Hello again. I haven't gotten much input about continued interest in this tutorial. I may discontinue it if the interest doesn't pick up. I am planning on taking this tutorial through all of the Pascal data structures, and maybe special topics (advanced ones, too, listen up self-professed Pascal experts -- you may even learn something. :>). I haven't gotten any suggestions on the special topics!!! Please give comments to [email protected].

An example of a solution of last week's programming problem:

program part2;
  { This program accompanies part 2.  It is a program designed to take a
    number representing the dimensions of a multiplication table from the
    user, and then write a multiplication table out to the screen. }

  var
    dimension: integer;
    i, j: integer;

  begin { part 2 }

    {take input for the dimension}
    write('What dimension number do you want for a table? ');
    readln(dimension);
    writeln;

    { if dimension not greater than 15, process table }
    if dimension <= 15 then
      begin

        { write top line of table }
        write('   ');
        for i := 1 to dimension do
          write(i:4);
        writeln;

        { write top rule line }
        write(#201:3);
        for i := 1 to (dimension * 4) do
          write(#205);
        writeln;

        { write rest of table }
        for i := 1 to dimension do
          begin
            if i < 10 then
              write(' ');
            write(i,#186);
            for j := 1 to dimension do
              write(i*j :4);
            writeln;
          end;
      end

    { else it's greater than 15, write an error message }
    else
      writeln('You must give a dimension that''s 15 or lower.');
  end.

If there are any questions as to understanding, or difficulties in solving the practice problems I pose (only way to improve your own programming talent is to practice...), write [email protected].

It is possible to have a loop to perform a set of commands a non-set number of times, until a condition is met. It can be a contrived one (we can code the basic idea of a FOR loop using a while loop or the repeat loop. We see this in the recode of tutorial6 from last time I will make using a while loop instead of a for loop.

program tutorial7;
  var
    i: integer;
  begin
    writeln('I''m going to write something 10 times.');
    i := 1;
    while i <= 10 do
      begin
        writeln('something', '(Time #':15, i, ')');
        i := i + 1;
      end;
  end.

As we see when we run this program, it produces the same output as program tutorial6 did. The statements in the while loop function while the condition is true. When i becomes 11, the loop breaks off and the program ends. Like the IF statements, we can place multiple conditions by connecting them like before with the AND or OR identifiers...

This is another loop we can use. The WHILE loop will function while a condition is true. The REPEAT loop stops fun- ctioning when a condition is true. We see the idea of this again, when we reconstruct program tutorial6 using the repeat loop...

program tutorial8;
  var
    i: integer;
  begin
    writeln('I''m going to write something 10 times.');
    i := 1;
    repeat
      writeln('something', '(Time #':15, i, ')');
      i := i + 1;
    until i > 10;
  end.

The programs tutorial6, tutorial7, and tutorial8 perform the same things, with loops, using different ideas. The difference of the while and repeat loops over the for loop is that UNDER NO CIRCUMSTANCES IS THE INDEX VARIABLE FOR A FOR LOOP TO BE CHANGED WHILE INSIDE THE LOOP. The conditional variable for a while or repeat loop can be easily changed as we saw in tutorial7 and tutorial8. There are many choices and options we can implement with the while and repeat loops. Menu systems are often implemented like this Continue until user wants to quit. is the basic logic.).

As we saw in tutorial5, we may want to make a choice based on many, multiple options. This statement is analogous to a series of IF statements on the same variable. The CASE statement reduces the wordiness of such a construct, and makes things easier. I was eluding to this statement before when I mentioned then that there is a better way of doing it. Well, here it is. We use the example of tutorial9 below, which is a rewrite of tutorial5, to illustrate the use and syntax of a case statement. Keep in mind that the operator we use in the case statement (like option below) must be a character or an integer...

program tutorial9;
  var
    one, two: integer;
    option: char;
  begin
    writeln('Enter an integer.');
    readln(one);
    writeln('Enter another integer.');
    readln(two);
    writeln('Use a mathematical symbol to indicate what you want to do');
    writeln('with these two numbers.');
    readln(option);
    case option of
      '+': begin   { sub procedures can be coded as well }
             writeln(one, ' + ', two, ' = ', one + two, '.');
             writeln('See, I can add.');
           end
      '-': writeln(one, ' - ', two, ' = ', one - two, '.');
      '*': writeln(one, ' * ', two, ' = ', one * two, '.');
      '/': writeln(one, ' / ', two, ' = ', one / two :0:3, '.');
    else {catch rest}
      writeln('Use +, -, *, or / as your operator.  Try again.');
    end; {case includes an implied begin.  We MUST end. }
  end.

As I said in the note, case statements include an implied BEGIN. We must say end; to complete the case statement. The case statement is formatted for each of our choices above, and the else can be used as a catch-all in case the user places something in there that we don't account for in the program, so we can write an error message to the user. The syntax of the case statement is basically as above. You see everything that can be done with the case statement under Pascal.

We can generate random numbers by doing the following. At the beginning of the program, call randomize. Then do random(). What will happen is it will produce an integer from 0 and less than the number you put in. Random(3) will produce a random number from 0-2. Example.

program tutorial10;
  { write 10 random numbers between 1 and 20 }
  var
    number: integer;
    i: integer;
  begin
    randomize;
    { start the random number generator.  Only call once, but must call! }
    for i := 1 to 10 do
      writeln('Random number (#', i,') = ', random(20) + 1);
      { produce random number from 1-20 instead of 0-19 like random(20) only
        does }
  end.

I'm thinking of a number between 1 and 100.  What is it?
50
It's higher. (5 guesses remaining)
75
It's lower.  (4 guesses remaining)
63
It's higher. (3 guesses remaining)

If right:
Congratulations!  You got the number right!

If wrong:
Sorry, you ran out of choices.  The number I was thinking of is 68.

Play again:
Do you want to play again? (Y/N)

We will discuss functions and procedures and their use next time. Please refer your comments to [email protected].