Class Crib Notes for CPSC 110 Brent Dingle

Wednesday, February 20, 2002

Reminders:

Quiz Next Week à Wed., Feb. 27

No class on Friday, Feb. 22 due to relocation =).

Review:

Last time we went through Top Down Design

New Stuff for today

Procedures (chap 4)

Some Key Words to watch for:

Global Variables, Local Variables, Scope, Formal Parameters, Actual Parameters, Variable Parameters, Value Parameters, Parameter List

Example 1:

Task:

Write a program that will infinitely ask for people’s names and say hello to them.

PROGRAM ProcEx;

VAR

first_name, last_name : string;

BEGIN { main body }

While (true) DO { notice this is an infinite loop }

Begin

{ Get Name }

write(‘Please enter your first name -> ‘);

readln(first_name);

write(‘Please enter your last name -> ‘);

readln(last_name);

{ Say hello }

writeln(first_name, last_name, ‘… such a nice name.’);

writeln(‘You must be a great person.’);

writeln;

End;

END.

So we could break our main task into 2 subtasks:

GetName and SayHello

Notice both subtasks SHOULD require parameters of first_name and and last_name. We will get back to that.

 
So let’s create a new program using PROCEDURES to implement our subtasks
Our first version will NOT use parameters but make use of the global scope of GLOBAL variables
 

PROGRAM ProcEx2;

VAR

first_name, last_name : string;

PROCEDURE GetName

BEGIN

write(‘Please enter your first name -> ‘);

readln(first_name);

write(‘Please enter your last name -> ‘);

readln(last_name);

END;

PROCEDURE SayHello;

BEGIN

writeln(first_name, last_name, ‘… such a nice name.’);

writeln(‘You must be a great person.’);

END; { end of proc Message }

BEGIN { main body }

WHILE (true) DO

Begin

GetName;

SayHello;

End;

END.

Now using global variables in procedures is BAD, VERY BAD.

Altering global variables in procedures is even WORSE.

 
So we want to send parameters to the procedures instead. Notice a couple things:

• Both GetName and SayHello would need parameters for first name and last name
• GetName would need to be able to ALTER the contents of parameters (i.e. set them to what the user enters)
• SayHello would NOT need to alter them.

If a procedure can alter a given parameter that parameter is called a VARIABLE PARAMETER.

If a procedure cannot alter a given parameter that parameter is called a VALUE PARAMETER.

So now let’s write that program:

PROGRAM ProcEx3;

VAR

first_name, last_name : string;

{ notice the VAR in front of the variable name means VARIABLE parameter }

{ notice also the variable names duplicate the global var names – they don’t have to }

PROCEDURE GetName(VAR first_name : string; VAR last_name : string);

BEGIN

write(‘Please enter your first name -> ‘);

readln(first_name);

write(‘Please enter your last name -> ‘);

readln(last_name);

END;

{ Note: there is no VAR before either variable name, note also the names are NOT the

same as the global names }

PROCEDURE SayHello(fname : string; lname : string);

BEGIN

writeln(fname, lname, ‘… such a nice name.’);

writeln(‘You must be a great person.’);

END; { end of proc Message }

BEGIN { main body }

WHILE (true) DO

Begin

GetName(first_name, last_name); { note we MUST send first_name first and last_name second }

SayHello(first_name, last_name); { else we will get them mixed around Bob Smith becomes Smith Bob }

End;

END.

 
And we may get into an more complicated example.
 

Next time

Procedures, maybe identifiers some more definitions about scope, continuing examples