Console Applications, part 3

Variables, program loop

If we want our console application to run for some time, instead of terminating after outputting a few lines, we have to set up a program loop.

Let's introduce a Boolean variable Quit for controlling the loop. In the loop, we set Quit to True when the user types "Q" followed by ENTER:

program ConsoleTest;
{$APPTYPE CONSOLE}
{$R *.res}
uses
  System.SysUtils;
var
  Quit: Boolean;
  S: string;
begin
  try
    WriteLn('To quit, press Q followed by ENTER');
    Quit := False;
    while not Quit do begin
      ReadLn(S);
      Quit := (S = 'q') OR (S = 'Q');
    end;	  
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.

In a GUI application (Windows program), we don't have to set up such a "program loop". But an archaic console application is not a "real time" application, it is not "event driven" by events, such as mouse clicks and keyboard presses. Just as in the old days, you as the programmer have to instruct the program to wait for key presses, examine which keys were pressed, and make the program act accordingly.

Let's make the program actually do something. We assume that the code speaks for itself:

program ConsoleTest;
{$APPTYPE CONSOLE}
{$R *.res}
uses
  System.SysUtils;
var
  Quit: Boolean;
  S: string;
  R, Guess: integer;  
begin
  try
    Randomize;
    R := Random(10);
    WriteLn('Guess a number');
    WriteLn('To stop, press Q followed by ENTER');
    Quit := False;
    while not Quit do begin
      ReadLn(S);
      Quit := (S = 'q') OR (S = 'Q');
      if not Quit then begin
        if S[1] in ['0'..'9'] then begin	  
          Guess := StrToInt(S);
          if Guess = R then begin
            WriteLn('Correct! Press ENTER to quit');
            ReadLn;
            Quit := True;
          end
          else if Guess < R then
            WriteLn('Wrong... try higher:')
          else
            WriteLn('Wrong... try lower:')
        end
        else
          WriteLn('Try again...'); 		
      end;
    end;	  
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.

« Part 1: Setting up and Compiling a Console Application
« Part 2: Structure; handling Input and Output