>Menu choices are');
	     writeln(' 1: Edit text file');
	     writeln(' 2: Load text file');
	     writeln(' 3: Save text file');
	     writeln(' 4: Copy text file');
	     writeln(' 5: Print text file')
	end;

The above procedure called DISPLAY_MENU, simply executes each of the statements in turn. To use this in a program, we write the name of the procedure, eg,


	program PROC1 (output);
	
	PROCEDURE  DISPLAY_MENU;
	begin
	     writeln('<14>Menu choices are');
	     writeln(' 1: Edit text file');
	     writeln(' 2: Load text file');
	     writeln(' 3: Save text file');
	     writeln(' 4: Copy text file');
	     writeln(' 5: Print text file')
	end;

	begin
		writeln('About to call the procedure');
		DISPLAY_MENU;
		writeln('Now back from the procedure')
	end.

In the main portion of the program, it executes the statement


		writeln('About to call the procedure');

then calls the procedure DISPLAY_MENU. All the statements in this procedure are executed, at which point we go back to the statement which follows the call to the procedure in the main section, which is,


		writeln('Now back from the procedure')

The sample output of the program is

	About to call the procedure
	Menu choices are
	 1: Edit text file
	 2: Load text file
	 3: Save text file
	 4: Copy text file
	 5: Print text file
	Now back from the procedure

SELF TEST 20: SIMPLE PROCEDURES
What does this program display?


	program  SIMPLE_PROCEDURES (input,output);
	var      time, distance, speed : real;

	procedure display_title;
	begin
	     writeln('This program calculates the distance travelled based');
	     writeln('on two variables entered from the keyboard, speed and');
	     writeln('time.')
	end;

	procedure get_choice;
	begin
	      writeln('Please enter the speed in MPH');
	      readln( speed );
	      writeln('Please enter the time in hours');
	      readln( time )
	end;

	procedure calculate_distance;
	begin
	      distance := speed * time
	end;

	procedure display_answer;
	begin
	      writeln('The distance travelled is ', distance:5:2,' miles.')
	end;

	begin      {This is the actual start of the program}
	      display_title;
	      get_choice;
	      calculate_distance;
	      display_answer
	end.

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{Note that the three variables, time, speed and distance, are available to all procedures. They may be updated by any procedure, and are known as GLOBAL variables}.

Variables which are declared external (outside of) to any procedure are accessible anywhere in the program. The use of global variables is limited. In a large program, it is difficult to determine which procedure updates the value of a global variable.

PROGRAM FIFTEEN
Convert the calculator program (program 12), using simple procedures, to perform the various calculations. Use global variables for number1, operator and number2.

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