Keys Without ENTER

Introduction

The Turbo course read a single key with Read(Kbd, Ch), which waited for one, and tested whether one was waiting with KeyPressed. HiSoft Pascal has one function that does both jobs - and a catch, which is the reason S5 spent so long on (*$C-*).

INCH

INCH looks at the keyboard and returns at once: the key being pressed, if there is one, or CHR(0) if not. It does not wait and it does not echo the key on the screen. So:

C:=INCH;
IF C<>CHR(0) THEN ...

is Turbo's if KeyPressed then Read(Kbd, Ch), and

REPEAT C:=INCH UNTIL C<>CHR(0);

is Turbo's Read(Kbd, Ch): wait for a key.

The special keys send what they sent in Turbo:

Letters come as the keyboard types them - capitals, while CAPS LOCK is on.

The Catch

Under the compiler's normal setting, the CTRL-C check in every loop (S5) looks at the keyboard too - and a key it sees there, INCH does not. Keys go missing, and some of them turn up printed on the screen. Every program that uses INCH needs (*$C-*) as its first line.

And then CTRL-C no longer stops it. So give the program its own way out - a key it watches for, like Q below - and save it before you run it the first time.

INCH itself is not fast: about 400 calls a second. A game loop that calls it once a pass will not notice; a loop that calls it many times will.

The Code

A * in the middle of the screen, moved by the arrow keys:

10 (*$C-*)
20 PROGRAM MOVER;
30 VAR C:CHAR; X,Y:INTEGER;
40 PROCEDURE GOTOXY(X,Y:INTEGER);
50 BEGIN
60 WRITE(CHR(29),CHR(X-1),CHR(Y-1))
70 END;
80 BEGIN
90 PAGE;
100 WRITE('ARROWS MOVE, Q STOPS');
110 X:=20; Y:=12;
120 REPEAT
130 GOTOXY(X,Y); WRITE('*');
140 REPEAT C:=INCH UNTIL C<>CHR(0);
150 GOTOXY(X,Y); WRITE(' ');
160 IF (C='[') AND (X>1) THEN X:=X-1;
170 IF (C=']') AND (X<40) THEN X:=X+1;
180 IF (C='^') AND (Y>2) THEN Y:=Y-1;
190 IF (C=CHR(10)) AND (Y<23) THEN Y:=Y+1
200 UNTIL C='Q';
210 GOTOXY(1,23)
220 END.

In XBAS the [, ] and ^ in lines 160 to 180 are typed with the LEFT, RIGHT and UP arrow keys - the very keys the program will be watching for.

Starting from

MOVER.ASC saved from XBAS; HPEIN MOVER.

What you should see

A clear screen, the instructions at the top, and a * in the middle that moves one place for each arrow key and stops at the edges. CTRL-C does nothing. Q ends it.

Change One Thing

• Take out line 10. Press the arrow keys a few times. What goes wrong - and where on the screen do the missing keys turn up?
• Put line 10 back and change line 200 to stop on ESC instead of Q.
• Change line 150 to write a . instead of a space. What does the * leave behind?

Exercises

15.1 Count key presses: show the number of keys pressed so far in the top right corner, and stop on Q.

15.2 Make the * keep moving by itself in the last direction chosen, one place every few hundred passes of a loop, turning when an arrow key is pressed.

Worked solutions are in Appendix II.

When It Goes Wrong

Summary

INCH returns the key being pressed, or CHR(0), without waiting or echoing - KeyPressed and Read(Kbd, Ch) in one. The arrow keys give [, ], ^ and CHR(10). A program using INCH needs (*$C-*), and so needs its own way to stop.

Next

S16, Chance - RANDOM, which needs no Randomize.