Teaching Programming I

Introduction

An algorithm is a list of instructions which when carried out completes some task or solves a particular problem. Examples of algorithms include a recipe, a knitting pattern, the instructions to program your video recorder and computer programs.

A computer program is a list of instructions, which are executed by a computer. There are only three types of structure which make up a typical computer program and they are:

• Sequence - Unless ordered otherwise, a computer will execute a program line by line, in order, starting from the top.
• Selection - Sometimes you need to decide what to do next, from a list of alternatives.
• Repetition - Often you need to do the same thing, or nearly the same thing, over and over again.

Basic Structures

The students are shown the basic structure of a Pascal program:

program <Program Name>;

  uses <Library 1>,<Library 2>,...;  {The libraries you will use are listed here.}

  const max=10;                      {Program constants are declared here}

  type ....;                         {Don't worry about this section, it can be left out}

  var i: integer;                    {Main program variables are declared here}
      name: string;

  {All your procedures and functions will come next, in this section}
{------------------------------------------------------------------------------------------------------------------------}
  procedure <Procedure Name1>;
    begin
    .
    .
    end;
{------------------------------------------------------------------------------------------------------------------------}
  procedure <Procedure Name2>;
    begin
    .
    .
    end;
{------------------------------------------------------------------------------------------------------------------------}
  .
  .                                    {Other procedures and functions}
  .
{------------------------------------------------------------------------------------------------------------------------}
  begin      {<--Start of Main Program. When the program is run, it starts here}
  .
  .                                    {The main program is usually a menu}
  .
  .
  end.                                 {Note that the program ends with a full stop}

The basic Hello World! program, which simply writes the message "Hello World!" on the screen, is useful in showing what a minimal program must contain. All programs must build on from here. At the same time, I cover logging on to the network, loading the programming editor, compiling and running the program. This program also introduces their first procedure, writeln.

The programming structures are now introduced individually. The students first take notes, which explain how each one is written, with a general form and several specific examples as illustration. I then give them a simple programming task to complete, that involves using just that one structure, for example "Write a program that will output your name on the screen five times." I like to work it so that I break up the note taking with short practical exercises, like this:

Notes on FOR
Write a program using a FOR statement
Notes on REPEAT UNTIL
Write a program using a REPEAT loop, etc.

The initial exercises are very trivial, deliberately so, as it is important that students clearly understand these basic building blocks. It is especially important not to leave students feeling that they do not really understand what is happening at this stage, some find this introductory work quite daunting. The focus is totally on the particular statement/structure being studied, with no peripheral demands to cloud the issue.

Others do find some of these exercises very trivial and it is also important for them not to lose interest. This is easily dealt with, as the basic exercises can be readily extended and made more complex, for those students who are obviously coping well. I may give them additional requirements, or I may show the quicker students how to do additional things, such as how to change text and background colours and leave them to embelish their work. This has the knock-on benefit that this knowledge usually gets passed on to the rest of the class, who see what their peers are producing and want to be let in on the secret, without it having to be formally taught!

for i:=1 to 3 do
  write('Fred ');
write('Mary ');

for i:=1 to 3 do
  begin
  write('Fred ');
  write('Mary ');
  end;

count:=0;
REPEAT
    .
    .
    .
    count:=count+1;
UNTIL count=5;                 {<--- simple condition}

count:=0;
REPEAT
    readln(ch);
    .
    .
    count:=count+1;
UNTIL (count=5) or (ch='Q');   {<--- compound condition}

if MenuChoice = 1 then procedure1
  else if MenuChoice = 2 then procedure2
    else if MenuChoice = 3 then procedure3;

case MenuChoice of
  1: procedure1;
  2: procedure2;
  3: procedure3;
  end;

case MenuChoice of
  1: begin
     procedure1a;
     procedure1b;
     end;
  2: procedure2;
  3: procedure3
  else procedure4;
  end;