Basic Pascal Tutorial/Chapter 3/Solution

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3Ea - Solutions (author: Tao Yue, state: unchanged)

Solution to Fibonacci Sequence Problem

(* Author:    Tao Yue
   Date:      19 July 1997
   Description:
      Find the first 10 Fibonacci numbers
   Version:
      1.0 - original version
*)

program Fibonacci;

var
   Fibonacci1, Fibonacci2 : integer;
   temp : integer;
   count : integer;

begin    (* Main *)
   writeln ('First ten Fibonacci numbers are:');
   count := 0;
   Fibonacci1 := 0;
   Fibonacci2 := 1;
   repeat
      write (Fibonacci2:7);
      temp := Fibonacci2;
      Fibonacci2 := Fibonacci1 + Fibonacci2;
      Fibonacci1 := Temp;
      count := count + 1
   until count = 10;
   writeln;

   (* Of course, you could use a FOR loop or a WHILE loop
      to solve this problem. *)

end.     (* Main *)

Solution to Powers of Two Problem

(* Author:    Tao Yue
   Date:      13 July 2000
   Description:
      Display all powers of two up to 20000, five per line
   Version:
      1.0 - original version
*)

program PowersofTwo;

const
   numperline = 5;
   maxnum = 20000;
   base = 2;

var
   number : longint;
   linecount : integer;

begin    (* Main *)
   writeln ('Powers of ', base, ', 1 <= x <= ', maxnum, ':');
   (* Set up for loop *)
   number := 1;
   linecount := 0;
   (* Loop *)
   while number <= maxnum do
      begin
         linecount := linecount + 1;
         (* Print a comma and space unless this is the first
            number on the line *)
         if linecount > 1 then
            write (', ');
         (* Display the number *)
         write (number);
         (* Print a comma and go to the next line if this is
            the last number on the line UNLESS it is the
            last number of the series *)
         if (linecount = numperline) and not (number * 2 > maxnum) then
            begin
               writeln (',');
               linecount := 0
            end;
         (* Increment number *)
         number := number * base;
      end;  (* while *)
   writeln;

   (* This program can also be written using a
      REPEAT..UNTIL loop. *)

end.     (* Main *)

Note that I used three constants: the base, the number of powers to display on each line, and the maximum number. This ensures that the program can be easily adaptable in the future.

Using constants rather than literals is a good programming habit to form. When you write really long programs, you may refer to certain numbers thousands of times. If you hardcoded them into your code, you'd have to search them out. Also, you might use the same value in a different context, so you can't simply do a global Search-and-Replace. Using a constant makes it simpler to expand the program.

Also note that I used the longint type for the number variable. This is because to fail the test number <= 20000, number would have to reach 32768, the next power of two after 16384. This exceeds the range of the integer type: -32768 to 32767. (try it without longint and see what happens)